

You can find recipes for using Google Mock here. If you haven't yet,
please read the [ForDummies](ForDummies.md) document first to make sure you understand
the basics.

**Note:** Google Mock lives in the `testing` name space. For
readability, it is recommended to write `using ::testing::Foo;` once in
your file before using the name `Foo` defined by Google Mock. We omit
such `using` statements in this page for brevity, but you should do it
in your own code.

# Creating Mock Classes #

## Mocking Private or Protected Methods ##

You must always put a mock method definition (`MOCK_METHOD*`) in a
`public:` section of the mock class, regardless of the method being
mocked being `public`, `protected`, or `private` in the base class.
This allows `ON_CALL` and `EXPECT_CALL` to reference the mock function
from outside of the mock class.  (Yes, C++ allows a subclass to change
the access level of a virtual function in the base class.)  Example:

```
class Foo {
 public:
  ...
  virtual bool Transform(Gadget* g) = 0;

 protected:
  virtual void Resume();

 private:
  virtual int GetTimeOut();
};

class MockFoo : public Foo {
 public:
  ...
  MOCK_METHOD1(Transform, bool(Gadget* g));

  // The following must be in the public section, even though the
  // methods are protected or private in the base class.
  MOCK_METHOD0(Resume, void());
  MOCK_METHOD0(GetTimeOut, int());
};
```

## Mocking Overloaded Methods ##

You can mock overloaded functions as usual. No special attention is required:

```
class Foo {
  ...

  // Must be virtual as we'll inherit from Foo.
  virtual ~Foo();

  // Overloaded on the types and/or numbers of arguments.
  virtual int Add(Element x);
  virtual int Add(int times, Element x);

  // Overloaded on the const-ness of this object.
  virtual Bar& GetBar();
  virtual const Bar& GetBar() const;
};

class MockFoo : public Foo {
  ...
  MOCK_METHOD1(Add, int(Element x));
  MOCK_METHOD2(Add, int(int times, Element x);

  MOCK_METHOD0(GetBar, Bar&());
  MOCK_CONST_METHOD0(GetBar, const Bar&());
};
```

**Note:** if you don't mock all versions of the overloaded method, the
compiler will give you a warning about some methods in the base class
being hidden. To fix that, use `using` to bring them in scope:

```
class MockFoo : public Foo {
  ...
  using Foo::Add;
  MOCK_METHOD1(Add, int(Element x));
  // We don't want to mock int Add(int times, Element x);
  ...
};
```

## Mocking Class Templates ##

To mock a class template, append `_T` to the `MOCK_*` macros:

```
template <typename Elem>
class StackInterface {
  ...
  // Must be virtual as we'll inherit from StackInterface.
  virtual ~StackInterface();

  virtual int GetSize() const = 0;
  virtual void Push(const Elem& x) = 0;
};

template <typename Elem>
class MockStack : public StackInterface<Elem> {
  ...
  MOCK_CONST_METHOD0_T(GetSize, int());
  MOCK_METHOD1_T(Push, void(const Elem& x));
};
```

## Mocking Nonvirtual Methods ##

Google Mock can mock non-virtual functions to be used in what we call _hi-perf
dependency injection_.

In this case, instead of sharing a common base class with the real
class, your mock class will be _unrelated_ to the real class, but
contain methods with the same signatures.  The syntax for mocking
non-virtual methods is the _same_ as mocking virtual methods:

```
// A simple packet stream class.  None of its members is virtual.
class ConcretePacketStream {
 public:
  void AppendPacket(Packet* new_packet);
  const Packet* GetPacket(size_t packet_number) const;
  size_t NumberOfPackets() const;
  ...
};

// A mock packet stream class.  It inherits from no other, but defines
// GetPacket() and NumberOfPackets().
class MockPacketStream {
 public:
  MOCK_CONST_METHOD1(GetPacket, const Packet*(size_t packet_number));
  MOCK_CONST_METHOD0(NumberOfPackets, size_t());
  ...
};
```

Note that the mock class doesn't define `AppendPacket()`, unlike the
real class. That's fine as long as the test doesn't need to call it.

Next, you need a way to say that you want to use
`ConcretePacketStream` in production code, and use `MockPacketStream`
in tests.  Since the functions are not virtual and the two classes are
unrelated, you must specify your choice at _compile time_ (as opposed
to run time).

One way to do it is to templatize your code that needs to use a packet
stream.  More specifically, you will give your code a template type
argument for the type of the packet stream.  In production, you will
instantiate your template with `ConcretePacketStream` as the type
argument.  In tests, you will instantiate the same template with
`MockPacketStream`.  For example, you may write:

```
template <class PacketStream>
void CreateConnection(PacketStream* stream) { ... }

template <class PacketStream>
class PacketReader {
 public:
  void ReadPackets(PacketStream* stream, size_t packet_num);
};
```

Then you can use `CreateConnection<ConcretePacketStream>()` and
`PacketReader<ConcretePacketStream>` in production code, and use
`CreateConnection<MockPacketStream>()` and
`PacketReader<MockPacketStream>` in tests.

```
  MockPacketStream mock_stream;
  EXPECT_CALL(mock_stream, ...)...;
  .. set more expectations on mock_stream ...
  PacketReader<MockPacketStream> reader(&mock_stream);
  ... exercise reader ...
```

## Mocking Free Functions ##

It's possible to use Google Mock to mock a free function (i.e. a
C-style function or a static method).  You just need to rewrite your
code to use an interface (abstract class).

