option(BUILD_GTEST "Builds the googletest subproject" OFF)
#Note that googlemock target already builds googletest
option(BUILD_GMOCK "Builds the googlemock subproject" ON)
cmake_dependent_option(INSTALL_GTEST "Enable installation of googletest. (Projects embedding googletest may want to turn this OFF.)" ON "BUILD_GTEST OR BUILD_GMOCK" OFF)
cmake_dependent_option(INSTALL_GMOCK "Enable installation of googlemock. (Projects embedding googlemock may want to turn this OFF.)" ON "BUILD_GMOCK" OFF)
Welcome to **Google Test**, Google's C++ test framework!
...
...
@@ -12,7 +12,7 @@ maintain and release them together.
Please see the project page above for more information as well as the
mailing list for questions, discussions, and development. There is
also an IRC channel on OFTC (irc.oftc.net) #gtest available. Please
also an IRC channel on [OFTC](https://webchat.oftc.net/)(irc.oftc.net) #gtest available. Please
join us!
Getting started information for **Google Test** is available in the
...
...
@@ -60,10 +60,12 @@ the following notable projects:
*[Protocol Buffers](https://github.com/google/protobuf), Google's data
interchange format.
* The [OpenCV](http://opencv.org/) computer vision library.
*[tiny-dnn](https://github.com/tiny-dnn/tiny-dnn): header only, dependency-free deep learning framework in C++11
*[tiny-dnn](https://github.com/tiny-dnn/tiny-dnn): header only, dependency-free deep learning framework in C++11.
## Related Open Source Projects ##
[GTest Runner](https://github.com/nholthaus/gtest-runner) is a Qt5 based automated test-runner and Graphical User Interface with powerful features for Windows and Linux platforms.
[Google Test UI](https://github.com/ospector/gtest-gbar) is test runner that runs
your test binary, allows you to track its progress via a progress bar, and
displays a list of test failures. Clicking on one shows failure text. Google
...
...
@@ -86,7 +88,7 @@ effort to support other platforms (e.g. Solaris, AIX, and z/OS).
However, since core members of the Google Test project have no access
to these platforms, Google Test may have outstanding issues there. If
you notice any problems on your platform, please notify
<googletestframework@googlegroups.com>. Patches for fixing them are
[googletestframework@googlegroups.com](https://groups.google.com/forum/#!forum/googletestframework). Patches for fixing them are
@@ -23,8 +23,8 @@ Using Google Mock involves three basic steps:
# Why Google Mock? #
While mock objects help you remove unnecessary dependencies in tests and make them fast and reliable, using mocks manually in C++ is _hard_:
* Someone has to implement the mocks. The job is usually tedious and error-prone. No wonder people go great distance to avoid it.
* The quality of those manually written mocks is a bit, uh, unpredictable. You may see some really polished ones, but you may also see some that were hacked up in a hurry and have all sorts of adhoc restrictions.
* Someone has to implement the mocks. The job is usually tedious and error-prone. No wonder people go great distances to avoid it.
* The quality of those manually written mocks is a bit, uh, unpredictable. You may see some really polished ones, but you may also see some that were hacked up in a hurry and have all sorts of ad-hoc restrictions.
* The knowledge you gained from using one mock doesn't transfer to the next.
In contrast, Java and Python programmers have some fine mock frameworks, which automate the creation of mocks. As a result, mocking is a proven effective technique and widely adopted practice in those communities. Having the right tool absolutely makes the difference.
|`A<type>()` or `An<type>()`|`argument` can be any value of type `type`. |
## Generic Comparison ##
|`Eq(value)` or `value`|`argument == value`|
|:---------------------|:------------------|
|`Ge(value)` |`argument >= value`|
|`Gt(value)` |`argument > value` |
|`Le(value)` |`argument <= value`|
|`Lt(value)` |`argument < value` |
|`Ne(value)` |`argument != value`|
|`IsNull()` |`argument` is a `NULL` pointer (raw or smart).|
|`NotNull()` |`argument` is a non-null pointer (raw or smart).|
|`Ref(variable)` |`argument` is a reference to `variable`.|
|`TypedEq<type>(value)`|`argument` has type `type` and is equal to `value`. You may need to use this instead of `Eq(value)` when the mock function is overloaded.|
Except `Ref()`, these matchers make a _copy_ of `value` in case it's
modified or destructed later. If the compiler complains that `value`
doesn't have a public copy constructor, try wrap it in `ByRef()`,
e.g. `Eq(ByRef(non_copyable_value))`. If you do that, make sure
`non_copyable_value` is not changed afterwards, or the meaning of your
matcher will be changed.
## Floating-Point Matchers ##
|`DoubleEq(a_double)`|`argument` is a `double` value approximately equal to `a_double`, treating two NaNs as unequal.|
|`EndsWith(suffix)` |`argument` ends with string `suffix`. |
|`HasSubstr(string)` |`argument` contains `string` as a sub-string. |
|`MatchesRegex(string)` |`argument` matches the given regular expression with the match starting at the first character and ending at the last character.|
|`StartsWith(prefix)` |`argument` starts with string `prefix`. |
|`StrCaseEq(string)` |`argument` is equal to `string`, ignoring case. |
|`StrCaseNe(string)` |`argument` is not equal to `string`, ignoring case.|
|`StrEq(string)` |`argument` is equal to `string`. |
|`StrNe(string)` |`argument` is not equal to `string`. |
`StrCaseEq()`, `StrCaseNe()`, `StrEq()`, and `StrNe()` work for wide
strings as well.
## Container Matchers ##
Most STL-style containers support `==`, so you can use
`Eq(expected_container)` or simply `expected_container` to match a
container exactly. If you want to write the elements in-line,
match them more flexibly, or get more informative messages, you can use:
| `Contains(e)` | `argument` contains an element that matches `e`, which can be either a value or a matcher. |
|`ElementsAre(e0, e1, ..., en)`|`argument` has `n + 1` elements, where the i-th element matches `ei`, which can be a value or a matcher. 0 to 10 arguments are allowed.|
|`ElementsAreArray(array)` or `ElementsAreArray(array, count)`|The same as `ElementsAre()` except that the expected element values/matchers come from a C-style array.|
| `ContainerEq(container)` | The same as `Eq(container)` except that the failure message also includes which elements are in one container but not the other. |
These matchers can also match:
1. a native array passed by reference (e.g. in `Foo(const int (&a)[5])`), and
1. an array passed as a pointer and a count (e.g. in `Bar(const T* buffer, int len)` -- see [Multi-argument Matchers](#Multiargument_Matchers.md)).
where the array may be multi-dimensional (i.e. its elements can be arrays).
## Member Matchers ##
|`Field(&class::field, m)`|`argument.field` (or `argument->field` when `argument` is a plain pointer) matches matcher `m`, where `argument` is an object of type _class_.|
|`Key(e)` |`argument.first` matches `e`, which can be either a value or a matcher. E.g. `Contains(Key(Le(5)))` can verify that a `map` contains a key `<= 5`.|
|`Pair(m1, m2)` |`argument` is an `std::pair` whose `first` field matches `m1` and `second` field matches `m2`. |
|`Property(&class::property, m)`|`argument.property()` (or `argument->property()` when `argument` is a plain pointer) matches matcher `m`, where `argument` is an object of type _class_.|
## Matching the Result of a Function or Functor ##
|`ResultOf(f, m)`|`f(argument)` matches matcher `m`, where `f` is a function or functor.|
| `MATCHER_P(IsDivisibleBy, n, "") { *result_listener << "where the remainder is " << (arg % n); return (arg % n) == 0; }` | Defines a macher `IsDivisibleBy(n)` to match a number divisible by `n`. |
| `MATCHER_P2(IsBetween, a, b, "is between %(a)s and %(b)s") { return a <= arg && arg <= b; }` | Defines a matcher `IsBetween(a, b)` to match a value in the range [`a`, `b`]. |
**Notes:**
1. The `MATCHER*` macros cannot be used inside a function or class.
1. The matcher body must be _purely functional_ (i.e. it cannot have any side effect, and the result must not depend on anything other than the value being matched and the matcher parameters).
1. You can use `PrintToString(x)` to convert a value `x` of any type to a string.
## Matchers as Test Assertions ##
|`ASSERT_THAT(expression, m)`|Generates a [fatal failure](http://code.google.com/p/googletest/wiki/GoogleTestPrimer#Assertions) if the value of `expression` doesn't match matcher `m`.|
| `DeleteArg<N>()` | Delete the `N`-th (0-based) argument, which must be a pointer. |
| `SaveArg<N>(pointer)` | Save the `N`-th (0-based) argument to `*pointer`. |
| `SetArgReferee<N>(value)` | Assign value to the variable referenced by the `N`-th (0-based) argument. |
|`SetArgumentPointee<N>(value)`|Assign `value` to the variable pointed by the `N`-th (0-based) argument.|
|`SetArrayArgument<N>(first, last)`|Copies the elements in source range [`first`, `last`) to the array pointed to by the `N`-th (0-based) argument, which can be either a pointer or an iterator. The action does not take ownership of the elements in the source range.|
|`SetErrnoAndReturn(error, value)`|Set `errno` to `error` and return `value`.|
|`Throw(exception)` |Throws the given exception, which can be any copyable value. Available since v1.1.0.|
## Using a Function or a Functor as an Action ##
|`Invoke(f)`|Invoke `f` with the arguments passed to the mock function, where `f` can be a global/static function or a functor.|
|`Invoke(object_pointer, &class::method)`|Invoke the {method on the object with the arguments passed to the mock function. |
|`InvokeWithoutArgs(f)`|Invoke `f`, which can be a global/static function or a functor. `f` must take no arguments. |
|`InvokeWithoutArgs(object_pointer, &class::method)`|Invoke the method on the object, which takes no arguments. |
|`InvokeArgument<N>(arg1, arg2, ..., argk)`|Invoke the mock function's `N`-th (0-based) argument, which must be a function or a functor, with the `k` arguments.|
The return value of the invoked function is used as the return value
of the action.
When defining a function or functor to be used with `Invoke*()`, you can declare any unused parameters as `Unused`:
**Note:** due to technical reasons, `DoDefault()` cannot be used inside a composite action - trying to do so will result in a run-time error.
## Composite Actions ##
|`DoAll(a1, a2, ..., an)`|Do all actions `a1` to `an` and return the result of `an` in each invocation. The first `n - 1` sub-actions must return void. |
Your code should talk to `FileInterface` to open a file. Now it's
easy to mock out the function.
This may seem much hassle, but in practice you often have multiple
related functions that you can put in the same interface, so the
per-function syntactic overhead will be much lower.
If you are concerned about the performance overhead incurred by
virtual functions, and profiling confirms your concern, you can
combine this with the recipe for [mocking non-virtual methods](#Mocking_Nonvirtual_Methods.md).
## Nice Mocks and Strict Mocks ##
If a mock method has no `EXPECT_CALL` spec but is called, Google Mock
will print a warning about the "uninteresting call". The rationale is:
* New methods may be added to an interface after a test is written. We shouldn't fail a test just because a method it doesn't know about is called.
* However, this may also mean there's a bug in the test, so Google Mock shouldn't be silent either. If the user believes these calls are harmless, he can add an `EXPECT_CALL()` to suppress the warning.
However, sometimes you may want to suppress all "uninteresting call"
warnings, while sometimes you may want the opposite, i.e. to treat all
of them as errors. Google Mock lets you make the decision on a
per-mock-object basis.
Suppose your test uses a mock class `MockFoo`:
```
TEST(...) {
MockFoo mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
```
If a method of `mock_foo` other than `DoThis()` is called, it will be
reported by Google Mock as a warning. However, if you rewrite your
test to use `NiceMock<MockFoo>` instead, the warning will be gone,
resulting in a cleaner test output:
```
using ::testing::NiceMock;
TEST(...) {
NiceMock<MockFoo> mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
```
`NiceMock<MockFoo>` is a subclass of `MockFoo`, so it can be used
wherever `MockFoo` is accepted.
It also works if `MockFoo`'s constructor takes some arguments, as
The usage of `StrictMock` is similar, except that it makes all
uninteresting calls failures:
```
using ::testing::StrictMock;
TEST(...) {
StrictMock<MockFoo> mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
// The test will fail if a method of mock_foo other than DoThis()
// is called.
}
```
There are some caveats though (I don't like them just as much as the
next guy, but sadly they are side effects of C++'s limitations):
1.`NiceMock<MockFoo>` and `StrictMock<MockFoo>` only work for mock methods defined using the `MOCK_METHOD*` family of macros **directly** in the `MockFoo` class. If a mock method is defined in a **base class** of `MockFoo`, the "nice" or "strict" modifier may not affect it, depending on the compiler. In particular, nesting `NiceMock` and `StrictMock` (e.g. `NiceMock<StrictMock<MockFoo> >`) is **not** supported.
1. The constructors of the base mock (`MockFoo`) cannot have arguments passed by non-const reference, which happens to be banned by the [Google C++ style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml).
1. During the constructor or destructor of `MockFoo`, the mock object is _not_ nice or strict. This may cause surprises if the constructor or destructor calls a mock method on `this` object. (This behavior, however, is consistent with C++'s general rule: if a constructor or destructor calls a virtual method of `this` object, that method is treated as non-virtual. In other words, to the base class's constructor or destructor, `this` object behaves like an instance of the base class, not the derived class. This rule is required for safety. Otherwise a base constructor may use members of a derived class before they are initialized, or a base destructor may use members of a derived class after they have been destroyed.)
Finally, you should be **very cautious** when using this feature, as the
decision you make applies to **all** future changes to the mock
class. If an important change is made in the interface you are mocking
(and thus in the mock class), it could break your tests (if you use
`StrictMock`) or let bugs pass through without a warning (if you use
`NiceMock`). Therefore, try to specify the mock's behavior using
explicit `EXPECT_CALL` first, and only turn to `NiceMock` or
`StrictMock` as the last resort.
## Simplifying the Interface without Breaking Existing Code ##
Sometimes a method has a long list of arguments that is mostly
By defining a new mock method with a trimmed argument list, we make
the mock class much more user-friendly.
## Alternative to Mocking Concrete Classes ##
Often you may find yourself using classes that don't implement
interfaces. In order to test your code that uses such a class (let's
call it `Concrete`), you may be tempted to make the methods of
`Concrete` virtual and then mock it.
Try not to do that.
Making a non-virtual function virtual is a big decision. It creates an
extension point where subclasses can tweak your class' behavior. This
weakens your control on the class because now it's harder to maintain
the class' invariants. You should make a function virtual only when
there is a valid reason for a subclass to override it.
Mocking concrete classes directly is problematic as it creates a tight
coupling between the class and the tests - any small change in the
class may invalidate your tests and make test maintenance a pain.
To avoid such problems, many programmers have been practicing "coding
to interfaces": instead of talking to the `Concrete` class, your code
would define an interface and talk to it. Then you implement that
interface as an adaptor on top of `Concrete`. In tests, you can easily
mock that interface to observe how your code is doing.
This technique incurs some overhead:
* You pay the cost of virtual function calls (usually not a problem).
* There is more abstraction for the programmers to learn.
However, it can also bring significant benefits in addition to better
testability:
*`Concrete`'s API may not fit your problem domain very well, as you may not be the only client it tries to serve. By designing your own interface, you have a chance to tailor it to your need - you may add higher-level functionalities, rename stuff, etc instead of just trimming the class. This allows you to write your code (user of the interface) in a more natural way, which means it will be more readable, more maintainable, and you'll be more productive.
* If `Concrete`'s implementation ever has to change, you don't have to rewrite everywhere it is used. Instead, you can absorb the change in your implementation of the interface, and your other code and tests will be insulated from this change.
Some people worry that if everyone is practicing this technique, they
will end up writing lots of redundant code. This concern is totally
understandable. However, there are two reasons why it may not be the
case:
* Different projects may need to use `Concrete` in different ways, so the best interfaces for them will be different. Therefore, each of them will have its own domain-specific interface on top of `Concrete`, and they will not be the same code.
* If enough projects want to use the same interface, they can always share it, just like they have been sharing `Concrete`. You can check in the interface and the adaptor somewhere near `Concrete` (perhaps in a `contrib` sub-directory) and let many projects use it.