Instead of calling a free function (say, `OpenFile`) directly,
introduce an interface for it and have a concrete subclass that calls
the free function:

```
class FileInterface {
 public:
  ...
  virtual bool Open(const char* path, const char* mode) = 0;
};

class File : public FileInterface {
 public:
  ...
  virtual bool Open(const char* path, const char* mode) {
    return OpenFile(path, mode);
  }
};
```

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).

## The Nice, the Strict, and the Naggy ##

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
`NiceMock<MockFoo>` "inherits" `MockFoo`'s constructors:

```
using ::testing::NiceMock;

TEST(...) {
  NiceMock<MockFoo> mock_foo(5, "hi");  // Calls MockFoo(5, "hi").
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}
```

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** about when to use naggy or strict mocks, as they tend to make tests more brittle and harder to maintain. When you refactor your code without changing its externally visible behavior, ideally you should't need to update any tests. If your code interacts with a naggy mock, however, you may start to get spammed with warnings as the result of your change. Worse, if your code interacts with a strict mock, your tests may start to fail and you'll be forced to fix them. Our general recommendation is to use nice mocks (not yet the default) most of the time, use naggy mocks (the current default) when developing or debugging tests, and use strict mocks only as the last resort.

## Simplifying the Interface without Breaking Existing Code ##

Sometimes a method has a long list of arguments that is mostly
uninteresting. For example,

```
class LogSink {
 public:
  ...
  virtual void send(LogSeverity severity, const char* full_filename,
                    const char* base_filename, int line,
                    const struct tm* tm_time,
                    const char* message, size_t message_len) = 0;
};
```

This method's argument list is lengthy and hard to work with (let's
say that the `message` argument is not even 0-terminated). If we mock
it as is, using the mock will be awkward. If, however, we try to
simplify this interface, we'll need to fix all clients depending on
it, which is often infeasible.

The trick is to re-dispatch the method in the mock class:

```
class ScopedMockLog : public LogSink {
 public:
  ...
  virtual void send(LogSeverity severity, const char* full_filename,
                    const char* base_filename, int line, const tm* tm_time,
                    const char* message, size_t message_len) {
    // We are only interested in the log severity, full file name, and
    // log message.
    Log(severity, full_filename, std::string(message, message_len));
  }

  // Implements the mock method:
  //
  //   void Log(LogSeverity severity,
  //            const string& file_path,
  //            const string& message);
  MOCK_METHOD3(Log, void(LogSeverity severity, const string& file_path,
                         const string& message));
};
```

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.
  MOCK_METHOD1(DoThis, char(int n));
  MOCK_METHOD2(DoThat, void(const char* s, int* p));

  // 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. For instance, if class `Foo` has methods `char DoThis(int n)` and `bool DoThis(double x) const`, and you want to invoke the latter, you need to write `Invoke(&fake_, static_cast<bool (FakeFoo::*)(double) const>(&FakeFoo::DoThis))` instead of `Invoke(&fake_, &FakeFoo::DoThis)` (The strange-looking thing inside the angled brackets of `static_cast` is the type of a function pointer to the second `DoThis()` method.).
  * 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()`:

```
using ::testing::Invoke;
...
  ON_CALL(foo, Concrete(_))
      .WillByDefault(Invoke(&foo, &MockFoo::FooConcrete));
```

(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.

Here's one example:

```
using ::testing::SafeMatcherCast;

// A base class and a child class.
class Base { ... };
class Derived : public Base { ... };

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(DoThis, void(Derived* derived));
};
...

  MockFoo foo;
  // m is a Matcher<Base*> we got from somewhere.
  EXPECT_CALL(foo, DoThis(SafeMatcherCast<Derived*>(m)));
```

If you find `SafeMatcherCast<T>(m)` too limiting, you can use a similar
function `MatcherCast<T>(m)`. The difference is that `MatcherCast` works
as long as you can `static_cast` type `T` to type `U`.

`MatcherCast` essentially lets you bypass C++'s type system
(`static_cast` isn't always safe as it could throw away information,
for example), so be careful not to misuse/abuse it.

## Selecting Between Overloaded Functions ##

If you expect an overloaded function to be called, the compiler may
need some help on which overloaded version it is.