You need to weigh the pros and cons carefully for your particular
problem, but I'd like to assure you that the Java community has been
practicing this for a long time and it's a proven effective technique
applicable in a wide variety of situations. :-)
## Delegating Calls to a Fake ##
Some times you have a non-trivial fake implementation of an
interface. For example:
```
class Foo {
public:
virtual ~Foo() {}
virtual char DoThis(int n) = 0;
virtual void DoThat(const char* s, int* p) = 0;
};
class FakeFoo : public Foo {
public:
virtual char DoThis(int n) {
return (n > 0) ? '+' :
(n < 0) ? '-' : '0';
}
virtual void DoThat(const char* s, int* p) {
*p = strlen(s);
}
};
```
Now you want to mock this interface such that you can set expectations
on it. However, you also want to use `FakeFoo` for the default
behavior, as duplicating it in the mock object is, well, a lot of
work.
When you define the mock class using Google Mock, you can have it
delegate its default action to a fake class you already have, using
this pattern:
```
using ::testing::_;
using ::testing::Invoke;
class MockFoo : public Foo {
public:
// Normal mock method definitions using Google Mock.
// Delegates the default actions of the methods to a FakeFoo object.
// This must be called *before* the custom ON_CALL() statements.
void DelegateToFake() {
ON_CALL(*this, DoThis(_))
.WillByDefault(Invoke(&fake_, &FakeFoo::DoThis));
ON_CALL(*this, DoThat(_, _))
.WillByDefault(Invoke(&fake_, &FakeFoo::DoThat));
}
private:
FakeFoo fake_; // Keeps an instance of the fake in the mock.
};
```
With that, you can use `MockFoo` in your tests as usual. Just remember
that if you don't explicitly set an action in an `ON_CALL()` or
`EXPECT_CALL()`, the fake will be called upon to do it:
```
using ::testing::_;
TEST(AbcTest, Xyz) {
MockFoo foo;
foo.DelegateToFake(); // Enables the fake for delegation.
// Put your ON_CALL(foo, ...)s here, if any.
// No action specified, meaning to use the default action.
EXPECT_CALL(foo, DoThis(5));
EXPECT_CALL(foo, DoThat(_, _));
int n = 0;
EXPECT_EQ('+', foo.DoThis(5)); // FakeFoo::DoThis() is invoked.
foo.DoThat("Hi", &n); // FakeFoo::DoThat() is invoked.
EXPECT_EQ(2, n);
}
```
**Some tips:**
* If you want, you can still override the default action by providing your own `ON_CALL()` or using `.WillOnce()` / `.WillRepeatedly()` in `EXPECT_CALL()`.
* In `DelegateToFake()`, you only need to delegate the methods whose fake implementation you intend to use.
* The general technique discussed here works for overloaded methods, but you'll need to tell the compiler which version you mean. To disambiguate a mock function (the one you specify inside the parentheses of `ON_CALL()`), see the "Selecting Between Overloaded Functions" section on this page; to disambiguate a fake function (the one you place inside `Invoke()`), use a `static_cast` to specify the function's type.
* Having to mix a mock and a fake is often a sign of something gone wrong. Perhaps you haven't got used to the interaction-based way of testing yet. Or perhaps your interface is taking on too many roles and should be split up. Therefore, **don't abuse this**. We would only recommend to do it as an intermediate step when you are refactoring your code.
Regarding the tip on mixing a mock and a fake, here's an example on
why it may be a bad sign: Suppose you have a class `System` for
low-level system operations. In particular, it does file and I/O
operations. And suppose you want to test how your code uses `System`
to do I/O, and you just want the file operations to work normally. If
you mock out the entire `System` class, you'll have to provide a fake
implementation for the file operation part, which suggests that
`System` is taking on too many roles.
Instead, you can define a `FileOps` interface and an `IOOps` interface
and split `System`'s functionalities into the two. Then you can mock
`IOOps` without mocking `FileOps`.
## Delegating Calls to a Real Object ##
When using testing doubles (mocks, fakes, stubs, and etc), sometimes
their behaviors will differ from those of the real objects. This
difference could be either intentional (as in simulating an error such
that you can test the error handling code) or unintentional. If your
mocks have different behaviors than the real objects by mistake, you
could end up with code that passes the tests but fails in production.
You can use the _delegating-to-real_ technique to ensure that your
mock has the same behavior as the real object while retaining the
ability to validate calls. This technique is very similar to the
delegating-to-fake technique, the difference being that we use a real
object instead of a fake. Here's an example:
```
using ::testing::_;
using ::testing::AtLeast;
using ::testing::Invoke;
class MockFoo : public Foo {
public:
MockFoo() {
// By default, all calls are delegated to the real object.
ON_CALL(*this, DoThis())
.WillByDefault(Invoke(&real_, &Foo::DoThis));
ON_CALL(*this, DoThat(_))
.WillByDefault(Invoke(&real_, &Foo::DoThat));
...
}
MOCK_METHOD0(DoThis, ...);
MOCK_METHOD1(DoThat, ...);
...
private:
Foo real_;
};
...
MockFoo mock;
EXPECT_CALL(mock, DoThis())
.Times(3);
EXPECT_CALL(mock, DoThat("Hi"))
.Times(AtLeast(1));
... use mock in test ...
```
With this, Google Mock will verify that your code made the right calls
(with the right arguments, in the right order, called the right number
of times, etc), and a real object will answer the calls (so the
behavior will be the same as in production). This gives you the best
of both worlds.
## Delegating Calls to a Parent Class ##
Ideally, you should code to interfaces, whose methods are all pure
virtual. In reality, sometimes you do need to mock a virtual method
that is not pure (i.e, it already has an implementation). For example:
```
class Foo {
public:
virtual ~Foo();
virtual void Pure(int n) = 0;
virtual int Concrete(const char* str) { ... }
};
class MockFoo : public Foo {
public:
// Mocking a pure method.
MOCK_METHOD1(Pure, void(int n));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD1(Concrete, int(const char* str));
};
```
Sometimes you may want to call `Foo::Concrete()` instead of
`MockFoo::Concrete()`. Perhaps you want to do it as part of a stub
action, or perhaps your test doesn't need to mock `Concrete()` at all
(but it would be oh-so painful to have to define a new mock class
whenever you don't need to mock one of its methods).
The trick is to leave a back door in your mock class for accessing the
real methods in the base class:
```
class MockFoo : public Foo {
public:
// Mocking a pure method.
MOCK_METHOD1(Pure, void(int n));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD1(Concrete, int(const char* str));
// Use this to call Concrete() defined in Foo.
int FooConcrete(const char* str) { return Foo::Concrete(str); }
};
```
Now, you can call `Foo::Concrete()` inside an action by:
```
using ::testing::_;
using ::testing::Invoke;
...
EXPECT_CALL(foo, Concrete(_))
.WillOnce(Invoke(&foo, &MockFoo::FooConcrete));
```
or tell the mock object that you don't want to mock `Concrete()`:
(Why don't we just write `Invoke(&foo, &Foo::Concrete)`? If you do
that, `MockFoo::Concrete()` will be called (and cause an infinite
recursion) since `Foo::Concrete()` is virtual. That's just how C++
works.)
# Using Matchers #
## Matching Argument Values Exactly ##
You can specify exactly which arguments a mock method is expecting:
```
using ::testing::Return;
...
EXPECT_CALL(foo, DoThis(5))
.WillOnce(Return('a'));
EXPECT_CALL(foo, DoThat("Hello", bar));
```
## Using Simple Matchers ##
You can use matchers to match arguments that have a certain property:
```
using ::testing::Ge;
using ::testing::NotNull;
using ::testing::Return;
...
EXPECT_CALL(foo, DoThis(Ge(5))) // The argument must be >= 5.
.WillOnce(Return('a'));
EXPECT_CALL(foo, DoThat("Hello", NotNull()));
// The second argument must not be NULL.
```
A frequently used matcher is `_`, which matches anything:
```
using ::testing::_;
using ::testing::NotNull;
...
EXPECT_CALL(foo, DoThat(_, NotNull()));
```
## Combining Matchers ##
You can build complex matchers from existing ones using `AllOf()`,
`AnyOf()`, and `Not()`:
```
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::HasSubstr;
using ::testing::Ne;
using ::testing::Not;
...
// The argument must be > 5 and != 10.
EXPECT_CALL(foo, DoThis(AllOf(Gt(5),
Ne(10))));
// The first argument must not contain sub-string "blah".
EXPECT_CALL(foo, DoThat(Not(HasSubstr("blah")),
NULL));
```
## Casting Matchers ##
Google Mock matchers are statically typed, meaning that the compiler
can catch your mistake if you use a matcher of the wrong type (for
example, if you use `Eq(5)` to match a `string` argument). Good for
you!
Sometimes, however, you know what you're doing and want the compiler
to give you some slack. One example is that you have a matcher for
`long` and the argument you want to match is `int`. While the two
types aren't exactly the same, there is nothing really wrong with
using a `Matcher<long>` to match an `int` - after all, we can first
convert the `int` argument to a `long` before giving it to the
matcher.
To support this need, Google Mock gives you the
`SafeMatcherCast<T>(m)` function. It casts a matcher `m` to type
`Matcher<T>`. To ensure safety, Google Mock checks that (let `U` be the
type `m` accepts):
1. Type `T` can be implicitly cast to type `U`;
1. When both `T` and `U` are built-in arithmetic types (`bool`, integers, and floating-point numbers), the conversion from `T` to `U` is not lossy (in other words, any value representable by `T` can also be represented by `U`); and
1. When `U` is a reference, `T` must also be a reference (as the underlying matcher may be interested in the address of the `U` value).
The code won't compile if any of these conditions isn't met.
*`ElementAre*()` works with _any_ container that implements the STL iterator concept (i.e. it has a `const_iterator` type and supports `begin()/end()`) and supports `size()`, not just the ones defined in STL. It will even work with container types yet to be written - as long as they follows the above pattern.
* You can use nested `ElementAre*()` to match nested (multi-dimensional) containers.
* If the container is passed by pointer instead of by reference, just write `Pointee(ElementsAre*(...))`.
* The order of elements _matters_ for `ElementsAre*()`. Therefore don't use it with containers whose element order is undefined (e.g. `hash_map`).
## Sharing Matchers ##
Under the hood, a Google Mock matcher object consists of a pointer to
a ref-counted implementation object. Copying matchers is allowed and
very efficient, as only the pointer is copied. When the last matcher
that references the implementation object dies, the implementation
object will be deleted.
Therefore, if you have some complex matcher that you want to use again
and again, there is no need to build it everytime. Just assign it to a
matcher variable and use that variable repeatedly! For example,
```
Matcher<int> in_range = AllOf(Gt(5), Le(10));
... use in_range as a matcher in multiple EXPECT_CALLs ...
```
# Setting Expectations #
## Ignoring Uninteresting Calls ##
If you are not interested in how a mock method is called, just don't
say anything about it. In this case, if the method is ever called,
Google Mock will perform its default action to allow the test program
to continue. If you are not happy with the default action taken by
Google Mock, you can override it using `DefaultValue<T>::Set()`
(described later in this document) or `ON_CALL()`.
Please note that once you expressed interest in a particular mock
method (via `EXPECT_CALL()`), all invocations to it must match some
expectation. If this function is called but the arguments don't match
any `EXPECT_CALL()` statement, it will be an error.
## Disallowing Unexpected Calls ##
If a mock method shouldn't be called at all, explicitly say so:
```
using ::testing::_;
...
EXPECT_CALL(foo, Bar(_))
.Times(0);
```
If some calls to the method are allowed, but the rest are not, just
list all the expected calls:
```
using ::testing::AnyNumber;
using ::testing::Gt;
...
EXPECT_CALL(foo, Bar(5));
EXPECT_CALL(foo, Bar(Gt(10)))
.Times(AnyNumber());
```
A call to `foo.Bar()` that doesn't match any of the `EXPECT_CALL()`
statements will be an error.
## Expecting Ordered Calls ##
Although an `EXPECT_CALL()` statement defined earlier takes precedence
when Google Mock tries to match a function call with an expectation,
by default calls don't have to happen in the order `EXPECT_CALL()`
statements are written. For example, if the arguments match the
matchers in the third `EXPECT_CALL()`, but not those in the first two,
then the third expectation will be used.
If you would rather have all calls occur in the order of the
expectations, put the `EXPECT_CALL()` statements in a block where you
define a variable of type `InSequence`:
```
using ::testing::_;
using ::testing::InSequence;
{
InSequence s;
EXPECT_CALL(foo, DoThis(5));
EXPECT_CALL(bar, DoThat(_))
.Times(2);
EXPECT_CALL(foo, DoThis(6));
}
```
In this example, we expect a call to `foo.DoThis(5)`, followed by two
calls to `bar.DoThat()` where the argument can be anything, which are
in turn followed by a call to `foo.DoThis(6)`. If a call occurred
out-of-order, Google Mock will report an error.
## Expecting Partially Ordered Calls ##
Sometimes requiring everything to occur in a predetermined order can
lead to brittle tests. For example, we may care about `A` occurring
before both `B` and `C`, but aren't interested in the relative order
of `B` and `C`. In this case, the test should reflect our real intent,
instead of being overly constraining.
Google Mock allows you to impose an arbitrary DAG (directed acyclic
graph) on the calls. One way to express the DAG is to use the
[After](V1_5_CheatSheet#The_After_Clause.md) clause of `EXPECT_CALL`.
Another way is via the `InSequence()` clause (not the same as the
`InSequence` class), which we borrowed from jMock 2. It's less
flexible than `After()`, but more convenient when you have long chains
of sequential calls, as it doesn't require you to come up with
different names for the expectations in the chains. Here's how it
works:
If we view `EXPECT_CALL()` statements as nodes in a graph, and add an
edge from node A to node B wherever A must occur before B, we can get
a DAG. We use the term "sequence" to mean a directed path in this
DAG. Now, if we decompose the DAG into sequences, we just need to know
which sequences each `EXPECT_CALL()` belongs to in order to be able to
reconstruct the orginal DAG.
So, to specify the partial order on the expectations we need to do two
things: first to define some `Sequence` objects, and then for each
`EXPECT_CALL()` say which `Sequence` objects it is part
of. Expectations in the same sequence must occur in the order they are
written. For example,
```
using ::testing::Sequence;
Sequence s1, s2;
EXPECT_CALL(foo, A())
.InSequence(s1, s2);
EXPECT_CALL(bar, B())
.InSequence(s1);
EXPECT_CALL(bar, C())
.InSequence(s2);
EXPECT_CALL(foo, D())
.InSequence(s2);
```
specifies the following DAG (where `s1` is `A -> B`, and `s2` is `A ->
C -> D`):
```
+---> B
|
A ---|
|
+---> C ---> D
```
This means that A must occur before B and C, and C must occur before
D. There's no restriction about the order other than these.
## Controlling When an Expectation Retires ##
When a mock method is called, Google Mock only consider expectations
that are still active. An expectation is active when created, and
becomes inactive (aka _retires_) when a call that has to occur later
has occurred. For example, in
```
using ::testing::_;
using ::testing::Sequence;
Sequence s1, s2;
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #1
.Times(AnyNumber())
.InSequence(s1, s2);
EXPECT_CALL(log, Log(WARNING, _, "Data set is empty.")) // #2
.InSequence(s1);
EXPECT_CALL(log, Log(WARNING, _, "User not found.")) // #3
.InSequence(s2);
```
as soon as either #2 or #3 is matched, #1 will retire. If a warning
`"File too large."` is logged after this, it will be an error.
Note that an expectation doesn't retire automatically when it's
saturated. For example,
```
using ::testing::_;
...
EXPECT_CALL(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")); // #2
```
says that there will be exactly one warning with the message `"File
too large."`. If the second warning contains this message too, #2 will
match again and result in an upper-bound-violated error.
If this is not what you want, you can ask an expectation to retire as
soon as it becomes saturated:
```
using ::testing::_;
...
EXPECT_CALL(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #2
.RetiresOnSaturation();
```
Here #2 can be used only once, so if you have two warnings with the
message `"File too large."`, the first will match #2 and the second
will match #1 - there will be no error.
# Using Actions #
## Returning References from Mock Methods ##
If a mock function's return type is a reference, you need to use
`ReturnRef()` instead of `Return()` to return a result:
```
using ::testing::ReturnRef;
class MockFoo : public Foo {
public:
MOCK_METHOD0(GetBar, Bar&());
};
...
MockFoo foo;
Bar bar;
EXPECT_CALL(foo, GetBar())
.WillOnce(ReturnRef(bar));
```
## Combining Actions ##
Want to do more than one thing when a function is called? That's
fine. `DoAll()` allow you to do sequence of actions every time. Only
the return value of the last action in the sequence will be used.
```
using ::testing::DoAll;
class MockFoo : public Foo {
public:
MOCK_METHOD1(Bar, bool(int n));
};
...
EXPECT_CALL(foo, Bar(_))
.WillOnce(DoAll(action_1,
action_2,
...
action_n));
```
## Mocking Side Effects ##
Sometimes a method exhibits its effect not via returning a value but
via side effects. For example, it may change some global state or
modify an output argument. To mock side effects, in general you can
define your own action by implementing `::testing::ActionInterface`.
If all you need to do is to change an output argument, the built-in
## Changing a Mock Object's Behavior Based on the State ##
If you expect a call to change the behavior of a mock object, you can use `::testing::InSequence` to specify different behaviors before and after the call:
```
using ::testing::InSequence;
using ::testing::Return;
...
{
InSequence seq;
EXPECT_CALL(my_mock, IsDirty())
.WillRepeatedly(Return(true));
EXPECT_CALL(my_mock, Flush());
EXPECT_CALL(my_mock, IsDirty())
.WillRepeatedly(Return(false));
}
my_mock.FlushIfDirty();
```
This makes `my_mock.IsDirty()` return `true` before `my_mock.Flush()` is called and return `false` afterwards.
If the behavior change is more complex, you can store the effects in a variable and make a mock method get its return value from that variable:
```
using ::testing::_;
using ::testing::SaveArg;
using ::testing::Return;
ACTION_P(ReturnPointee, p) { return *p; }
...
int previous_value = 0;
EXPECT_CALL(my_mock, GetPrevValue())
.WillRepeatedly(ReturnPointee(&previous_value));
EXPECT_CALL(my_mock, UpdateValue(_))
.WillRepeatedly(SaveArg<0>(&previous_value));
my_mock.DoSomethingToUpdateValue();
```
Here `my_mock.GetPrevValue()` will always return the argument of the last `UpdateValue()` call.
## Setting the Default Value for a Return Type ##
If a mock method's return type is a built-in C++ type or pointer, by
default it will return 0 when invoked. You only need to specify an
action if this default value doesn't work for you.
Sometimes, you may want to change this default value, or you may want
to specify a default value for types Google Mock doesn't know
about. You can do this using the `::testing::DefaultValue` class
template:
```
class MockFoo : public Foo {
public:
MOCK_METHOD0(CalculateBar, Bar());
};
...
Bar default_bar;
// Sets the default return value for type Bar.
DefaultValue<Bar>::Set(default_bar);
MockFoo foo;
// We don't need to specify an action here, as the default
// return value works for us.
EXPECT_CALL(foo, CalculateBar());
foo.CalculateBar(); // This should return default_bar.
// Unsets the default return value.
DefaultValue<Bar>::Clear();
```
Please note that changing the default value for a type can make you
tests hard to understand. We recommend you to use this feature
judiciously. For example, you may want to make sure the `Set()` and
`Clear()` calls are right next to the code that uses your mock.
## Setting the Default Actions for a Mock Method ##
You've learned how to change the default value of a given
type. However, this may be too coarse for your purpose: perhaps you
have two mock methods with the same return type and you want them to
have different behaviors. The `ON_CALL()` macro allows you to
customize your mock's behavior at the method level:
```
using ::testing::_;
using ::testing::AnyNumber;
using ::testing::Gt;
using ::testing::Return;
...
ON_CALL(foo, Sign(_))
.WillByDefault(Return(-1));
ON_CALL(foo, Sign(0))
.WillByDefault(Return(0));
ON_CALL(foo, Sign(Gt(0)))
.WillByDefault(Return(1));
EXPECT_CALL(foo, Sign(_))
.Times(AnyNumber());
foo.Sign(5); // This should return 1.
foo.Sign(-9); // This should return -1.
foo.Sign(0); // This should return 0.
```
As you may have guessed, when there are more than one `ON_CALL()`
statements, the news order take precedence over the older ones. In
other words, the **last** one that matches the function arguments will
be used. This matching order allows you to set up the common behavior
in a mock object's constructor or the test fixture's set-up phase and
specialize the mock's behavior later.
## Using Functions/Methods/Functors as Actions ##
If the built-in actions don't suit you, you can easily use an existing
For better readability, Google Mock also gives you:
*`WithoutArgs(action)` when the inner `action` takes _no_ argument, and
*`WithArg<N>(action)` (no `s` after `Arg`) when the inner `action` takes _one_ argument.
As you may have realized, `InvokeWithoutArgs(...)` is just syntactic
sugar for `WithoutArgs(Inovke(...))`.
Here are more tips:
* The inner action used in `WithArgs` and friends does not have to be `Invoke()` -- it can be anything.
* You can repeat an argument in the argument list if necessary, e.g. `WithArgs<2, 3, 3, 5>(...)`.
* You can change the order of the arguments, e.g. `WithArgs<3, 2, 1>(...)`.
* The types of the selected arguments do _not_ have to match the signature of the inner action exactly. It works as long as they can be implicitly converted to the corresponding arguments of the inner action. For example, if the 4-th argument of the mock function is an `int` and `my_action` takes a `double`, `WithArg<4>(my_action)` will work.
## Ignoring Arguments in Action Functions ##
The selecting-an-action's-arguments recipe showed us one way to make a
mock function and an action with incompatible argument lists fit
together. The downside is that wrapping the action in
`WithArgs<...>()` can get tedious for people writing the tests.
If you are defining a function, method, or functor to be used with
`Invoke*()`, and you are not interested in some of its arguments, an
alternative to `WithArgs` is to declare the uninteresting arguments as
`Unused`. This makes the definition less cluttered and less fragile in
case the types of the uninteresting arguments change. It could also
increase the chance the action function can be reused. For example,
The expectation spec says that the first `Bar("a")` must happen before
check point "1", the second `Bar("a")` must happen after check point "2",
and nothing should happen between the two check points. The explicit
check points make it easy to tell which `Bar("a")` is called by which
call to `Foo()`.
## Mocking Destructors ##
Sometimes you want to make sure a mock object is destructed at the
right time, e.g. after `bar->A()` is called but before `bar->B()` is
called. We already know that you can specify constraints on the order
of mock function calls, so all we need to do is to mock the destructor
of the mock function.
This sounds simple, except for one problem: a destructor is a special
function with special syntax and special semantics, and the
`MOCK_METHOD0` macro doesn't work for it:
```
MOCK_METHOD0(~MockFoo, void()); // Won't compile!
```
The good news is that you can use a simple pattern to achieve the same
effect. First, add a mock function `Die()` to your mock class and call
it in the destructor, like this:
```
class MockFoo : public Foo {
...
// Add the following two lines to the mock class.
MOCK_METHOD0(Die, void());
virtual ~MockFoo() { Die(); }
};
```
(If the name `Die()` clashes with an existing symbol, choose another
name.) Now, we have translated the problem of testing when a `MockFoo`
object dies to testing when its `Die()` method is called:
```
MockFoo* foo = new MockFoo;
MockBar* bar = new MockBar;
...
{
InSequence s;
// Expects *foo to die after bar->A() and before bar->B().
EXPECT_CALL(*bar, A());
EXPECT_CALL(*foo, Die());
EXPECT_CALL(*bar, B());
}
```
And that's that.
## Using Google Mock and Threads ##
**IMPORTANT NOTE:** What we describe in this recipe is **NOT** true yet,
as Google Mock is not currently thread-safe. However, all we need to
make it thread-safe is to implement some synchronization operations in
`<gtest/internal/gtest-port.h>` - and then the information below will
become true.
In a **unit** test, it's best if you could isolate and test a piece of
code in a single-threaded context. That avoids race conditions and
dead locks, and makes debugging your test much easier.
Yet many programs are multi-threaded, and sometimes to test something
we need to pound on it from more than one thread. Google Mock works
for this purpose too.
Remember the steps for using a mock:
1. Create a mock object `foo`.
1. Set its default actions and expectations using `ON_CALL()` and `EXPECT_CALL()`.
1. The code under test calls methods of `foo`.
1. Optionally, verify and reset the mock.
1. Destroy the mock yourself, or let the code under test destroy it. The destructor will automatically verify it.
If you follow the following simple rules, your mocks and threads can
live happily togeter:
* Execute your _test code_ (as opposed to the code being tested) in _one_ thread. This makes your test easy to follow.
* Obviously, you can do step #1 without locking.
* When doing step #2 and #5, make sure no other thread is accessing `foo`. Obvious too, huh?
* #3 and #4 can be done either in one thread or in multiple threads - anyway you want. Google Mock takes care of the locking, so you don't have to do any - unless required by your test logic.
If you violate the rules (for example, if you set expectations on a
mock while another thread is calling its methods), you get undefined
behavior. That's not fun, so don't do it.
Google Mock guarantees that the action for a mock function is done in
the same thread that called the mock function. For example, in
```
EXPECT_CALL(mock, Foo(1))
.WillOnce(action1);
EXPECT_CALL(mock, Foo(2))
.WillOnce(action2);
```
if `Foo(1)` is called in thread 1 and `Foo(2)` is called in thread 2,
Google Mock will execute `action1` in thread 1 and `action2` in thread
2.
Google Mock does _not_ impose a sequence on actions performed in
different threads (doing so may create deadlocks as the actions may
need to cooperate). This means that the execution of `action1` and
`action2` in the above example _may_ interleave. If this is a problem,
you should add proper synchronization logic to `action1` and `action2`
to make the test thread-safe.
Also, remember that `DefaultValue<T>` is a global resource that
potentially affects _all_ living mock objects in your
program. Naturally, you won't want to mess with it from multiple
threads or when there still are mocks in action.
## Controlling How Much Information Google Mock Prints ##
When Google Mock sees something that has the potential of being an
error (e.g. a mock function with no expectation is called, a.k.a. an
uninteresting call, which is allowed but perhaps you forgot to
explicitly ban the call), it prints some warning messages, including
the arguments of the function and the return value. Hopefully this
will remind you to take a look and see if there is indeed a problem.
Sometimes you are confident that your tests are correct and may not
appreciate such friendly messages. Some other times, you are debugging
your tests or learning about the behavior of the code you are testing,
and wish you could observe every mock call that happens (including
argument values and the return value). Clearly, one size doesn't fit
all.
You can control how much Google Mock tells you using the
`--gmock_verbose=LEVEL` command-line flag, where `LEVEL` is a string
with three possible values:
*`info`: Google Mock will print all informational messages, warnings, and errors (most verbose). At this setting, Google Mock will also log any calls to the `ON_CALL/EXPECT_CALL` macros.
*`warning`: Google Mock will print both warnings and errors (less verbose). This is the default.
*`error`: Google Mock will print errors only (least verbose).
Alternatively, you can adjust the value of that flag from within your
tests like so:
```
::testing::FLAGS_gmock_verbose = "error";
```
Now, judiciously use the right flag to enable Google Mock serve you better!
## Running Tests in Emacs ##
If you build and run your tests in Emacs, the source file locations of
Google Mock and [Google Test](http://code.google.com/p/googletest/)
errors will be highlighted. Just press `<Enter>` on one of them and
you'll be taken to the offending line. Or, you can just type `C-x ``
to jump to the next error.
To make it even easier, you can add the following lines to your
// Expects mock_foo.Bar(n) to be called where n is divisible by 7.
EXPECT_CALL(mock_foo, Bar(IsDivisibleBy7()));
```
or,
```
// Verifies that the value of some_expression is divisible by 7.
EXPECT_THAT(some_expression, IsDivisibleBy7());
```
If the above assertion fails, it will print something like:
```
Value of: some_expression
Expected: is divisible by 7
Actual: 27
```
where the description `"is divisible by 7"` is automatically calculated from the
matcher name `IsDivisibleBy7`.
Optionally, you can stream additional information to a hidden argument
named `result_listener` to explain the match result. For example, a
better definition of `IsDivisibleBy7` is:
```
MATCHER(IsDivisibleBy7, "") {
if ((arg % 7) == 0)
return true;
*result_listener << "the remainder is " << (arg % 7);
return false;
}
```
With this definition, the above assertion will give a better message:
```
Value of: some_expression
Expected: is divisible by 7
Actual: 27 (the remainder is 6)
```
You should let `MatchAndExplain()` print _any additional information_
that can help a user understand the match result. Note that it should
explain why the match succeeds in case of a success (unless it's
obvious) - this is useful when the matcher is used inside
`Not()`. There is no need to print the argument value itself, as
Google Mock already prints it for you.
**Notes:**
1. The type of the value being matched (`arg_type`) is determined by the context in which you use the matcher and is supplied to you by the compiler, so you don't need to worry about declaring it (nor can you). This allows the matcher to be polymorphic. For example, `IsDivisibleBy7()` can be used to match any type where the value of `(arg % 7) == 0` can be implicitly converted to a `bool`. In the `Bar(IsDivisibleBy7())` example above, if method `Bar()` takes an `int`, `arg_type` will be `int`; if it takes an `unsigned long`, `arg_type` will be `unsigned long`; and so on.
1. Google Mock doesn't guarantee when or how many times a matcher will be invoked. Therefore the matcher logic must be _purely functional_ (i.e. it cannot have any side effect, and the result must not depend on anything other than the value being matched and the matcher parameters). This requirement must be satisfied no matter how you define the matcher (e.g. using one of the methods described in the following recipes). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and Google Mock.
## Writing New Parameterized Matchers Quickly ##
Sometimes you'll want to define a matcher that has parameters. For that you
*os << foo.DebugString(); // Whatever needed to print foo to os.
}
} // namespace foo
```
What if you have both `<<` and `PrintTo()`? In this case, the latter
will override the former when Google Mock is concerned. This allows
you to customize how the value should appear in Google Mock's output
without affecting code that relies on the behavior of its `<<`
operator.
**Note:** When printing a pointer of type `T*`, Google Mock calls
`PrintTo(T*, std::ostream* os)` instead of `operator<<(std::ostream&, T*)`.
Therefore the only way to affect how a pointer is printed by Google
Mock is to define `PrintTo()` for it. Also note that `T*` and `const T*`
are different types, so you may need to define `PrintTo()` for both.
Why does Google Mock treat pointers specially? There are several reasons:
* We cannot use `operator<<` to print a `signed char*` or `unsigned char*`, since it will print the pointer as a NUL-terminated C string, which likely will cause an access violation.
* We want `NULL` pointers to be printed as `"NULL"`, but `operator<<` prints it as `"0"`, `"nullptr"`, or something else, depending on the compiler.
* With some compilers, printing a `NULL` `char*` using `operator<<` will segfault.
* `operator<<` prints a function pointer as a `bool` (hence it always prints `"1"`), which is not very useful.
This page lists all documentation wiki pages for Google Mock **version 1.5.0** -- **if you use a different version of Google Mock, please read the documentation for that specific version instead.**
*[ForDummies](V1_5_ForDummies.md) -- start here if you are new to Google Mock.
*[CheatSheet](V1_5_CheatSheet.md) -- a quick reference.
*[CookBook](V1_5_CookBook.md) -- recipes for doing various tasks using Google Mock.
*[FrequentlyAskedQuestions](V1_5_FrequentlyAskedQuestions.md) -- check here before asking a question on the mailing list.
To contribute code to Google Mock, read:
* DevGuide -- read this _before_ writing your first patch.
*[Pump Manual](http://code.google.com/p/googletest/wiki/PumpManual) -- how we generate some of Google Mock's source files.
(**Note:** If you get compiler errors that you don't understand, be sure to consult [Google Mock Doctor](V1_5_FrequentlyAskedQuestions#How_am_I_supposed_to_make_sense_of_these_horrible_template_error.md).)
# What Is Google C++ Mocking Framework? #
When you write a prototype or test, often it's not feasible or wise to rely on real objects entirely. A **mock object** implements the same interface as a real object (so it can be used as one), but lets you specify at run time how it will be used and what it should do (which methods will be called? in which order? how many times? with what arguments? what will they return? etc).
**Note:** It is easy to confuse the term _fake objects_ with mock objects. Fakes and mocks actually mean very different things in the Test-Driven Development (TDD) community:
***Fake** objects have working implementations, but usually take some shortcut (perhaps to make the operations less expensive), which makes them not suitable for production. An in-memory file system would be an example of a fake.
***Mocks** are objects pre-programmed with _expectations_, which form a specification of the calls they are expected to receive.
If all this seems too abstract for you, don't worry - the most important thing to remember is that a mock allows you to check the _interaction_ between itself and code that uses it. The difference between fakes and mocks will become much clearer once you start to use mocks.