To disambiguate functions overloaded on the const-ness of this object,
use the `Const()` argument wrapper.

```
using ::testing::ReturnRef;

class MockFoo : public Foo {
  ...
  MOCK_METHOD0(GetBar, Bar&());
  MOCK_CONST_METHOD0(GetBar, const Bar&());
};
...

  MockFoo foo;
  Bar bar1, bar2;
  EXPECT_CALL(foo, GetBar())         // The non-const GetBar().
      .WillOnce(ReturnRef(bar1));
  EXPECT_CALL(Const(foo), GetBar())  // The const GetBar().
      .WillOnce(ReturnRef(bar2));
```

(`Const()` is defined by Google Mock and returns a `const` reference
to its argument.)

To disambiguate overloaded functions with the same number of arguments
but different argument types, you may need to specify the exact type
of a matcher, either by wrapping your matcher in `Matcher<type>()`, or
using a matcher whose type is fixed (`TypedEq<type>`, `An<type>()`,
etc):

```
using ::testing::An;
using ::testing::Lt;
using ::testing::Matcher;
using ::testing::TypedEq;

class MockPrinter : public Printer {
 public:
  MOCK_METHOD1(Print, void(int n));
  MOCK_METHOD1(Print, void(char c));
};

TEST(PrinterTest, Print) {
  MockPrinter printer;

  EXPECT_CALL(printer, Print(An<int>()));            // void Print(int);
  EXPECT_CALL(printer, Print(Matcher<int>(Lt(5))));  // void Print(int);
  EXPECT_CALL(printer, Print(TypedEq<char>('a')));   // void Print(char);

  printer.Print(3);
  printer.Print(6);
  printer.Print('a');
}
```

## Performing Different Actions Based on the Arguments ##

When a mock method is called, the _last_ matching expectation that's
still active will be selected (think "newer overrides older"). So, you
can make a method do different things depending on its argument values
like this:

```
using ::testing::_;
using ::testing::Lt;
using ::testing::Return;
...
  // The default case.
  EXPECT_CALL(foo, DoThis(_))
      .WillRepeatedly(Return('b'));

  // The more specific case.
  EXPECT_CALL(foo, DoThis(Lt(5)))
      .WillRepeatedly(Return('a'));
```

Now, if `foo.DoThis()` is called with a value less than 5, `'a'` will
be returned; otherwise `'b'` will be returned.

## Matching Multiple Arguments as a Whole ##

Sometimes it's not enough to match the arguments individually. For
example, we may want to say that the first argument must be less than
the second argument. The `With()` clause allows us to match
all arguments of a mock function as a whole. For example,

```
using ::testing::_;
using ::testing::Lt;
using ::testing::Ne;
...
  EXPECT_CALL(foo, InRange(Ne(0), _))
      .With(Lt());
```

says that the first argument of `InRange()` must not be 0, and must be
less than the second argument.

The expression inside `With()` must be a matcher of type
`Matcher< ::testing::tuple<A1, ..., An> >`, where `A1`, ..., `An` are the
types of the function arguments.

You can also write `AllArgs(m)` instead of `m` inside `.With()`. The
two forms are equivalent, but `.With(AllArgs(Lt()))` is more readable
than `.With(Lt())`.

You can use `Args<k1, ..., kn>(m)` to match the `n` selected arguments
(as a tuple) against `m`. For example,

```
using ::testing::_;
using ::testing::AllOf;
using ::testing::Args;
using ::testing::Lt;
...
  EXPECT_CALL(foo, Blah(_, _, _))
      .With(AllOf(Args<0, 1>(Lt()), Args<1, 2>(Lt())));
```

says that `Blah()` will be called with arguments `x`, `y`, and `z` where
`x < y < z`.

As a convenience and example, Google Mock provides some matchers for
2-tuples, including the `Lt()` matcher above. See the [CheatSheet](CheatSheet.md) for
the complete list.

Note that if you want to pass the arguments to a predicate of your own
(e.g. `.With(Args<0, 1>(Truly(&MyPredicate)))`), that predicate MUST be
written to take a `::testing::tuple` as its argument; Google Mock will pass the `n` selected arguments as _one_ single tuple to the predicate.

## Using Matchers as Predicates ##

Have you noticed that a matcher is just a fancy predicate that also
knows how to describe itself? Many existing algorithms take predicates
as arguments (e.g. those defined in STL's `<algorithm>` header), and
it would be a shame if Google Mock matchers are not allowed to
participate.