**Google C++ Mocking Framework** (or **Google Mock** for short) is a library (sometimes we also call it a "framework" to make it sound cool) for creating mock classes and using them. It does to C++ what [jMock](http://www.jmock.org/) and [EasyMock](http://www.easymock.org/) do to Java.
Using Google Mock involves three basic steps:
1. Use some simple macros to describe the interface you want to mock, and they will expand to the implementation of your mock class;
1. Create some mock objects and specify its expectations and behavior using an intuitive syntax;
1. Exercise code that uses the mock objects. Google Mock will catch any violation of the expectations as soon as it arises.
# Why Google Mock? #
While mock objects help you remove unnecessary dependencies in tests and make them fast and reliable, using mocks manually in C++ is _hard_:
* Someone has to implement the mocks. The job is usually tedious and error-prone. No wonder people go great distance to avoid it.
* The quality of those manually written mocks is a bit, uh, unpredictable. You may see some really polished ones, but you may also see some that were hacked up in a hurry and have all sorts of ad hoc restrictions.
* The knowledge you gained from using one mock doesn't transfer to the next.
In contrast, Java and Python programmers have some fine mock frameworks, which automate the creation of mocks. As a result, mocking is a proven effective technique and widely adopted practice in those communities. Having the right tool absolutely makes the difference.
Google Mock was built to help C++ programmers. It was inspired by [jMock](http://www.jmock.org/) and [EasyMock](http://www.easymock.org/), but designed with C++'s specifics in mind. It is your friend if any of the following problems is bothering you:
* You are stuck with a sub-optimal design and wish you had done more prototyping before it was too late, but prototyping in C++ is by no means "rapid".
* Your tests are slow as they depend on too many libraries or use expensive resources (e.g. a database).
* Your tests are brittle as some resources they use are unreliable (e.g. the network).
* You want to test how your code handles a failure (e.g. a file checksum error), but it's not easy to cause one.
* You need to make sure that your module interacts with other modules in the right way, but it's hard to observe the interaction; therefore you resort to observing the side effects at the end of the action, which is awkward at best.
* You want to "mock out" your dependencies, except that they don't have mock implementations yet; and, frankly, you aren't thrilled by some of those hand-written mocks.
We encourage you to use Google Mock as:
* a _design_ tool, for it lets you experiment with your interface design early and often. More iterations lead to better designs!
* a _testing_ tool to cut your tests' outbound dependencies and probe the interaction between your module and its collaborators.
# Getting Started #
Using Google Mock is easy! Inside your C++ source file, just `#include``<gtest/gtest.h>` and `<gmock/gmock.h>`, and you are ready to go.
# A Case for Mock Turtles #
Let's look at an example. Suppose you are developing a graphics program that relies on a LOGO-like API for drawing. How would you test that it does the right thing? Well, you can run it and compare the screen with a golden screen snapshot, but let's admit it: tests like this are expensive to run and fragile (What if you just upgraded to a shiny new graphics card that has better anti-aliasing? Suddenly you have to update all your golden images.). It would be too painful if all your tests are like this. Fortunately, you learned about Dependency Injection and know the right thing to do: instead of having your application talk to the drawing API directly, wrap the API in an interface (say, `Turtle`) and code to that interface:
```
class Turtle {
...
virtual ~Turtle() {}
virtual void PenUp() = 0;
virtual void PenDown() = 0;
virtual void Forward(int distance) = 0;
virtual void Turn(int degrees) = 0;
virtual void GoTo(int x, int y) = 0;
virtual int GetX() const = 0;
virtual int GetY() const = 0;
};
```
(Note that the destructor of `Turtle`**must** be virtual, as is the case for **all** classes you intend to inherit from - otherwise the destructor of the derived class will not be called when you delete an object through a base pointer, and you'll get corrupted program states like memory leaks.)
You can control whether the turtle's movement will leave a trace using `PenUp()` and `PenDown()`, and control its movement using `Forward()`, `Turn()`, and `GoTo()`. Finally, `GetX()` and `GetY()` tell you the current position of the turtle.
Your program will normally use a real implementation of this interface. In tests, you can use a mock implementation instead. This allows you to easily check what drawing primitives your program is calling, with what arguments, and in which order. Tests written this way are much more robust (they won't break because your new machine does anti-aliasing differently), easier to read and maintain (the intent of a test is expressed in the code, not in some binary images), and run _much, much faster_.
# Writing the Mock Class #
If you are lucky, the mocks you need to use have already been implemented by some nice people. If, however, you find yourself in the position to write a mock class, relax - Google Mock turns this task into a fun game! (Well, almost.)
## How to Define It ##
Using the `Turtle` interface as example, here are the simple steps you need to follow:
1. Derive a class `MockTurtle` from `Turtle`.
1. Take a virtual function of `Turtle`. Count how many arguments it has.
1. In the `public:` section of the child class, write `MOCK_METHODn();` (or `MOCK_CONST_METHODn();` if you are mocking a `const` method), where `n` is the number of the arguments; if you counted wrong, shame on you, and a compiler error will tell you so.
1. Now comes the fun part: you take the function signature, cut-and-paste the _function name_ as the _first_ argument to the macro, and leave what's left as the _second_ argument (in case you're curious, this is the _type of the function_).
1. Repeat until all virtual functions you want to mock are done.
After the process, you should have something like:
```
#include <gmock/gmock.h> // Brings in Google Mock.
class MockTurtle : public Turtle {
public:
...
MOCK_METHOD0(PenUp, void());
MOCK_METHOD0(PenDown, void());
MOCK_METHOD1(Forward, void(int distance));
MOCK_METHOD1(Turn, void(int degrees));
MOCK_METHOD2(GoTo, void(int x, int y));
MOCK_CONST_METHOD0(GetX, int());
MOCK_CONST_METHOD0(GetY, int());
};
```
You don't need to define these mock methods somewhere else - the `MOCK_METHOD*` macros will generate the definitions for you. It's that simple! Once you get the hang of it, you can pump out mock classes faster than your source-control system can handle your check-ins.
**Tip:** If even this is too much work for you, you'll find the
`gmock_gen.py` tool in Google Mock's `scripts/generator/` directory (courtesy of the [cppclean](http://code.google.com/p/cppclean/) project) useful. This command-line
tool requires that you have Python 2.4 installed. You give it a C++ file and the name of an abstract class defined in it,
and it will print the definition of the mock class for you. Due to the
complexity of the C++ language, this script may not always work, but
it can be quite handy when it does. For more details, read the [user documentation](http://code.google.com/p/googlemock/source/browse/trunk/scripts/generator/README).
## Where to Put It ##
When you define a mock class, you need to decide where to put its definition. Some people put it in a `*_test.cc`. This is fine when the interface being mocked (say, `Foo`) is owned by the same person or team. Otherwise, when the owner of `Foo` changes it, your test could break. (You can't really expect `Foo`'s maintainer to fix every test that uses `Foo`, can you?)
So, the rule of thumb is: if you need to mock `Foo` and it's owned by others, define the mock class in `Foo`'s package (better, in a `testing` sub-package such that you can clearly separate production code and testing utilities), and put it in a `mock_foo.h`. Then everyone can reference `mock_foo.h` from their tests. If `Foo` ever changes, there is only one copy of `MockFoo` to change, and only tests that depend on the changed methods need to be fixed.
Another way to do it: you can introduce a thin layer `FooAdaptor` on top of `Foo` and code to this new interface. Since you own `FooAdaptor`, you can absorb changes in `Foo` much more easily. While this is more work initially, carefully choosing the adaptor interface can make your code easier to write and more readable (a net win in the long run), as you can choose `FooAdaptor` to fit your specific domain much better than `Foo` does.
# Using Mocks in Tests #
Once you have a mock class, using it is easy. The typical work flow is:
1. Import the Google Mock names from the `testing` namespace such that you can use them unqualified (You only have to do it once per file. Remember that namespaces are a good idea and good for your health.).
1. Create some mock objects.
1. Specify your expectations on them (How many times will a method be called? With what arguments? What should it do? etc.).
1. Exercise some code that uses the mocks; optionally, check the result using Google Test assertions. If a mock method is called more than expected or with wrong arguments, you'll get an error immediately.
1. When a mock is destructed, Google Mock will automatically check whether all expectations on it have been satisfied.
Here's an example:
```
#include "path/to/mock-turtle.h"
#include <gmock/gmock.h>
#include <gtest/gtest.h>
using ::testing::AtLeast; // #1
TEST(PainterTest, CanDrawSomething) {
MockTurtle turtle; // #2
EXPECT_CALL(turtle, PenDown()) // #3
.Times(AtLeast(1));
Painter painter(&turtle); // #4
EXPECT_TRUE(painter.DrawCircle(0, 0, 10));
} // #5
int main(int argc, char** argv) {
// The following line must be executed to initialize Google Mock
// (and Google Test) before running the tests.
::testing::InitGoogleMock(&argc, argv);
return RUN_ALL_TESTS();
}
```
As you might have guessed, this test checks that `PenDown()` is called at least once. If the `painter` object didn't call this method, your test will fail with a message like this:
```
path/to/my_test.cc:119: Failure
Actual function call count doesn't match this expectation:
Actually: never called;
Expected: called at least once.
```
**Tip 1:** If you run the test from an Emacs buffer, you can hit `<Enter>` on the line number displayed in the error message to jump right to the failed expectation.
**Tip 2:** If your mock objects are never deleted, the final verification won't happen. Therefore it's a good idea to use a heap leak checker in your tests when you allocate mocks on the heap.
**Important note:** Google Mock requires expectations to be set **before** the mock functions are called, otherwise the behavior is **undefined**. In particular, you mustn't interleave `EXPECT_CALL()`s and calls to the mock functions.
This means `EXPECT_CALL()` should be read as expecting that a call will occur _in the future_, not that a call has occurred. Why does Google Mock work like that? Well, specifying the expectation beforehand allows Google Mock to report a violation as soon as it arises, when the context (stack trace, etc) is still available. This makes debugging much easier.
Admittedly, this test is contrived and doesn't do much. You can easily achieve the same effect without using Google Mock. However, as we shall reveal soon, Google Mock allows you to do _much more_ with the mocks.
## Using Google Mock with Any Testing Framework ##
If you want to use something other than Google Test (e.g. [CppUnit](http://apps.sourceforge.net/mediawiki/cppunit/index.php?title=Main_Page) or
[CxxTest](http://cxxtest.tigris.org/)) as your testing framework, just change the `main()` function in the previous section to:
```
int main(int argc, char** argv) {
// The following line causes Google Mock to throw an exception on failure,
// which will be interpreted by your testing framework as a test failure.
::testing::GTEST_FLAG(throw_on_failure) = true;
::testing::InitGoogleMock(&argc, argv);
... whatever your testing framework requires ...
}
```
This approach has a catch: it makes Google Mock throw an exception
from a mock object's destructor sometimes. With some compilers, this
sometimes causes the test program to crash. You'll still be able to
notice that the test has failed, but it's not a graceful failure.
to report a test failure to your testing framework properly. You'll need to
implement the `OnTestPartResult()` method of the event listener interface, but it
should be straightforward.
If this turns out to be too much work, we suggest that you stick with
Google Test, which works with Google Mock seamlessly (in fact, it is
technically part of Google Mock.). If there is a reason that you
cannot use Google Test, please let us know.
# Setting Expectations #
The key to using a mock object successfully is to set the _right expectations_ on it. If you set the expectations too strict, your test will fail as the result of unrelated changes. If you set them too loose, bugs can slip through. You want to do it just right such that your test can catch exactly the kind of bugs you intend it to catch. Google Mock provides the necessary means for you to do it "just right."
## General Syntax ##
In Google Mock we use the `EXPECT_CALL()` macro to set an expectation on a mock method. The general syntax is:
```
EXPECT_CALL(mock_object, method(matchers))
.Times(cardinality)
.WillOnce(action)
.WillRepeatedly(action);
```
The macro has two arguments: first the mock object, and then the method and its arguments. Note that the two are separated by a comma (`,`), not a period (`.`). (Why using a comma? The answer is that it was necessary for technical reasons.)
The macro can be followed by some optional _clauses_ that provide more information about the expectation. We'll discuss how each clause works in the coming sections.
This syntax is designed to make an expectation read like English. For example, you can probably guess that
```
using ::testing::Return;...
EXPECT_CALL(turtle, GetX())
.Times(5)
.WillOnce(Return(100))
.WillOnce(Return(150))
.WillRepeatedly(Return(200));
```
says that the `turtle` object's `GetX()` method will be called five times, it will return 100 the first time, 150 the second time, and then 200 every time. Some people like to call this style of syntax a Domain-Specific Language (DSL).
**Note:** Why do we use a macro to do this? It serves two purposes: first it makes expectations easily identifiable (either by `grep` or by a human reader), and second it allows Google Mock to include the source file location of a failed expectation in messages, making debugging easier.
## Matchers: What Arguments Do We Expect? ##
When a mock function takes arguments, we must specify what arguments we are expecting; for example:
```
// Expects the turtle to move forward by 100 units.
EXPECT_CALL(turtle, Forward(100));
```
Sometimes you may not want to be too specific (Remember that talk about tests being too rigid? Over specification leads to brittle tests and obscures the intent of tests. Therefore we encourage you to specify only what's necessary - no more, no less.). If you care to check that `Forward()` will be called but aren't interested in its actual argument, write `_` as the argument, which means "anything goes":
```
using ::testing::_;
...
// Expects the turtle to move forward.
EXPECT_CALL(turtle, Forward(_));
```
`_` is an instance of what we call **matchers**. A matcher is like a predicate and can test whether an argument is what we'd expect. You can use a matcher inside `EXPECT_CALL()` wherever a function argument is expected.
A list of built-in matchers can be found in the [CheatSheet](V1_5_CheatSheet.md). For example, here's the `Ge` (greater than or equal) matcher:
```
using ::testing::Ge;...
EXPECT_CALL(turtle, Forward(Ge(100)));
```
This checks that the turtle will be told to go forward by at least 100 units.
## Cardinalities: How Many Times Will It Be Called? ##
The first clause we can specify following an `EXPECT_CALL()` is `Times()`. We call its argument a **cardinality** as it tells _how many times_ the call should occur. It allows us to repeat an expectation many times without actually writing it as many times. More importantly, a cardinality can be "fuzzy", just like a matcher can be. This allows a user to express the intent of a test exactly.
An interesting special case is when we say `Times(0)`. You may have guessed - it means that the function shouldn't be called with the given arguments at all, and Google Mock will report a Google Test failure whenever the function is (wrongfully) called.
We've seen `AtLeast(n)` as an example of fuzzy cardinalities earlier. For the list of built-in cardinalities you can use, see the [CheatSheet](V1_5_CheatSheet.md).
The `Times()` clause can be omitted. **If you omit `Times()`, Google Mock will infer the cardinality for you.** The rules are easy to remember:
* If **neither**`WillOnce()`**nor**`WillRepeatedly()` is in the `EXPECT_CALL()`, the inferred cardinality is `Times(1)`.
* If there are `n WillOnce()`'s but **no**`WillRepeatedly()`, where `n` >= 1, the cardinality is `Times(n)`.
* If there are `n WillOnce()`'s and **one**`WillRepeatedly()`, where `n` >= 0, the cardinality is `Times(AtLeast(n))`.
**Quick quiz:** what do you think will happen if a function is expected to be called twice but actually called four times?
## Actions: What Should It Do? ##
Remember that a mock object doesn't really have a working implementation? We as users have to tell it what to do when a method is invoked. This is easy in Google Mock.
First, if the return type of a mock function is a built-in type or a pointer, the function has a **default action** (a `void` function will just return, a `bool` function will return `false`, and other functions will return 0). If you don't say anything, this behavior will be used.
Second, if a mock function doesn't have a default action, or the default action doesn't suit you, you can specify the action to be taken each time the expectation matches using a series of `WillOnce()` clauses followed by an optional `WillRepeatedly()`. For example,
```
using ::testing::Return;...
EXPECT_CALL(turtle, GetX())
.WillOnce(Return(100))
.WillOnce(Return(200))
.WillOnce(Return(300));
```
This says that `turtle.GetX()` will be called _exactly three times_ (Google Mock inferred this from how many `WillOnce()` clauses we've written, since we didn't explicitly write `Times()`), and will return 100, 200, and 300 respectively.
```
using ::testing::Return;...
EXPECT_CALL(turtle, GetY())
.WillOnce(Return(100))
.WillOnce(Return(200))
.WillRepeatedly(Return(300));
```
says that `turtle.GetY()` will be called _at least twice_ (Google Mock knows this as we've written two `WillOnce()` clauses and a `WillRepeatedly()` while having no explicit `Times()`), will return 100 the first time, 200 the second time, and 300 from the third time on.