Luckily, you can use a matcher where a unary predicate functor is
expected by wrapping it inside the `Matches()` function. For example,

```
#include <algorithm>
#include <vector>

std::vector<int> v;
...
// How many elements in v are >= 10?
const int count = count_if(v.begin(), v.end(), Matches(Ge(10)));
```

Since you can build complex matchers from simpler ones easily using
Google Mock, this gives you a way to conveniently construct composite
predicates (doing the same using STL's `<functional>` header is just
painful). For example, here's a predicate that's satisfied by any
number that is >= 0, <= 100, and != 50:

```
Matches(AllOf(Ge(0), Le(100), Ne(50)))
```

## Using Matchers in Google Test Assertions ##

Since matchers are basically predicates that also know how to describe
themselves, there is a way to take advantage of them in
[Google Test](http://code.google.com/p/googletest/) assertions. It's
called `ASSERT_THAT` and `EXPECT_THAT`:

```
  ASSERT_THAT(value, matcher);  // Asserts that value matches matcher.
  EXPECT_THAT(value, matcher);  // The non-fatal version.
```

For example, in a Google Test test you can write:

```
#include "gmock/gmock.h"

using ::testing::AllOf;
using ::testing::Ge;
using ::testing::Le;
using ::testing::MatchesRegex;
using ::testing::StartsWith;
...

  EXPECT_THAT(Foo(), StartsWith("Hello"));
  EXPECT_THAT(Bar(), MatchesRegex("Line \\d+"));
  ASSERT_THAT(Baz(), AllOf(Ge(5), Le(10)));
```

which (as you can probably guess) executes `Foo()`, `Bar()`, and
`Baz()`, and verifies that:

  * `Foo()` returns a string that starts with `"Hello"`.
  * `Bar()` returns a string that matches regular expression `"Line \\d+"`.
  * `Baz()` returns a number in the range [5, 10].

The nice thing about these macros is that _they read like
English_. They generate informative messages too. For example, if the
first `EXPECT_THAT()` above fails, the message will be something like:

```
Value of: Foo()
  Actual: "Hi, world!"
Expected: starts with "Hello"
```

**Credit:** The idea of `(ASSERT|EXPECT)_THAT` was stolen from the
[Hamcrest](http://code.google.com/p/hamcrest/) project, which adds
`assertThat()` to JUnit.

## Using Predicates as Matchers ##

Google Mock provides a built-in set of matchers. In case you find them
lacking, you can use an arbitray unary predicate function or functor
as a matcher - as long as the predicate accepts a value of the type
you want. You do this by wrapping the predicate inside the `Truly()`
function, for example:

```
using ::testing::Truly;

int IsEven(int n) { return (n % 2) == 0 ? 1 : 0; }
...

  // Bar() must be called with an even number.
  EXPECT_CALL(foo, Bar(Truly(IsEven)));
```

Note that the predicate function / functor doesn't have to return
`bool`. It works as long as the return value can be used as the
condition in statement `if (condition) ...`.

## Matching Arguments that Are Not Copyable ##

When you do an `EXPECT_CALL(mock_obj, Foo(bar))`, Google Mock saves
away a copy of `bar`. When `Foo()` is called later, Google Mock
compares the argument to `Foo()` with the saved copy of `bar`. This
way, you don't need to worry about `bar` being modified or destroyed
after the `EXPECT_CALL()` is executed. The same is true when you use
matchers like `Eq(bar)`, `Le(bar)`, and so on.

But what if `bar` cannot be copied (i.e. has no copy constructor)? You
could define your own matcher function and use it with `Truly()`, as
the previous couple of recipes have shown. Or, you may be able to get
away from it if you can guarantee that `bar` won't be changed after
the `EXPECT_CALL()` is executed. Just tell Google Mock that it should
save a reference to `bar`, instead of a copy of it. Here's how:

```
using ::testing::Eq;
using ::testing::ByRef;
using ::testing::Lt;
...
  // Expects that Foo()'s argument == bar.
  EXPECT_CALL(mock_obj, Foo(Eq(ByRef(bar))));

  // Expects that Foo()'s argument < bar.
  EXPECT_CALL(mock_obj, Foo(Lt(ByRef(bar))));
```

Remember: if you do this, don't change `bar` after the
`EXPECT_CALL()`, or the result is undefined.

## Validating a Member of an Object ##

Often a mock function takes a reference to object as an argument. When
matching the argument, you may not want to compare the entire object
against a fixed object, as that may be over-specification. Instead,
you may need to validate a certain member variable or the result of a
certain getter method of the object. You can do this with `Field()`
and `Property()`. More specifically,

```
Field(&Foo::bar, m)
```

is a matcher that matches a `Foo` object whose `bar` member variable
satisfies matcher `m`.

```
Property(&Foo::baz, m)
```

is a matcher that matches a `Foo` object whose `baz()` method returns
a value that satisfies matcher `m`.

For example:

> | `Field(&Foo::number, Ge(3))` | Matches `x` where `x.number >= 3`. |
|:-----------------------------|:-----------------------------------|
> | `Property(&Foo::name, StartsWith("John "))` | Matches `x` where `x.name()` starts with `"John "`. |

Note that in `Property(&Foo::baz, ...)`, method `baz()` must take no
argument and be declared as `const`.

BTW, `Field()` and `Property()` can also match plain pointers to
objects. For instance,

```
Field(&Foo::number, Ge(3))
```

matches a plain pointer `p` where `p->number >= 3`. If `p` is `NULL`,
the match will always fail regardless of the inner matcher.

What if you want to validate more than one members at the same time?
Remember that there is `AllOf()`.

## Validating the Value Pointed to by a Pointer Argument ##

C++ functions often take pointers as arguments. You can use matchers
like `IsNull()`, `NotNull()`, and other comparison matchers to match a
pointer, but what if you want to make sure the value _pointed to_ by
the pointer, instead of the pointer itself, has a certain property?
Well, you can use the `Pointee(m)` matcher.

`Pointee(m)` matches a pointer iff `m` matches the value the pointer
points to. For example:

```
using ::testing::Ge;
using ::testing::Pointee;
...
  EXPECT_CALL(foo, Bar(Pointee(Ge(3))));
```

expects `foo.Bar()` to be called with a pointer that points to a value
greater than or equal to 3.

One nice thing about `Pointee()` is that it treats a `NULL` pointer as
a match failure, so you can write `Pointee(m)` instead of

```
  AllOf(NotNull(), Pointee(m))
```

without worrying that a `NULL` pointer will crash your test.

Also, did we tell you that `Pointee()` works with both raw pointers
**and** smart pointers (`linked_ptr`, `shared_ptr`, `scoped_ptr`, and
etc)?

What if you have a pointer to pointer? You guessed it - you can use
nested `Pointee()` to probe deeper inside the value. For example,
`Pointee(Pointee(Lt(3)))` matches a pointer that points to a pointer
that points to a number less than 3 (what a mouthful...).

## Testing a Certain Property of an Object ##

Sometimes you want to specify that an object argument has a certain
property, but there is no existing matcher that does this. If you want
good error messages, you should define a matcher. If you want to do it
quick and dirty, you could get away with writing an ordinary function.

Let's say you have a mock function that takes an object of type `Foo`,
which has an `int bar()` method and an `int baz()` method, and you
want to constrain that the argument's `bar()` value plus its `baz()`
value is a given number. Here's how you can define a matcher to do it:

```
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;

class BarPlusBazEqMatcher : public MatcherInterface<const Foo&> {
 public:
  explicit BarPlusBazEqMatcher(int expected_sum)
      : expected_sum_(expected_sum) {}

  virtual bool MatchAndExplain(const Foo& foo,
                               MatchResultListener* listener) const {
    return (foo.bar() + foo.baz()) == expected_sum_;
  }

  virtual void DescribeTo(::std::ostream* os) const {
    *os << "bar() + baz() equals " << expected_sum_;
  }

  virtual void DescribeNegationTo(::std::ostream* os) const {
    *os << "bar() + baz() does not equal " << expected_sum_;
  }
 private:
  const int expected_sum_;
};

inline Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
  return MakeMatcher(new BarPlusBazEqMatcher(expected_sum));
}

...

  EXPECT_CALL(..., DoThis(BarPlusBazEq(5)))...;
```

## Matching Containers ##

Sometimes an STL container (e.g. list, vector, map, ...) is passed to
a mock function and you may want to validate it. Since most STL
containers support the `==` operator, you can write
`Eq(expected_container)` or simply `expected_container` to match a
container exactly.

Sometimes, though, you may want to be more flexible (for example, the
first element must be an exact match, but the second element can be
any positive number, and so on). Also, containers used in tests often
have a small number of elements, and having to define the expected
container out-of-line is a bit of a hassle.

You can use the `ElementsAre()` or `UnorderedElementsAre()` matcher in
such cases:

```
using ::testing::_;
using ::testing::ElementsAre;
using ::testing::Gt;
...

  MOCK_METHOD1(Foo, void(const vector<int>& numbers));
...

  EXPECT_CALL(mock, Foo(ElementsAre(1, Gt(0), _, 5)));
```

The above matcher says that the container must have 4 elements, which
must be 1, greater than 0, anything, and 5 respectively.

If you instead write:

```
using ::testing::_;
using ::testing::Gt;
using ::testing::UnorderedElementsAre;
...

  MOCK_METHOD1(Foo, void(const vector<int>& numbers));
...

  EXPECT_CALL(mock, Foo(UnorderedElementsAre(1, Gt(0), _, 5)));
```

It means that the container must have 4 elements, which under some
permutation must be 1, greater than 0, anything, and 5 respectively.

`ElementsAre()` and `UnorderedElementsAre()` are overloaded to take 0
to 10 arguments. If more are needed, you can place them in a C-style
array and use `ElementsAreArray()` or `UnorderedElementsAreArray()`
instead:

```
using ::testing::ElementsAreArray;
...

  // ElementsAreArray accepts an array of element values.
  const int expected_vector1[] = { 1, 5, 2, 4, ... };
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector1)));

  // Or, an array of element matchers.
  Matcher<int> expected_vector2 = { 1, Gt(2), _, 3, ... };
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector2)));
```

In case the array needs to be dynamically created (and therefore the
array size cannot be inferred by the compiler), you can give
`ElementsAreArray()` an additional argument to specify the array size:

```
using ::testing::ElementsAreArray;
...
  int* const expected_vector3 = new int[count];
  ... fill expected_vector3 with values ...
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector3, count)));
```

**Tips:**

  * `ElementsAre*()` can be used to match _any_ container that implements the STL iterator pattern (i.e. it has a `const_iterator` type and supports `begin()/end()`), 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 `ElementsAre*()` 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 #

## Knowing When to Expect ##

`ON_CALL` is likely the single most under-utilized construct in Google Mock.