Of course, if you explicitly write a `Times()`, Google Mock will not try to infer the cardinality itself. What if the number you specified is larger than there are `WillOnce()` clauses? Well, after all `WillOnce()`s are used up, Google Mock will do the _default_ action for the function every time (unless, of course, you have a `WillRepeatedly()`.).
What can we do inside `WillOnce()` besides `Return()`? You can return a reference using `ReturnRef(variable)`, or invoke a pre-defined function, among [others](V1_5_CheatSheet#Actions.md).
**Important note:** The `EXPECT_CALL()` statement evaluates the action clause only once, even though the action may be performed many times. Therefore you must be careful about side effects. The following may not do what you want:
```
int n = 100;
EXPECT_CALL(turtle, GetX())
.Times(4)
.WillOnce(Return(n++));
```
Instead of returning 100, 101, 102, ..., consecutively, this mock function will always return 100 as `n++` is only evaluated once. Similarly, `Return(new Foo)` will create a new `Foo` object when the `EXPECT_CALL()` is executed, and will return the same pointer every time. If you want the side effect to happen every time, you need to define a custom action, which we'll teach in the [CookBook](V1_5_CookBook.md).
Time for another quiz! What do you think the following means?
```
using ::testing::Return;...
EXPECT_CALL(turtle, GetY())
.Times(4)
.WillOnce(Return(100));
```
Obviously `turtle.GetY()` is expected to be called four times. But if you think it will return 100 every time, think twice! Remember that one `WillOnce()` clause will be consumed each time the function is invoked and the default action will be taken afterwards. So the right answer is that `turtle.GetY()` will return 100 the first time, but **return 0 from the second time on**, as returning 0 is the default action for `int` functions.
## Using Multiple Expectations ##
So far we've only shown examples where you have a single expectation. More realistically, you're going to specify expectations on multiple mock methods, which may be from multiple mock objects.
By default, when a mock method is invoked, Google Mock will search the expectations in the **reverse order** they are defined, and stop when an active expectation that matches the arguments is found (you can think of it as "newer rules override older ones."). If the matching expectation cannot take any more calls, you will get an upper-bound-violated failure. Here's an example:
```
using ::testing::_;...
EXPECT_CALL(turtle, Forward(_)); // #1
EXPECT_CALL(turtle, Forward(10)) // #2
.Times(2);
```
If `Forward(10)` is called three times in a row, the third time it will be an error, as the last matching expectation (#2) has been saturated. If, however, the third `Forward(10)` call is replaced by `Forward(20)`, then it would be OK, as now #1 will be the matching expectation.
**Side note:** Why does Google Mock search for a match in the _reverse_ order of the expectations? The reason is that this allows a user to set up the default expectations in a mock object's constructor or the test fixture's set-up phase and then customize the mock by writing more specific expectations in the test body. So, if you have two expectations on the same method, you want to put the one with more specific matchers **after** the other, or the more specific rule would be shadowed by the more general one that comes after it.
## Ordered vs Unordered Calls ##
By default, an expectation can match a call even though an earlier expectation hasn't been satisfied. In other words, the calls don't have to occur in the order the expectations are specified.
Sometimes, you may want all the expected calls to occur in a strict order. To say this in Google Mock is easy:
```
using ::testing::InSequence;...
TEST(FooTest, DrawsLineSegment) {
...
{
InSequence dummy;
EXPECT_CALL(turtle, PenDown());
EXPECT_CALL(turtle, Forward(100));
EXPECT_CALL(turtle, PenUp());
}
Foo();
}
```
By creating an object of type `InSequence`, all expectations in its scope are put into a _sequence_ and have to occur _sequentially_. Since we are just relying on the constructor and destructor of this object to do the actual work, its name is really irrelevant.
In this example, we test that `Foo()` calls the three expected functions in the order as written. If a call is made out-of-order, it will be an error.
(What if you care about the relative order of some of the calls, but not all of them? Can you specify an arbitrary partial order? The answer is ... yes! If you are impatient, the details can be found in the [CookBook](V1_5_CookBook.md).)
## All Expectations Are Sticky (Unless Said Otherwise) ##
Now let's do a quick quiz to see how well you can use this mock stuff already. How would you test that the turtle is asked to go to the origin _exactly twice_ (you want to ignore any other instructions it receives)?
After you've come up with your answer, take a look at ours and compare notes (solve it yourself first - don't cheat!):
```
using ::testing::_;...
EXPECT_CALL(turtle, GoTo(_, _)) // #1
.Times(AnyNumber());
EXPECT_CALL(turtle, GoTo(0, 0)) // #2
.Times(2);
```
Suppose `turtle.GoTo(0, 0)` is called three times. In the third time, Google Mock will see that the arguments match expectation #2 (remember that we always pick the last matching expectation). Now, since we said that there should be only two such calls, Google Mock will report an error immediately. This is basically what we've told you in the "Using Multiple Expectations" section above.
This example shows that **expectations in Google Mock are "sticky" by default**, in the sense that they remain active even after we have reached their invocation upper bounds. This is an important rule to remember, as it affects the meaning of the spec, and is **different** to how it's done in many other mocking frameworks (Why'd we do that? Because we think our rule makes the common cases easier to express and understand.).
Simple? Let's see if you've really understood it: what does the following code say?
```
using ::testing::Return;
...
for (int i = n; i > 0; i--) {
EXPECT_CALL(turtle, GetX())
.WillOnce(Return(10*i));
}
```
If you think it says that `turtle.GetX()` will be called `n` times and will return 10, 20, 30, ..., consecutively, think twice! The problem is that, as we said, expectations are sticky. So, the second time `turtle.GetX()` is called, the last (latest) `EXPECT_CALL()` statement will match, and will immediately lead to an "upper bound exceeded" error - this piece of code is not very useful!
One correct way of saying that `turtle.GetX()` will return 10, 20, 30, ..., is to explicitly say that the expectations are _not_ sticky. In other words, they should _retire_ as soon as they are saturated:
```
using ::testing::Return;
...
for (int i = n; i > 0; i--) {
EXPECT_CALL(turtle, GetX())
.WillOnce(Return(10*i))
.RetiresOnSaturation();
}
```
And, there's a better way to do it: in this case, we expect the calls to occur in a specific order, and we line up the actions to match the order. Since the order is important here, we should make it explicit using a sequence:
```
using ::testing::InSequence;
using ::testing::Return;
...
{
InSequence s;
for (int i = 1; i <= n; i++) {
EXPECT_CALL(turtle, GetX())
.WillOnce(Return(10*i))
.RetiresOnSaturation();
}
}
```
By the way, the other situation where an expectation may _not_ be sticky is when it's in a sequence - as soon as another expectation that comes after it in the sequence has been used, it automatically retires (and will never be used to match any call).
## Uninteresting Calls ##
A mock object may have many methods, and not all of them are that interesting. For example, in some tests we may not care about how many times `GetX()` and `GetY()` get called.
In Google Mock, if you are not interested in a method, just don't say anything about it. If a call to this method occurs, you'll see a warning in the test output, but it won't be a failure.
# What Now? #
Congratulations! You've learned enough about Google Mock to start using it. Now, you might want to join the [googlemock](http://groups.google.com/group/googlemock) discussion group and actually write some tests using Google Mock - it will be fun. Hey, it may even be addictive - you've been warned.
Then, if you feel like increasing your mock quotient, you should move on to the [CookBook](V1_5_CookBook.md). You can learn many advanced features of Google Mock there -- and advance your level of enjoyment and testing bliss.
Or you can run `gmd` and copy-n-paste gcc's error messages to it.
## Can I mock a variadic function? ##
You cannot mock a variadic function (i.e. a function taking ellipsis
(`...`) arguments) directly in Google Mock.
The problem is that in general, there is _no way_ for a mock object to
know how many arguments are passed to the variadic method, and what
the arguments' types are. Only the _author of the base class_ knows
the protocol, and we cannot look into his head.
Therefore, to mock such a function, the _user_ must teach the mock
object how to figure out the number of arguments and their types. One
way to do it is to provide overloaded versions of the function.
Ellipsis arguments are inherited from C and not really a C++ feature.
They are unsafe to use and don't work with arguments that have
constructors or destructors. Therefore we recommend to avoid them in
C++ as much as possible.
## MSVC gives me warning C4301 or C4373 when I define a mock method with a const parameter. Why? ##
If you compile this using Microsoft Visual C++ 2005 SP1:
```
class Foo {
...
virtual void Bar(const int i) = 0;
};
class MockFoo : public Foo {
...
MOCK_METHOD1(Bar, void(const int i));
};
```
You may get the following warning:
```
warning C4301: 'MockFoo::Bar': overriding virtual function only differs from 'Foo::Bar' by const/volatile qualifier
```
This is a MSVC bug. The same code compiles fine with gcc ,for
example. If you use Visual C++ 2008 SP1, you would get the warning:
```
warning C4373: 'MockFoo::Bar': virtual function overrides 'Foo::Bar', previous versions of the compiler did not override when parameters only differed by const/volatile qualifiers
```
In C++, if you _declare_ a function with a `const` parameter, the
`const` modifier is _ignored_. Therefore, the `Foo` base class above
is equivalent to:
```
class Foo {
...
virtual void Bar(int i) = 0; // int or const int? Makes no difference.
};
```
In fact, you can _declare_ Bar() with an `int` parameter, and _define_
it with a `const int` parameter. The compiler will still match them
up.
Since making a parameter `const` is meaningless in the method
_declaration_, we recommend to remove it in both `Foo` and `MockFoo`.
That should workaround the VC bug.
Note that we are talking about the _top-level_ `const` modifier here.
If the function parameter is passed by pointer or reference, declaring
the _pointee_ or _referee_ as `const` is still meaningful. For
example, the following two declarations are _not_ equivalent:
```
void Bar(int* p); // Neither p nor *p is const.
void Bar(const int* p); // p is not const, but *p is.
```
## I have a huge mock class, and Microsoft Visual C++ runs out of memory when compiling it. What can I do? ##
We've noticed that when the `/clr` compiler flag is used, Visual C++
uses 5~6 times as much memory when compiling a mock class. We suggest
to avoid `/clr` when compiling native C++ mocks.
## I can't figure out why Google Mock thinks my expectations are not satisfied. What should I do? ##
You might want to run your test with
`--gmock_verbose=info`. This flag lets Google Mock print a trace
of every mock function call it receives. By studying the trace,
you'll gain insights on why the expectations you set are not met.
## How can I assert that a function is NEVER called? ##
```
EXPECT_CALL(foo, Bar(_))
.Times(0);
```
## I have a failed test where Google Mock tells me TWICE that a particular expectation is not satisfied. Isn't this redundant? ##
When Google Mock detects a failure, it prints relevant information
(the mock function arguments, the state of relevant expectations, and
etc) to help the user debug. If another failure is detected, Google
Mock will do the same, including printing the state of relevant
expectations.
Sometimes an expectation's state didn't change between two failures,
and you'll see the same description of the state twice. They are
however _not_ redundant, as they refer to _different points in time_.
The fact they are the same _is_ interesting information.
## I get a heap check failure when using a mock object, but using a real object is fine. What can be wrong? ##
Does the class (hopefully a pure interface) you are mocking have a
virtual destructor?
Whenever you derive from a base class, make sure its destructor is
virtual. Otherwise Bad Things will happen. Consider the following
code:
```
class Base {
public:
// Not virtual, but should be.
~Base() { ... }
...
};
class Derived : public Base {
public:
...
private:
std::string value_;
};
...
Base* p = new Derived;
...
delete p; // Surprise! ~Base() will be called, but ~Derived() will not
// - value_ is leaked.
```
By changing `~Base()` to virtual, `~Derived()` will be correctly
called when `delete p` is executed, and the heap checker
will be happy.
## The "newer expectations override older ones" rule makes writing expectations awkward. Why does Google Mock do that? ##
When people complain about this, often they are referring to code like:
```
// foo.Bar() should be called twice, return 1 the first time, and return
// 2 the second time. However, I have to write the expectations in the
// reverse order. This sucks big time!!!
EXPECT_CALL(foo, Bar())
.WillOnce(Return(2))
.RetiresOnSaturation();
EXPECT_CALL(foo, Bar())
.WillOnce(Return(1))
.RetiresOnSaturation();
```
The problem is that they didn't pick the **best** way to express the test's
intent.
By default, expectations don't have to be matched in _any_ particular
order. If you want them to match in a certain order, you need to be
explicit. This is Google Mock's (and jMock's) fundamental philosophy: it's
easy to accidentally over-specify your tests, and we want to make it
harder to do so.
There are two better ways to write the test spec. You could either
put the expectations in sequence:
```
// foo.Bar() should be called twice, return 1 the first time, and return
// 2 the second time. Using a sequence, we can write the expectations
// in their natural order.
{
InSequence s;
EXPECT_CALL(foo, Bar())
.WillOnce(Return(1))
.RetiresOnSaturation();
EXPECT_CALL(foo, Bar())
.WillOnce(Return(2))
.RetiresOnSaturation();
}
```
or you can put the sequence of actions in the same expectation:
```
// foo.Bar() should be called twice, return 1 the first time, and return
// 2 the second time.
EXPECT_CALL(foo, Bar())
.WillOnce(Return(1))
.WillOnce(Return(2))
.RetiresOnSaturation();
```
Back to the original questions: why does Google Mock search the
expectations (and `ON_CALL`s) from back to front? Because this
allows a user to set up a mock's behavior for the common case early
(e.g. in the mock's constructor or the test fixture's set-up phase)
and customize it with more specific rules later. If Google Mock
searches from front to back, this very useful pattern won't be
possible.
## Google Mock prints a warning when a function without EXPECT\_CALL is called, even if I have set its behavior using ON\_CALL. Would it be reasonable not to show the warning in this case? ##
When choosing between being neat and being safe, we lean toward the
latter. So the answer is that we think it's better to show the
warning.
Often people write `ON_CALL`s in the mock object's
constructor or `SetUp()`, as the default behavior rarely changes from
test to test. Then in the test body they set the expectations, which
are often different for each test. Having an `ON_CALL` in the set-up
part of a test doesn't mean that the calls are expected. If there's
no `EXPECT_CALL` and the method is called, it's possibly an error. If
we quietly let the call go through without notifying the user, bugs
may creep in unnoticed.
If, however, you are sure that the calls are OK, you can write
```
EXPECT_CALL(foo, Bar(_))
.WillRepeatedly(...);
```
instead of
```
ON_CALL(foo, Bar(_))
.WillByDefault(...);
```
This tells Google Mock that you do expect the calls and no warning should be
printed.
Also, you can control the verbosity using the `--gmock_verbose` flag.
If you find the output too noisy when debugging, just choose a less
verbose level.
## How can I delete the mock function's argument in an action? ##
If you find yourself needing to perform some action that's not
supported by Google Mock directly, remember that you can define your own
actions using
[MakeAction()](V1_5_CookBook#Writing_New_Actions.md) or
You still need a `typedef` if the return type contains an unprotected
comma, but that's much rarer.
Other advantages include:
1.`MOCK_METHOD1(Foo, int, bool)` can leave a reader wonder whether the method returns `int` or `bool`, while there won't be such confusion using Google Mock's syntax.
1. The way Google Mock describes a function type is nothing new, although many people may not be familiar with it. The same syntax was used in C, and the `function` library in `tr1` uses this syntax extensively. Since `tr1` will become a part of the new version of STL, we feel very comfortable to be consistent with it.
1. The function type syntax is also used in other parts of Google Mock's API (e.g. the action interface) in order to make the implementation tractable. A user needs to learn it anyway in order to utilize Google Mock's more advanced features. We'd as well stick to the same syntax in `MOCK_METHOD*`!
## My code calls a static/global function. Can I mock it? ##
You can, but you need to make some changes.
In general, if you find yourself needing to mock a static function,
it's a sign that your modules are too tightly coupled (and less
flexible, less reusable, less testable, etc). You are probably better
off defining a small interface and call the function through that
interface, which then can be easily mocked. It's a bit of work
## My mock object needs to do complex stuff. It's a lot of pain to specify the actions. Google Mock sucks! ##
I know it's not a question, but you get an answer for free any way. :-)
With Google Mock, you can create mocks in C++ easily. And people might be
tempted to use them everywhere. Sometimes they work great, and
sometimes you may find them, well, a pain to use. So, what's wrong in
the latter case?