There are basically two constructs for defining the behavior of a mock object: `ON_CALL` and `EXPECT_CALL`. The difference? `ON_CALL` defines what happens when a mock method is called, but _doesn't imply any expectation on the method being called._ `EXPECT_CALL` not only defines the behavior, but also sets an expectation that _the method will be called with the given arguments, for the given number of times_ (and _in the given order_ when you specify the order too).

Since `EXPECT_CALL` does more, isn't it better than `ON_CALL`? Not really. Every `EXPECT_CALL` adds a constraint on the behavior of the code under test. Having more constraints than necessary is _baaad_ - even worse than not having enough constraints.

This may be counter-intuitive. How could tests that verify more be worse than tests that verify less? Isn't verification the whole point of tests?

The answer, lies in _what_ a test should verify. **A good test verifies the contract of the code.** If a test over-specifies, it doesn't leave enough freedom to the implementation. As a result, changing the implementation without breaking the contract (e.g. refactoring and optimization), which should be perfectly fine to do, can break such tests. Then you have to spend time fixing them, only to see them broken again the next time the implementation is changed.

Keep in mind that one doesn't have to verify more than one property in one test. In fact, **it's a good style to verify only one thing in one test.** If you do that, a bug will likely break only one or two tests instead of dozens (which case would you rather debug?). If you are also in the habit of giving tests descriptive names that tell what they verify, you can often easily guess what's wrong just from the test log itself.

So use `ON_CALL` by default, and only use `EXPECT_CALL` when you actually intend to verify that the call is made. For example, you may have a bunch of `ON_CALL`s in your test fixture to set the common mock behavior shared by all tests in the same group, and write (scarcely) different `EXPECT_CALL`s in different `TEST_F`s to verify different aspects of the code's behavior. Compared with the style where each `TEST` has many `EXPECT_CALL`s, this leads to tests that are more resilient to implementational changes (and thus less likely to require maintenance) and makes the intent of the tests more obvious (so they are easier to maintain when you do need to maintain them).

If you are bothered by the "Uninteresting mock function call" message printed when a mock method without an `EXPECT_CALL` is called, you may use a `NiceMock` instead to suppress all such messages for the mock object, or suppress the message for specific methods by adding `EXPECT_CALL(...).Times(AnyNumber())`. DO NOT suppress it by blindly adding an `EXPECT_CALL(...)`, or you'll have a test that's a pain to maintain.

## 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.

## Understanding Uninteresting vs Unexpected Calls ##

_Uninteresting_ calls and _unexpected_ calls are different concepts in Google Mock. _Very_ different.

A call `x.Y(...)` is **uninteresting** if there's _not even a single_ `EXPECT_CALL(x, Y(...))` set. In other words, the test isn't interested in the `x.Y()` method at all, as evident in that the test doesn't care to say anything about it.

A call `x.Y(...)` is **unexpected** if there are some `EXPECT_CALL(x, Y(...))s` set, but none of them matches the call. Put another way, the test is interested in the `x.Y()` method (therefore it _explicitly_ sets some `EXPECT_CALL` to verify how it's called); however, the verification fails as the test doesn't expect this particular call to happen.

**An unexpected call is always an error,** as the code under test doesn't behave the way the test expects it to behave.

**By default, an uninteresting call is not an error,** as it violates no constraint specified by the test. (Google Mock's philosophy is that saying nothing means there is no constraint.) However, it leads to a warning, as it _might_ indicate a problem (e.g. the test author might have forgotten to specify a constraint).

In Google Mock, `NiceMock` and `StrictMock` can be used to make a mock class "nice" or "strict". How does this affect uninteresting calls and unexpected calls?

A **nice mock** suppresses uninteresting call warnings. It is less chatty than the default mock, but otherwise is the same. If a test fails with a default mock, it will also fail using a nice mock instead. And vice versa. Don't expect making a mock nice to change the test's result.

A **strict mock** turns uninteresting call warnings into errors. So making a mock strict may change the test's result.

Let's look at an example:

```
TEST(...) {
  NiceMock<MockDomainRegistry> mock_registry;
  EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
          .WillRepeatedly(Return("Larry Page"));

  // Use mock_registry in code under test.
  ... &mock_registry ...
}
```

The sole `EXPECT_CALL` here says that all calls to `GetDomainOwner()` must have `"google.com"` as the argument. If `GetDomainOwner("yahoo.com")` is called, it will be an unexpected call, and thus an error. Having a nice mock doesn't change the severity of an unexpected call.

So how do we tell Google Mock that `GetDomainOwner()` can be called with some other arguments as well? The standard technique is to add a "catch all" `EXPECT_CALL`:

```
  EXPECT_CALL(mock_registry, GetDomainOwner(_))
        .Times(AnyNumber());  // catches all other calls to this method.
  EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
        .WillRepeatedly(Return("Larry Page"));
```

Remember that `_` is the wildcard matcher that matches anything. With this, if `GetDomainOwner("google.com")` is called, it will do what the second `EXPECT_CALL` says; if it is called with a different argument, it will do what the first `EXPECT_CALL` says.

Note that the order of the two `EXPECT_CALLs` is important, as a newer `EXPECT_CALL` takes precedence over an older one.

For more on uninteresting calls, nice mocks, and strict mocks, read ["The Nice, the Strict, and the Naggy"](https://code.google.com/p/googlemock/wiki/CookBook#The_Nice,_the_Strict,_and_the_Naggy).

## 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/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
`SetArgPointee()` action is convenient:

```
using ::testing::SetArgPointee;

class MockMutator : public Mutator {
 public:
  MOCK_METHOD2(Mutate, void(bool mutate, int* value));
  ...
};
...

  MockMutator mutator;
  EXPECT_CALL(mutator, Mutate(true, _))
      .WillOnce(SetArgPointee<1>(5));
```

In this example, when `mutator.Mutate()` is called, we will assign 5
to the `int` variable pointed to by argument #1
(0-based).

`SetArgPointee()` conveniently makes an internal copy of the
value you pass to it, removing the need to keep the value in scope and
alive. The implication however is that the value must have a copy
constructor and assignment operator.

If the mock method also needs to return a value as well, you can chain
`SetArgPointee()` with `Return()` using `DoAll()`:

```
using ::testing::_;
using ::testing::Return;
using ::testing::SetArgPointee;

class MockMutator : public Mutator {
 public:
  ...
  MOCK_METHOD1(MutateInt, bool(int* value));
};
...

  MockMutator mutator;
  EXPECT_CALL(mutator, MutateInt(_))
      .WillOnce(DoAll(SetArgPointee<0>(5),
                      Return(true)));
```

If the output argument is an array, use the
`SetArrayArgument<N>(first, last)` action instead. It copies the
elements in source range `[first, last)` to the array pointed to by
the `N`-th (0-based) argument:

```
using ::testing::NotNull;
using ::testing::SetArrayArgument;

class MockArrayMutator : public ArrayMutator {
 public:
  MOCK_METHOD2(Mutate, void(int* values, int num_values));
  ...
};
...

  MockArrayMutator mutator;
  int values[5] = { 1, 2, 3, 4, 5 };
  EXPECT_CALL(mutator, Mutate(NotNull(), 5))
      .WillOnce(SetArrayArgument<0>(values, values + 5));
```

This also works when the argument is an output iterator:

```
using ::testing::_;
using ::testing::SeArrayArgument;

class MockRolodex : public Rolodex {
 public:
  MOCK_METHOD1(GetNames, void(std::back_insert_iterator<vector<string> >));
  ...
};
...

  MockRolodex rolodex;
  vector<string> names;
  names.push_back("George");
  names.push_back("John");
  names.push_back("Thomas");
  EXPECT_CALL(rolodex, GetNames(_))
      .WillOnce(SetArrayArgument<0>(names.begin(), names.end()));
```

## 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. Also, in C++ 11 and above, a mock
method whose return type has a default constructor will return a default-constructed
value by default.  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
function, method, or functor as an action:

```
using ::testing::_;
using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  MOCK_METHOD2(Sum, int(int x, int y));
  MOCK_METHOD1(ComplexJob, bool(int x));
};

int CalculateSum(int x, int y) { return x + y; }

class Helper {
 public:
  bool ComplexJob(int x);
};
...

  MockFoo foo;
  Helper helper;
  EXPECT_CALL(foo, Sum(_, _))
      .WillOnce(Invoke(CalculateSum));
  EXPECT_CALL(foo, ComplexJob(_))
      .WillOnce(Invoke(&helper, &Helper::ComplexJob));

  foo.Sum(5, 6);       // Invokes CalculateSum(5, 6).
  foo.ComplexJob(10);  // Invokes helper.ComplexJob(10);
```

The only requirement is that the type of the function, etc must be
_compatible_ with the signature of the mock function, meaning that the
latter's arguments can be implicitly converted to the corresponding
arguments of the former, and the former's return type can be
implicitly converted to that of the latter. So, you can invoke
something whose type is _not_ exactly the same as the mock function,
as long as it's safe to do so - nice, huh?

## Invoking a Function/Method/Functor Without Arguments ##

`Invoke()` is very useful for doing actions that are more complex. It
passes the mock function's arguments to the function or functor being
invoked such that the callee has the full context of the call to work
with. If the invoked function is not interested in some or all of the
arguments, it can simply ignore them.

Yet, a common pattern is that a test author wants to invoke a function
without the arguments of the mock function. `Invoke()` allows her to
do that using a wrapper function that throws away the arguments before
invoking an underlining nullary function. Needless to say, this can be
tedious and obscures the intent of the test.