When you write a test without using mocks, you exercise the code and
assert that it returns the correct value or that the system is in an
expected state. This is sometimes called "state-based testing".
Mocks are great for what some call "interaction-based" testing:
instead of checking the system state at the very end, mock objects
verify that they are invoked the right way and report an error as soon
as it arises, giving you a handle on the precise context in which the
error was triggered. This is often more effective and economical to
do than state-based testing.
If you are doing state-based testing and using a test double just to
simulate the real object, you are probably better off using a fake.
Using a mock in this case causes pain, as it's not a strong point for
mocks to perform complex actions. If you experience this and think
that mocks suck, you are just not using the right tool for your
problem. Or, you might be trying to solve the wrong problem. :-)
## I got a warning "Uninteresting function call encountered - default action taken.." Should I panic? ##
By all means, NO! It's just an FYI.
What it means is that you have a mock function, you haven't set any
expectations on it (by Google Mock's rule this means that you are not
interested in calls to this function and therefore it can be called
any number of times), and it is called. That's OK - you didn't say
it's not OK to call the function!
What if you actually meant to disallow this function to be called, but
forgot to write `EXPECT_CALL(foo, Bar()).Times(0)`? While
one can argue that it's the user's fault, Google Mock tries to be nice and
prints you a note.
So, when you see the message and believe that there shouldn't be any
uninteresting calls, you should investigate what's going on. To make
your life easier, Google Mock prints the function name and arguments
when an uninteresting call is encountered.
## I want to define a custom action. Should I use Invoke() or implement the action interface? ##
Either way is fine - you want to choose the one that's more convenient
for your circumstance.
Usually, if your action is for a particular function type, defining it
using `Invoke()` should be easier; if your action can be used in
functions of different types (e.g. if you are defining
`Return(value)`), `MakePolymorphicAction()` is
easiest. Sometimes you want precise control on what types of
functions the action can be used in, and implementing
`ActionInterface` is the way to go here. See the implementation of
`Return()` in `include/gmock/gmock-actions.h` for an example.
## I'm using the set-argument-pointee action, and the compiler complains about "conflicting return type specified". What does it mean? ##
You got this error as Google Mock has no idea what value it should return
when the mock method is called. `SetArgumentPointee()` says what the
side effect is, but doesn't say what the return value should be. You
need `DoAll()` to chain a `SetArgumentPointee()` with a `Return()`.
See this [recipe](V1_5_CookBook#Mocking_Side_Effects.md) for more details and an example.
## My question is not in your FAQ! ##
If you cannot find the answer to your question in this FAQ, there are
some other resources you can use:
1. read other [wiki pages](http://code.google.com/p/googlemock/w/list),
1. search the mailing list [archive](http://groups.google.com/group/googlemock/topics),
1. ask it on [googlemock@googlegroups.com](mailto:googlemock@googlegroups.com) and someone will answer it (to prevent spam, we require you to join the [discussion group](http://groups.google.com/group/googlemock) before you can post.).
Please note that creating an issue in the
[issue tracker](http://code.google.com/p/googlemock/issues/list) is _not_
a good way to get your answer, as it is monitored infrequently by a
very small number of people.
When asking a question, it's helpful to provide as much of the
following information as possible (people cannot help you if there's
not enough information in your question):
* the version (or the revision number if you check out from SVN directly) of Google Mock you use (Google Mock is under active development, so it's possible that your problem has been solved in a later version),
* your operating system,
* the name and version of your compiler,
* the complete command line flags you give to your compiler,
* the complete compiler error messages (if the question is about compilation),
* the _actual_ code (ideally, a minimal but complete program) that has the problem you encounter.
|`A<type>()` or `An<type>()`|`argument` can be any value of type `type`. |
## Generic Comparison ##
|`Eq(value)` or `value`|`argument == value`|
|:---------------------|:------------------|
|`Ge(value)` |`argument >= value`|
|`Gt(value)` |`argument > value` |
|`Le(value)` |`argument <= value`|
|`Lt(value)` |`argument < value` |
|`Ne(value)` |`argument != value`|
|`IsNull()` |`argument` is a `NULL` pointer (raw or smart).|
|`NotNull()` |`argument` is a non-null pointer (raw or smart).|
|`Ref(variable)` |`argument` is a reference to `variable`.|
|`TypedEq<type>(value)`|`argument` has type `type` and is equal to `value`. You may need to use this instead of `Eq(value)` when the mock function is overloaded.|
Except `Ref()`, these matchers make a _copy_ of `value` in case it's
modified or destructed later. If the compiler complains that `value`
doesn't have a public copy constructor, try wrap it in `ByRef()`,
e.g. `Eq(ByRef(non_copyable_value))`. If you do that, make sure
`non_copyable_value` is not changed afterwards, or the meaning of your
matcher will be changed.
## Floating-Point Matchers ##
|`DoubleEq(a_double)`|`argument` is a `double` value approximately equal to `a_double`, treating two NaNs as unequal.|
|`EndsWith(suffix)` |`argument` ends with string `suffix`. |
|`HasSubstr(string)` |`argument` contains `string` as a sub-string. |
|`MatchesRegex(string)` |`argument` matches the given regular expression with the match starting at the first character and ending at the last character.|
|`StartsWith(prefix)` |`argument` starts with string `prefix`. |
|`StrCaseEq(string)` |`argument` is equal to `string`, ignoring case. |
|`StrCaseNe(string)` |`argument` is not equal to `string`, ignoring case.|
|`StrEq(string)` |`argument` is equal to `string`. |
|`StrNe(string)` |`argument` is not equal to `string`. |
`ContainsRegex()` and `MatchesRegex()` use the regular expression
| `Each(e)` | `argument` is a container where _every_ element matches `e`, which can be either a value or a matcher. |
| `ElementsAre(e0, e1, ..., en)` | `argument` has `n + 1` elements, where the i-th element matches `ei`, which can be a value or a matcher. 0 to 10 arguments are allowed. |
| `ElementsAreArray(array)` or `ElementsAreArray(array, count)` | The same as `ElementsAre()` except that the expected element values/matchers come from a C-style array. |
| `ContainerEq(container)` | The same as `Eq(container)` except that the failure message also includes which elements are in one container but not the other. |
| `Pointwise(m, container)` | `argument` contains the same number of elements as in `container`, and for all i, (the i-th element in `argument`, the i-th element in `container`) match `m`, which is a matcher on 2-tuples. E.g. `Pointwise(Le(), upper_bounds)` verifies that each element in `argument` doesn't exceed the corresponding element in `upper_bounds`. |
These matchers can also match:
1. a native array passed by reference (e.g. in `Foo(const int (&a)[5])`), and
1. an array passed as a pointer and a count (e.g. in `Bar(const T* buffer, int len)` -- see [Multi-argument Matchers](#Multiargument_Matchers.md)).
where the array may be multi-dimensional (i.e. its elements can be arrays).
## Member Matchers ##
|`Field(&class::field, m)`|`argument.field` (or `argument->field` when `argument` is a plain pointer) matches matcher `m`, where `argument` is an object of type _class_.|
|`Key(e)` |`argument.first` matches `e`, which can be either a value or a matcher. E.g. `Contains(Key(Le(5)))` can verify that a `map` contains a key `<= 5`.|
|`Pair(m1, m2)` |`argument` is an `std::pair` whose `first` field matches `m1` and `second` field matches `m2`. |
|`Property(&class::property, m)`|`argument.property()` (or `argument->property()` when `argument` is a plain pointer) matches matcher `m`, where `argument` is an object of type _class_.|
## Matching the Result of a Function or Functor ##
|`ResultOf(f, m)`|`f(argument)` matches matcher `m`, where `f` is a function or functor.|
| `MATCHER_P(IsDivisibleBy, n, "") { *result_listener << "where the remainder is " << (arg % n); return (arg % n) == 0; }` | Defines a macher `IsDivisibleBy(n)` to match a number divisible by `n`. |
| `MATCHER_P2(IsBetween, a, b, std::string(negation ? "isn't" : "is") + " between " + PrintToString(a) + " and " + PrintToString(b)) { return a <= arg && arg <= b; }` | Defines a matcher `IsBetween(a, b)` to match a value in the range [`a`, `b`]. |
**Notes:**
1. The `MATCHER*` macros cannot be used inside a function or class.
1. The matcher body must be _purely functional_ (i.e. it cannot have any side effect, and the result must not depend on anything other than the value being matched and the matcher parameters).
1. You can use `PrintToString(x)` to convert a value `x` of any type to a string.
## Matchers as Test Assertions ##
|`ASSERT_THAT(expression, m)`|Generates a [fatal failure](http://code.google.com/p/googletest/wiki/V1_6_Primer#Assertions) if the value of `expression` doesn't match matcher `m`.|
|`EXPECT_THAT(expression, m)`|Generates a non-fatal failure if the value of `expression` doesn't match matcher `m`. |
# Actions #
**Actions** specify what a mock function should do when invoked.
## Returning a Value ##
|`Return()`|Return from a `void` mock function.|
|:---------|:----------------------------------|
|`Return(value)`|Return `value`. If the type of `value` is different to the mock function's return type, `value` is converted to the latter type <i>at the time the expectation is set</i>, not when the action is executed.|
|`ReturnArg<N>()`|Return the `N`-th (0-based) argument.|
|`ReturnNew<T>(a1, ..., ak)`|Return `new T(a1, ..., ak)`; a different object is created each time.|
|`ReturnNull()`|Return a null pointer. |
|`ReturnPointee(ptr)`|Return the value pointed to by `ptr`.|
|`ReturnRef(variable)`|Return a reference to `variable`. |
|`ReturnRefOfCopy(value)`|Return a reference to a copy of `value`; the copy lives as long as the action.|
## Side Effects ##
|`Assign(&variable, value)`|Assign `value` to variable.|
| `DeleteArg<N>()` | Delete the `N`-th (0-based) argument, which must be a pointer. |
| `SaveArg<N>(pointer)` | Save the `N`-th (0-based) argument to `*pointer`. |
| `SaveArgPointee<N>(pointer)` | Save the value pointed to by the `N`-th (0-based) argument to `*pointer`. |
| `SetArgReferee<N>(value)` | Assign value to the variable referenced by the `N`-th (0-based) argument. |
|`SetArgPointee<N>(value)` |Assign `value` to the variable pointed by the `N`-th (0-based) argument.|
|`SetArgumentPointee<N>(value)`|Same as `SetArgPointee<N>(value)`. Deprecated. Will be removed in v1.7.0.|
|`SetArrayArgument<N>(first, last)`|Copies the elements in source range [`first`, `last`) to the array pointed to by the `N`-th (0-based) argument, which can be either a pointer or an iterator. The action does not take ownership of the elements in the source range.|
|`SetErrnoAndReturn(error, value)`|Set `errno` to `error` and return `value`.|
|`Throw(exception)` |Throws the given exception, which can be any copyable value. Available since v1.1.0.|
## Using a Function or a Functor as an Action ##
|`Invoke(f)`|Invoke `f` with the arguments passed to the mock function, where `f` can be a global/static function or a functor.|
|`Invoke(object_pointer, &class::method)`|Invoke the {method on the object with the arguments passed to the mock function. |
|`InvokeWithoutArgs(f)`|Invoke `f`, which can be a global/static function or a functor. `f` must take no arguments. |
|`InvokeWithoutArgs(object_pointer, &class::method)`|Invoke the method on the object, which takes no arguments. |
|`InvokeArgument<N>(arg1, arg2, ..., argk)`|Invoke the mock function's `N`-th (0-based) argument, which must be a function or a functor, with the `k` arguments.|
The return value of the invoked function is used as the return value
of the action.
When defining a function or functor to be used with `Invoke*()`, you can declare any unused parameters as `Unused`:
**Note:** due to technical reasons, `DoDefault()` cannot be used inside a composite action - trying to do so will result in a run-time error.
## Composite Actions ##
|`DoAll(a1, a2, ..., an)`|Do all actions `a1` to `an` and return the result of `an` in each invocation. The first `n - 1` sub-actions must return void. |
Your code should talk to `FileInterface` to open a file. Now it's
easy to mock out the function.
This may seem much hassle, but in practice you often have multiple
related functions that you can put in the same interface, so the
per-function syntactic overhead will be much lower.
If you are concerned about the performance overhead incurred by
virtual functions, and profiling confirms your concern, you can
combine this with the recipe for [mocking non-virtual methods](#Mocking_Nonvirtual_Methods.md).
## Nice Mocks and Strict Mocks ##
If a mock method has no `EXPECT_CALL` spec but is called, Google Mock
will print a warning about the "uninteresting call". The rationale is:
* New methods may be added to an interface after a test is written. We shouldn't fail a test just because a method it doesn't know about is called.
* However, this may also mean there's a bug in the test, so Google Mock shouldn't be silent either. If the user believes these calls are harmless, he can add an `EXPECT_CALL()` to suppress the warning.
However, sometimes you may want to suppress all "uninteresting call"
warnings, while sometimes you may want the opposite, i.e. to treat all
of them as errors. Google Mock lets you make the decision on a
per-mock-object basis.
Suppose your test uses a mock class `MockFoo`:
```
TEST(...) {
MockFoo mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
```
If a method of `mock_foo` other than `DoThis()` is called, it will be
reported by Google Mock as a warning. However, if you rewrite your
test to use `NiceMock<MockFoo>` instead, the warning will be gone,
resulting in a cleaner test output:
```
using ::testing::NiceMock;
TEST(...) {
NiceMock<MockFoo> mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
```
`NiceMock<MockFoo>` is a subclass of `MockFoo`, so it can be used
wherever `MockFoo` is accepted.
It also works if `MockFoo`'s constructor takes some arguments, as
The usage of `StrictMock` is similar, except that it makes all
uninteresting calls failures:
```
using ::testing::StrictMock;
TEST(...) {
StrictMock<MockFoo> mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
// The test will fail if a method of mock_foo other than DoThis()
// is called.
}
```
There are some caveats though (I don't like them just as much as the
next guy, but sadly they are side effects of C++'s limitations):
1.`NiceMock<MockFoo>` and `StrictMock<MockFoo>` only work for mock methods defined using the `MOCK_METHOD*` family of macros **directly** in the `MockFoo` class. If a mock method is defined in a **base class** of `MockFoo`, the "nice" or "strict" modifier may not affect it, depending on the compiler. In particular, nesting `NiceMock` and `StrictMock` (e.g. `NiceMock<StrictMock<MockFoo> >`) is **not** supported.
1. The constructors of the base mock (`MockFoo`) cannot have arguments passed by non-const reference, which happens to be banned by the [Google C++ style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml).
1. During the constructor or destructor of `MockFoo`, the mock object is _not_ nice or strict. This may cause surprises if the constructor or destructor calls a mock method on `this` object. (This behavior, however, is consistent with C++'s general rule: if a constructor or destructor calls a virtual method of `this` object, that method is treated as non-virtual. In other words, to the base class's constructor or destructor, `this` object behaves like an instance of the base class, not the derived class. This rule is required for safety. Otherwise a base constructor may use members of a derived class before they are initialized, or a base destructor may use members of a derived class after they have been destroyed.)
Finally, you should be **very cautious** when using this feature, as the
decision you make applies to **all** future changes to the mock
class. If an important change is made in the interface you are mocking
(and thus in the mock class), it could break your tests (if you use
`StrictMock`) or let bugs pass through without a warning (if you use
`NiceMock`). Therefore, try to specify the mock's behavior using
explicit `EXPECT_CALL` first, and only turn to `NiceMock` or
`StrictMock` as the last resort.
## Simplifying the Interface without Breaking Existing Code ##
Sometimes a method has a long list of arguments that is mostly
By defining a new mock method with a trimmed argument list, we make
the mock class much more user-friendly.
## Alternative to Mocking Concrete Classes ##
Often you may find yourself using classes that don't implement
interfaces. In order to test your code that uses such a class (let's
call it `Concrete`), you may be tempted to make the methods of
`Concrete` virtual and then mock it.
Try not to do that.
Making a non-virtual function virtual is a big decision. It creates an
extension point where subclasses can tweak your class' behavior. This
weakens your control on the class because now it's harder to maintain
the class' invariants. You should make a function virtual only when
there is a valid reason for a subclass to override it.
Mocking concrete classes directly is problematic as it creates a tight
coupling between the class and the tests - any small change in the
class may invalidate your tests and make test maintenance a pain.
To avoid such problems, many programmers have been practicing "coding
to interfaces": instead of talking to the `Concrete` class, your code
would define an interface and talk to it. Then you implement that
interface as an adaptor on top of `Concrete`. In tests, you can easily
mock that interface to observe how your code is doing.
This technique incurs some overhead:
* You pay the cost of virtual function calls (usually not a problem).
* There is more abstraction for the programmers to learn.
However, it can also bring significant benefits in addition to better
testability:
*`Concrete`'s API may not fit your problem domain very well, as you may not be the only client it tries to serve. By designing your own interface, you have a chance to tailor it to your need - you may add higher-level functionalities, rename stuff, etc instead of just trimming the class. This allows you to write your code (user of the interface) in a more natural way, which means it will be more readable, more maintainable, and you'll be more productive.
* If `Concrete`'s implementation ever has to change, you don't have to rewrite everywhere it is used. Instead, you can absorb the change in your implementation of the interface, and your other code and tests will be insulated from this change.
Some people worry that if everyone is practicing this technique, they
will end up writing lots of redundant code. This concern is totally
understandable. However, there are two reasons why it may not be the
case:
* Different projects may need to use `Concrete` in different ways, so the best interfaces for them will be different. Therefore, each of them will have its own domain-specific interface on top of `Concrete`, and they will not be the same code.
* If enough projects want to use the same interface, they can always share it, just like they have been sharing `Concrete`. You can check in the interface and the adaptor somewhere near `Concrete` (perhaps in a `contrib` sub-directory) and let many projects use it.
You need to weigh the pros and cons carefully for your particular
problem, but I'd like to assure you that the Java community has been
practicing this for a long time and it's a proven effective technique
applicable in a wide variety of situations. :-)
## Delegating Calls to a Fake ##
Some times you have a non-trivial fake implementation of an
interface. For example:
```
class Foo {
public:
virtual ~Foo() {}
virtual char DoThis(int n) = 0;
virtual void DoThat(const char* s, int* p) = 0;
};
class FakeFoo : public Foo {
public:
virtual char DoThis(int n) {
return (n > 0) ? '+' :
(n < 0) ? '-' : '0';
}
virtual void DoThat(const char* s, int* p) {
*p = strlen(s);
}
};
```
Now you want to mock this interface such that you can set expectations
on it. However, you also want to use `FakeFoo` for the default
behavior, as duplicating it in the mock object is, well, a lot of
work.
When you define the mock class using Google Mock, you can have it
delegate its default action to a fake class you already have, using
this pattern:
```
using ::testing::_;
using ::testing::Invoke;
class MockFoo : public Foo {
public:
// Normal mock method definitions using Google Mock.
// Delegates the default actions of the methods to a FakeFoo object.
// This must be called *before* the custom ON_CALL() statements.
void DelegateToFake() {
ON_CALL(*this, DoThis(_))
.WillByDefault(Invoke(&fake_, &FakeFoo::DoThis));
ON_CALL(*this, DoThat(_, _))
.WillByDefault(Invoke(&fake_, &FakeFoo::DoThat));
}
private:
FakeFoo fake_; // Keeps an instance of the fake in the mock.
};
```
With that, you can use `MockFoo` in your tests as usual. Just remember
that if you don't explicitly set an action in an `ON_CALL()` or
`EXPECT_CALL()`, the fake will be called upon to do it:
```
using ::testing::_;
TEST(AbcTest, Xyz) {
MockFoo foo;
foo.DelegateToFake(); // Enables the fake for delegation.
// Put your ON_CALL(foo, ...)s here, if any.
// No action specified, meaning to use the default action.
EXPECT_CALL(foo, DoThis(5));
EXPECT_CALL(foo, DoThat(_, _));
int n = 0;
EXPECT_EQ('+', foo.DoThis(5)); // FakeFoo::DoThis() is invoked.
foo.DoThat("Hi", &n); // FakeFoo::DoThat() is invoked.
EXPECT_EQ(2, n);
}
```
**Some tips:**
* If you want, you can still override the default action by providing your own `ON_CALL()` or using `.WillOnce()` / `.WillRepeatedly()` in `EXPECT_CALL()`.
* In `DelegateToFake()`, you only need to delegate the methods whose fake implementation you intend to use.
* The general technique discussed here works for overloaded methods, but you'll need to tell the compiler which version you mean. To disambiguate a mock function (the one you specify inside the parentheses of `ON_CALL()`), see the "Selecting Between Overloaded Functions" section on this page; to disambiguate a fake function (the one you place inside `Invoke()`), use a `static_cast` to specify the function's type.
* Having to mix a mock and a fake is often a sign of something gone wrong. Perhaps you haven't got used to the interaction-based way of testing yet. Or perhaps your interface is taking on too many roles and should be split up. Therefore, **don't abuse this**. We would only recommend to do it as an intermediate step when you are refactoring your code.
Regarding the tip on mixing a mock and a fake, here's an example on
why it may be a bad sign: Suppose you have a class `System` for
low-level system operations. In particular, it does file and I/O
operations. And suppose you want to test how your code uses `System`
to do I/O, and you just want the file operations to work normally. If
you mock out the entire `System` class, you'll have to provide a fake
implementation for the file operation part, which suggests that
`System` is taking on too many roles.
Instead, you can define a `FileOps` interface and an `IOOps` interface
and split `System`'s functionalities into the two. Then you can mock
`IOOps` without mocking `FileOps`.
## Delegating Calls to a Real Object ##
When using testing doubles (mocks, fakes, stubs, and etc), sometimes
their behaviors will differ from those of the real objects. This
difference could be either intentional (as in simulating an error such
that you can test the error handling code) or unintentional. If your
mocks have different behaviors than the real objects by mistake, you
could end up with code that passes the tests but fails in production.
You can use the _delegating-to-real_ technique to ensure that your
mock has the same behavior as the real object while retaining the
ability to validate calls. This technique is very similar to the
delegating-to-fake technique, the difference being that we use a real
object instead of a fake. Here's an example:
```
using ::testing::_;
using ::testing::AtLeast;
using ::testing::Invoke;
class MockFoo : public Foo {
public:
MockFoo() {
// By default, all calls are delegated to the real object.
ON_CALL(*this, DoThis())
.WillByDefault(Invoke(&real_, &Foo::DoThis));
ON_CALL(*this, DoThat(_))
.WillByDefault(Invoke(&real_, &Foo::DoThat));
...
}
MOCK_METHOD0(DoThis, ...);
MOCK_METHOD1(DoThat, ...);
...
private:
Foo real_;
};
...
MockFoo mock;
EXPECT_CALL(mock, DoThis())
.Times(3);
EXPECT_CALL(mock, DoThat("Hi"))
.Times(AtLeast(1));
... use mock in test ...
```
With this, Google Mock will verify that your code made the right calls
(with the right arguments, in the right order, called the right number
of times, etc), and a real object will answer the calls (so the
behavior will be the same as in production). This gives you the best
of both worlds.
## Delegating Calls to a Parent Class ##
Ideally, you should code to interfaces, whose methods are all pure
virtual. In reality, sometimes you do need to mock a virtual method
that is not pure (i.e, it already has an implementation). For example:
```
class Foo {
public:
virtual ~Foo();
virtual void Pure(int n) = 0;
virtual int Concrete(const char* str) { ... }
};
class MockFoo : public Foo {
public:
// Mocking a pure method.
MOCK_METHOD1(Pure, void(int n));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD1(Concrete, int(const char* str));
};
```
Sometimes you may want to call `Foo::Concrete()` instead of
`MockFoo::Concrete()`. Perhaps you want to do it as part of a stub
action, or perhaps your test doesn't need to mock `Concrete()` at all
(but it would be oh-so painful to have to define a new mock class
whenever you don't need to mock one of its methods).
The trick is to leave a back door in your mock class for accessing the
real methods in the base class:
```
class MockFoo : public Foo {
public:
// Mocking a pure method.
MOCK_METHOD1(Pure, void(int n));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD1(Concrete, int(const char* str));
// Use this to call Concrete() defined in Foo.
int FooConcrete(const char* str) { return Foo::Concrete(str); }
};
```
Now, you can call `Foo::Concrete()` inside an action by:
```
using ::testing::_;
using ::testing::Invoke;
...
EXPECT_CALL(foo, Concrete(_))
.WillOnce(Invoke(&foo, &MockFoo::FooConcrete));
```
or tell the mock object that you don't want to mock `Concrete()`:
(Why don't we just write `Invoke(&foo, &Foo::Concrete)`? If you do
that, `MockFoo::Concrete()` will be called (and cause an infinite
recursion) since `Foo::Concrete()` is virtual. That's just how C++
works.)
# Using Matchers #
## Matching Argument Values Exactly ##
You can specify exactly which arguments a mock method is expecting:
```
using ::testing::Return;
...
EXPECT_CALL(foo, DoThis(5))
.WillOnce(Return('a'));
EXPECT_CALL(foo, DoThat("Hello", bar));
```
## Using Simple Matchers ##
You can use matchers to match arguments that have a certain property:
```
using ::testing::Ge;
using ::testing::NotNull;
using ::testing::Return;
...
EXPECT_CALL(foo, DoThis(Ge(5))) // The argument must be >= 5.
.WillOnce(Return('a'));
EXPECT_CALL(foo, DoThat("Hello", NotNull()));
// The second argument must not be NULL.
```
A frequently used matcher is `_`, which matches anything:
```
using ::testing::_;
using ::testing::NotNull;
...
EXPECT_CALL(foo, DoThat(_, NotNull()));
```
## Combining Matchers ##
You can build complex matchers from existing ones using `AllOf()`,
`AnyOf()`, and `Not()`:
```
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::HasSubstr;
using ::testing::Ne;
using ::testing::Not;
...
// The argument must be > 5 and != 10.
EXPECT_CALL(foo, DoThis(AllOf(Gt(5),
Ne(10))));
// The first argument must not contain sub-string "blah".
EXPECT_CALL(foo, DoThat(Not(HasSubstr("blah")),
NULL));
```
## Casting Matchers ##
Google Mock matchers are statically typed, meaning that the compiler
can catch your mistake if you use a matcher of the wrong type (for
example, if you use `Eq(5)` to match a `string` argument). Good for
you!
Sometimes, however, you know what you're doing and want the compiler
to give you some slack. One example is that you have a matcher for
`long` and the argument you want to match is `int`. While the two
types aren't exactly the same, there is nothing really wrong with
using a `Matcher<long>` to match an `int` - after all, we can first
convert the `int` argument to a `long` before giving it to the
matcher.
To support this need, Google Mock gives you the
`SafeMatcherCast<T>(m)` function. It casts a matcher `m` to type
`Matcher<T>`. To ensure safety, Google Mock checks that (let `U` be the
type `m` accepts):
1. Type `T` can be implicitly cast to type `U`;
1. When both `T` and `U` are built-in arithmetic types (`bool`, integers, and floating-point numbers), the conversion from `T` to `U` is not lossy (in other words, any value representable by `T` can also be represented by `U`); and
1. When `U` is a reference, `T` must also be a reference (as the underlying matcher may be interested in the address of the `U` value).
The code won't compile if any of these conditions isn't met.
*`ElementAre*()` works with _any_ container that implements the STL iterator concept (i.e. it has a `const_iterator` type and supports `begin()/end()`) and supports `size()`, not just the ones defined in STL. It will even work with container types yet to be written - as long as they follows the above pattern.
* You can use nested `ElementAre*()` to match nested (multi-dimensional) containers.
* If the container is passed by pointer instead of by reference, just write `Pointee(ElementsAre*(...))`.
* The order of elements _matters_ for `ElementsAre*()`. Therefore don't use it with containers whose element order is undefined (e.g. `hash_map`).
## Sharing Matchers ##
Under the hood, a Google Mock matcher object consists of a pointer to
a ref-counted implementation object. Copying matchers is allowed and
very efficient, as only the pointer is copied. When the last matcher
that references the implementation object dies, the implementation
object will be deleted.
Therefore, if you have some complex matcher that you want to use again
and again, there is no need to build it everytime. Just assign it to a
matcher variable and use that variable repeatedly! For example,
```
Matcher<int> in_range = AllOf(Gt(5), Le(10));
... use in_range as a matcher in multiple EXPECT_CALLs ...
```
# Setting Expectations #
## Ignoring Uninteresting Calls ##
If you are not interested in how a mock method is called, just don't
say anything about it. In this case, if the method is ever called,
Google Mock will perform its default action to allow the test program
to continue. If you are not happy with the default action taken by
Google Mock, you can override it using `DefaultValue<T>::Set()`
(described later in this document) or `ON_CALL()`.
Please note that once you expressed interest in a particular mock
method (via `EXPECT_CALL()`), all invocations to it must match some
expectation. If this function is called but the arguments don't match
any `EXPECT_CALL()` statement, it will be an error.
## Disallowing Unexpected Calls ##
If a mock method shouldn't be called at all, explicitly say so:
```
using ::testing::_;
...
EXPECT_CALL(foo, Bar(_))
.Times(0);
```
If some calls to the method are allowed, but the rest are not, just
list all the expected calls:
```
using ::testing::AnyNumber;
using ::testing::Gt;
...
EXPECT_CALL(foo, Bar(5));
EXPECT_CALL(foo, Bar(Gt(10)))
.Times(AnyNumber());
```
A call to `foo.Bar()` that doesn't match any of the `EXPECT_CALL()`
statements will be an error.
## Expecting Ordered Calls ##
Although an `EXPECT_CALL()` statement defined earlier takes precedence
when Google Mock tries to match a function call with an expectation,
by default calls don't have to happen in the order `EXPECT_CALL()`
statements are written. For example, if the arguments match the
matchers in the third `EXPECT_CALL()`, but not those in the first two,
then the third expectation will be used.
If you would rather have all calls occur in the order of the
expectations, put the `EXPECT_CALL()` statements in a block where you
define a variable of type `InSequence`:
```
using ::testing::_;
using ::testing::InSequence;
{
InSequence s;
EXPECT_CALL(foo, DoThis(5));
EXPECT_CALL(bar, DoThat(_))
.Times(2);
EXPECT_CALL(foo, DoThis(6));
}
```
In this example, we expect a call to `foo.DoThis(5)`, followed by two
calls to `bar.DoThat()` where the argument can be anything, which are
in turn followed by a call to `foo.DoThis(6)`. If a call occurred
out-of-order, Google Mock will report an error.
## Expecting Partially Ordered Calls ##
Sometimes requiring everything to occur in a predetermined order can
lead to brittle tests. For example, we may care about `A` occurring
before both `B` and `C`, but aren't interested in the relative order
of `B` and `C`. In this case, the test should reflect our real intent,
instead of being overly constraining.
Google Mock allows you to impose an arbitrary DAG (directed acyclic
graph) on the calls. One way to express the DAG is to use the
[After](http://code.google.com/p/googlemock/wiki/V1_6_CheatSheet#The_After_Clause) clause of `EXPECT_CALL`.
Another way is via the `InSequence()` clause (not the same as the
`InSequence` class), which we borrowed from jMock 2. It's less
flexible than `After()`, but more convenient when you have long chains
of sequential calls, as it doesn't require you to come up with
different names for the expectations in the chains. Here's how it
works:
If we view `EXPECT_CALL()` statements as nodes in a graph, and add an
edge from node A to node B wherever A must occur before B, we can get
a DAG. We use the term "sequence" to mean a directed path in this
DAG. Now, if we decompose the DAG into sequences, we just need to know
which sequences each `EXPECT_CALL()` belongs to in order to be able to
reconstruct the orginal DAG.
So, to specify the partial order on the expectations we need to do two
things: first to define some `Sequence` objects, and then for each
`EXPECT_CALL()` say which `Sequence` objects it is part
of. Expectations in the same sequence must occur in the order they are
written. For example,
```
using ::testing::Sequence;
Sequence s1, s2;
EXPECT_CALL(foo, A())
.InSequence(s1, s2);
EXPECT_CALL(bar, B())
.InSequence(s1);
EXPECT_CALL(bar, C())
.InSequence(s2);
EXPECT_CALL(foo, D())
.InSequence(s2);
```
specifies the following DAG (where `s1` is `A -> B`, and `s2` is `A ->
C -> D`):
```
+---> B
|
A ---|
|
+---> C ---> D
```
This means that A must occur before B and C, and C must occur before
D. There's no restriction about the order other than these.
## Controlling When an Expectation Retires ##
When a mock method is called, Google Mock only consider expectations
that are still active. An expectation is active when created, and
becomes inactive (aka _retires_) when a call that has to occur later
has occurred. For example, in
```
using ::testing::_;
using ::testing::Sequence;
Sequence s1, s2;
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #1
.Times(AnyNumber())
.InSequence(s1, s2);
EXPECT_CALL(log, Log(WARNING, _, "Data set is empty.")) // #2
.InSequence(s1);
EXPECT_CALL(log, Log(WARNING, _, "User not found.")) // #3
.InSequence(s2);
```
as soon as either #2 or #3 is matched, #1 will retire. If a warning
`"File too large."` is logged after this, it will be an error.
Note that an expectation doesn't retire automatically when it's
saturated. For example,
```
using ::testing::_;
...
EXPECT_CALL(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")); // #2
```
says that there will be exactly one warning with the message `"File
too large."`. If the second warning contains this message too, #2 will
match again and result in an upper-bound-violated error.
If this is not what you want, you can ask an expectation to retire as
soon as it becomes saturated:
```
using ::testing::_;
...
EXPECT_CALL(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #2
.RetiresOnSaturation();
```
Here #2 can be used only once, so if you have two warnings with the
message `"File too large."`, the first will match #2 and the second
will match #1 - there will be no error.
# Using Actions #
## Returning References from Mock Methods ##
If a mock function's return type is a reference, you need to use
`ReturnRef()` instead of `Return()` to return a result:
```
using ::testing::ReturnRef;
class MockFoo : public Foo {
public:
MOCK_METHOD0(GetBar, Bar&());
};
...
MockFoo foo;
Bar bar;
EXPECT_CALL(foo, GetBar())
.WillOnce(ReturnRef(bar));
```
## Returning Live Values from Mock Methods ##
The `Return(x)` action saves a copy of `x` when the action is
_created_, and always returns the same value whenever it's
executed. Sometimes you may want to instead return the _live_ value of
`x` (i.e. its value at the time when the action is _executed_.).
If the mock function's return type is a reference, you can do it using
`ReturnRef(x)`, as shown in the previous recipe ("Returning References
from Mock Methods"). However, Google Mock doesn't let you use
`ReturnRef()` in a mock function whose return type is not a reference,
as doing that usually indicates a user error. So, what shall you do?
You may be tempted to try `ByRef()`:
```
using testing::ByRef;
using testing::Return;
class MockFoo : public Foo {
public:
MOCK_METHOD0(GetValue, int());
};
...
int x = 0;
MockFoo foo;
EXPECT_CALL(foo, GetValue())
.WillRepeatedly(Return(ByRef(x)));
x = 42;
EXPECT_EQ(42, foo.GetValue());
```
Unfortunately, it doesn't work here. The above code will fail with error:
```
Value of: foo.GetValue()
Actual: 0
Expected: 42
```
The reason is that `Return(value)` converts `value` to the actual
return type of the mock function at the time when the action is
_created_, not when it is _executed_. (This behavior was chosen for
the action to be safe when `value` is a proxy object that references
some temporary objects.) As a result, `ByRef(x)` is converted to an
`int` value (instead of a `const int&`) when the expectation is set,
and `Return(ByRef(x))` will always return 0.
`ReturnPointee(pointer)` was provided to solve this problem
specifically. It returns the value pointed to by `pointer` at the time
the action is _executed_:
```
using testing::ReturnPointee;
...
int x = 0;
MockFoo foo;
EXPECT_CALL(foo, GetValue())
.WillRepeatedly(ReturnPointee(&x)); // Note the & here.
x = 42;
EXPECT_EQ(42, foo.GetValue()); // This will succeed now.
```
## Combining Actions ##
Want to do more than one thing when a function is called? That's
fine. `DoAll()` allow you to do sequence of actions every time. Only
the return value of the last action in the sequence will be used.
```
using ::testing::DoAll;
class MockFoo : public Foo {
public:
MOCK_METHOD1(Bar, bool(int n));
};
...
EXPECT_CALL(foo, Bar(_))
.WillOnce(DoAll(action_1,
action_2,
...
action_n));
```
## Mocking Side Effects ##
Sometimes a method exhibits its effect not via returning a value but
via side effects. For example, it may change some global state or
modify an output argument. To mock side effects, in general you can
define your own action by implementing `::testing::ActionInterface`.
If all you need to do is to change an output argument, the built-in
## Changing a Mock Object's Behavior Based on the State ##
If you expect a call to change the behavior of a mock object, you can use `::testing::InSequence` to specify different behaviors before and after the call:
```
using ::testing::InSequence;
using ::testing::Return;
...
{
InSequence seq;
EXPECT_CALL(my_mock, IsDirty())
.WillRepeatedly(Return(true));
EXPECT_CALL(my_mock, Flush());
EXPECT_CALL(my_mock, IsDirty())
.WillRepeatedly(Return(false));
}
my_mock.FlushIfDirty();
```
This makes `my_mock.IsDirty()` return `true` before `my_mock.Flush()` is called and return `false` afterwards.
If the behavior change is more complex, you can store the effects in a variable and make a mock method get its return value from that variable:
```
using ::testing::_;
using ::testing::SaveArg;
using ::testing::Return;
ACTION_P(ReturnPointee, p) { return *p; }
...
int previous_value = 0;
EXPECT_CALL(my_mock, GetPrevValue())
.WillRepeatedly(ReturnPointee(&previous_value));
EXPECT_CALL(my_mock, UpdateValue(_))
.WillRepeatedly(SaveArg<0>(&previous_value));
my_mock.DoSomethingToUpdateValue();
```
Here `my_mock.GetPrevValue()` will always return the argument of the last `UpdateValue()` call.
## Setting the Default Value for a Return Type ##
If a mock method's return type is a built-in C++ type or pointer, by
default it will return 0 when invoked. You only need to specify an
action if this default value doesn't work for you.
Sometimes, you may want to change this default value, or you may want
to specify a default value for types Google Mock doesn't know
about. You can do this using the `::testing::DefaultValue` class
template:
```
class MockFoo : public Foo {
public:
MOCK_METHOD0(CalculateBar, Bar());
};
...
Bar default_bar;
// Sets the default return value for type Bar.
DefaultValue<Bar>::Set(default_bar);
MockFoo foo;
// We don't need to specify an action here, as the default
// return value works for us.
EXPECT_CALL(foo, CalculateBar());
foo.CalculateBar(); // This should return default_bar.
// Unsets the default return value.
DefaultValue<Bar>::Clear();
```
Please note that changing the default value for a type can make you
tests hard to understand. We recommend you to use this feature
judiciously. For example, you may want to make sure the `Set()` and
`Clear()` calls are right next to the code that uses your mock.
## Setting the Default Actions for a Mock Method ##
You've learned how to change the default value of a given
type. However, this may be too coarse for your purpose: perhaps you
have two mock methods with the same return type and you want them to
have different behaviors. The `ON_CALL()` macro allows you to
customize your mock's behavior at the method level:
```
using ::testing::_;
using ::testing::AnyNumber;
using ::testing::Gt;
using ::testing::Return;
...
ON_CALL(foo, Sign(_))
.WillByDefault(Return(-1));
ON_CALL(foo, Sign(0))
.WillByDefault(Return(0));
ON_CALL(foo, Sign(Gt(0)))
.WillByDefault(Return(1));
EXPECT_CALL(foo, Sign(_))
.Times(AnyNumber());
foo.Sign(5); // This should return 1.
foo.Sign(-9); // This should return -1.
foo.Sign(0); // This should return 0.
```
As you may have guessed, when there are more than one `ON_CALL()`
statements, the news order take precedence over the older ones. In
other words, the **last** one that matches the function arguments will
be used. This matching order allows you to set up the common behavior
in a mock object's constructor or the test fixture's set-up phase and
specialize the mock's behavior later.
## Using Functions/Methods/Functors as Actions ##
If the built-in actions don't suit you, you can easily use an existing
For better readability, Google Mock also gives you:
*`WithoutArgs(action)` when the inner `action` takes _no_ argument, and
*`WithArg<N>(action)` (no `s` after `Arg`) when the inner `action` takes _one_ argument.
As you may have realized, `InvokeWithoutArgs(...)` is just syntactic
sugar for `WithoutArgs(Inovke(...))`.
Here are more tips:
* The inner action used in `WithArgs` and friends does not have to be `Invoke()` -- it can be anything.
* You can repeat an argument in the argument list if necessary, e.g. `WithArgs<2, 3, 3, 5>(...)`.
* You can change the order of the arguments, e.g. `WithArgs<3, 2, 1>(...)`.
* The types of the selected arguments do _not_ have to match the signature of the inner action exactly. It works as long as they can be implicitly converted to the corresponding arguments of the inner action. For example, if the 4-th argument of the mock function is an `int` and `my_action` takes a `double`, `WithArg<4>(my_action)` will work.
## Ignoring Arguments in Action Functions ##
The selecting-an-action's-arguments recipe showed us one way to make a
mock function and an action with incompatible argument lists fit
together. The downside is that wrapping the action in
`WithArgs<...>()` can get tedious for people writing the tests.
If you are defining a function, method, or functor to be used with
`Invoke*()`, and you are not interested in some of its arguments, an
alternative to `WithArgs` is to declare the uninteresting arguments as
`Unused`. This makes the definition less cluttered and less fragile in
case the types of the uninteresting arguments change. It could also
increase the chance the action function can be reused. For example,
The expectation spec says that the first `Bar("a")` must happen before
check point "1", the second `Bar("a")` must happen after check point "2",
and nothing should happen between the two check points. The explicit
check points make it easy to tell which `Bar("a")` is called by which
call to `Foo()`.
## Mocking Destructors ##
Sometimes you want to make sure a mock object is destructed at the
right time, e.g. after `bar->A()` is called but before `bar->B()` is
called. We already know that you can specify constraints on the order
of mock function calls, so all we need to do is to mock the destructor
of the mock function.
This sounds simple, except for one problem: a destructor is a special
function with special syntax and special semantics, and the
`MOCK_METHOD0` macro doesn't work for it:
```
MOCK_METHOD0(~MockFoo, void()); // Won't compile!
```
The good news is that you can use a simple pattern to achieve the same
effect. First, add a mock function `Die()` to your mock class and call
it in the destructor, like this:
```
class MockFoo : public Foo {
...
// Add the following two lines to the mock class.
MOCK_METHOD0(Die, void());
virtual ~MockFoo() { Die(); }
};
```
(If the name `Die()` clashes with an existing symbol, choose another
name.) Now, we have translated the problem of testing when a `MockFoo`
object dies to testing when its `Die()` method is called:
```
MockFoo* foo = new MockFoo;
MockBar* bar = new MockBar;
...
{
InSequence s;
// Expects *foo to die after bar->A() and before bar->B().
EXPECT_CALL(*bar, A());
EXPECT_CALL(*foo, Die());
EXPECT_CALL(*bar, B());
}
```
And that's that.
## Using Google Mock and Threads ##
**IMPORTANT NOTE:** What we describe in this recipe is **ONLY** true on
platforms where Google Mock is thread-safe. Currently these are only
platforms that support the pthreads library (this includes Linux and Mac).
To make it thread-safe on other platforms we only need to implement
some synchronization operations in `"gtest/internal/gtest-port.h"`.
In a **unit** test, it's best if you could isolate and test a piece of
code in a single-threaded context. That avoids race conditions and
dead locks, and makes debugging your test much easier.
Yet many programs are multi-threaded, and sometimes to test something
we need to pound on it from more than one thread. Google Mock works
for this purpose too.
Remember the steps for using a mock:
1. Create a mock object `foo`.
1. Set its default actions and expectations using `ON_CALL()` and `EXPECT_CALL()`.
1. The code under test calls methods of `foo`.
1. Optionally, verify and reset the mock.
1. Destroy the mock yourself, or let the code under test destroy it. The destructor will automatically verify it.
If you follow the following simple rules, your mocks and threads can
live happily togeter:
* Execute your _test code_ (as opposed to the code being tested) in _one_ thread. This makes your test easy to follow.
* Obviously, you can do step #1 without locking.
* When doing step #2 and #5, make sure no other thread is accessing `foo`. Obvious too, huh?
* #3 and #4 can be done either in one thread or in multiple threads - anyway you want. Google Mock takes care of the locking, so you don't have to do any - unless required by your test logic.
If you violate the rules (for example, if you set expectations on a
mock while another thread is calling its methods), you get undefined
behavior. That's not fun, so don't do it.
Google Mock guarantees that the action for a mock function is done in
the same thread that called the mock function. For example, in
```
EXPECT_CALL(mock, Foo(1))
.WillOnce(action1);
EXPECT_CALL(mock, Foo(2))
.WillOnce(action2);
```
if `Foo(1)` is called in thread 1 and `Foo(2)` is called in thread 2,
Google Mock will execute `action1` in thread 1 and `action2` in thread
2.
Google Mock does _not_ impose a sequence on actions performed in
different threads (doing so may create deadlocks as the actions may
need to cooperate). This means that the execution of `action1` and
`action2` in the above example _may_ interleave. If this is a problem,
you should add proper synchronization logic to `action1` and `action2`
to make the test thread-safe.
Also, remember that `DefaultValue<T>` is a global resource that
potentially affects _all_ living mock objects in your
program. Naturally, you won't want to mess with it from multiple
threads or when there still are mocks in action.
## Controlling How Much Information Google Mock Prints ##
When Google Mock sees something that has the potential of being an
error (e.g. a mock function with no expectation is called, a.k.a. an
uninteresting call, which is allowed but perhaps you forgot to
explicitly ban the call), it prints some warning messages, including
the arguments of the function and the return value. Hopefully this
will remind you to take a look and see if there is indeed a problem.
Sometimes you are confident that your tests are correct and may not
appreciate such friendly messages. Some other times, you are debugging
your tests or learning about the behavior of the code you are testing,
and wish you could observe every mock call that happens (including
argument values and the return value). Clearly, one size doesn't fit
all.
You can control how much Google Mock tells you using the
`--gmock_verbose=LEVEL` command-line flag, where `LEVEL` is a string
with three possible values:
*`info`: Google Mock will print all informational messages, warnings, and errors (most verbose). At this setting, Google Mock will also log any calls to the `ON_CALL/EXPECT_CALL` macros.
*`warning`: Google Mock will print both warnings and errors (less verbose). This is the default.
*`error`: Google Mock will print errors only (least verbose).
Alternatively, you can adjust the value of that flag from within your
tests like so:
```
::testing::FLAGS_gmock_verbose = "error";
```
Now, judiciously use the right flag to enable Google Mock serve you better!
## Running Tests in Emacs ##
If you build and run your tests in Emacs, the source file locations of
Google Mock and [Google Test](http://code.google.com/p/googletest/)
errors will be highlighted. Just press `<Enter>` on one of them and
you'll be taken to the offending line. Or, you can just type `C-x ``
to jump to the next error.
To make it even easier, you can add the following lines to your
Optionally, you can stream additional information to a hidden argument
named `result_listener` to explain the match result. For example, a
better definition of `IsDivisibleBy7` is:
```
MATCHER(IsDivisibleBy7, "") {
if ((arg % 7) == 0)
return true;
*result_listener << "the remainder is " << (arg % 7);
return false;
}
```
With this definition, the above assertion will give a better message:
```
Value of: some_expression
Expected: is divisible by 7
Actual: 27 (the remainder is 6)
```
You should let `MatchAndExplain()` print _any additional information_
that can help a user understand the match result. Note that it should
explain why the match succeeds in case of a success (unless it's
obvious) - this is useful when the matcher is used inside
`Not()`. There is no need to print the argument value itself, as
Google Mock already prints it for you.
**Notes:**
1. The type of the value being matched (`arg_type`) is determined by the context in which you use the matcher and is supplied to you by the compiler, so you don't need to worry about declaring it (nor can you). This allows the matcher to be polymorphic. For example, `IsDivisibleBy7()` can be used to match any type where the value of `(arg % 7) == 0` can be implicitly converted to a `bool`. In the `Bar(IsDivisibleBy7())` example above, if method `Bar()` takes an `int`, `arg_type` will be `int`; if it takes an `unsigned long`, `arg_type` will be `unsigned long`; and so on.
1. Google Mock doesn't guarantee when or how many times a matcher will be invoked. Therefore the matcher logic must be _purely functional_ (i.e. it cannot have any side effect, and the result must not depend on anything other than the value being matched and the matcher parameters). This requirement must be satisfied no matter how you define the matcher (e.g. using one of the methods described in the following recipes). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and Google Mock.
## Writing New Parameterized Matchers Quickly ##
Sometimes you'll want to define a matcher that has parameters. For that you