`InvokeWithoutArgs()` solves this problem. It's like `Invoke()` except
that it doesn't pass the mock function's arguments to the
callee. Here's an example:

```
using ::testing::_;
using ::testing::InvokeWithoutArgs;

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(ComplexJob, bool(int n));
};

bool Job1() { ... }
...

  MockFoo foo;
  EXPECT_CALL(foo, ComplexJob(_))
      .WillOnce(InvokeWithoutArgs(Job1));

  foo.ComplexJob(10);  // Invokes Job1().
```

## Invoking an Argument of the Mock Function ##

Sometimes a mock function will receive a function pointer or a functor
(in other words, a "callable") as an argument, e.g.

```
class MockFoo : public Foo {
 public:
  MOCK_METHOD2(DoThis, bool(int n, bool (*fp)(int)));
};
```

and you may want to invoke this callable argument:

```
using ::testing::_;
...
  MockFoo foo;
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(...);
  // Will execute (*fp)(5), where fp is the
  // second argument DoThis() receives.
```

Arghh, you need to refer to a mock function argument but C++ has no
lambda (yet), so you have to define your own action. :-( Or do you
really?

Well, Google Mock has an action to solve _exactly_ this problem:

```
  InvokeArgument<N>(arg_1, arg_2, ..., arg_m)
```

will invoke the `N`-th (0-based) argument the mock function receives,
with `arg_1`, `arg_2`, ..., and `arg_m`. No matter if the argument is
a function pointer or a functor, Google Mock handles them both.

With that, you could write:

```
using ::testing::_;
using ::testing::InvokeArgument;
...
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(InvokeArgument<1>(5));
  // Will execute (*fp)(5), where fp is the
  // second argument DoThis() receives.
```

What if the callable takes an argument by reference? No problem - just
wrap it inside `ByRef()`:

```
...
  MOCK_METHOD1(Bar, bool(bool (*fp)(int, const Helper&)));
...
using ::testing::_;
using ::testing::ByRef;
using ::testing::InvokeArgument;
...

  MockFoo foo;
  Helper helper;
  ...
  EXPECT_CALL(foo, Bar(_))
      .WillOnce(InvokeArgument<0>(5, ByRef(helper)));
  // ByRef(helper) guarantees that a reference to helper, not a copy of it,
  // will be passed to the callable.
```

What if the callable takes an argument by reference and we do **not**
wrap the argument in `ByRef()`? Then `InvokeArgument()` will _make a
copy_ of the argument, and pass a _reference to the copy_, instead of
a reference to the original value, to the callable. This is especially
handy when the argument is a temporary value:

```
...
  MOCK_METHOD1(DoThat, bool(bool (*f)(const double& x, const string& s)));
...
using ::testing::_;
using ::testing::InvokeArgument;
...

  MockFoo foo;
  ...
  EXPECT_CALL(foo, DoThat(_))
      .WillOnce(InvokeArgument<0>(5.0, string("Hi")));
  // Will execute (*f)(5.0, string("Hi")), where f is the function pointer
  // DoThat() receives.  Note that the values 5.0 and string("Hi") are
  // temporary and dead once the EXPECT_CALL() statement finishes.  Yet
  // it's fine to perform this action later, since a copy of the values
  // are kept inside the InvokeArgument action.
```

## Ignoring an Action's Result ##

Sometimes you have an action that returns _something_, but you need an
action that returns `void` (perhaps you want to use it in a mock
function that returns `void`, or perhaps it needs to be used in
`DoAll()` and it's not the last in the list). `IgnoreResult()` lets
you do that. For example:

```
using ::testing::_;
using ::testing::Invoke;
using ::testing::Return;

int Process(const MyData& data);
string DoSomething();

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(Abc, void(const MyData& data));
  MOCK_METHOD0(Xyz, bool());
};
...

  MockFoo foo;
  EXPECT_CALL(foo, Abc(_))
  // .WillOnce(Invoke(Process));
  // The above line won't compile as Process() returns int but Abc() needs
  // to return void.
      .WillOnce(IgnoreResult(Invoke(Process)));

  EXPECT_CALL(foo, Xyz())
      .WillOnce(DoAll(IgnoreResult(Invoke(DoSomething)),
      // Ignores the string DoSomething() returns.
                      Return(true)));
```

Note that you **cannot** use `IgnoreResult()` on an action that already
returns `void`. Doing so will lead to ugly compiler errors.

## Selecting an Action's Arguments ##

Say you have a mock function `Foo()` that takes seven arguments, and
you have a custom action that you want to invoke when `Foo()` is
called. Trouble is, the custom action only wants three arguments:

```
using ::testing::_;
using ::testing::Invoke;
...
  MOCK_METHOD7(Foo, bool(bool visible, const string& name, int x, int y,
                         const map<pair<int, int>, double>& weight,
                         double min_weight, double max_wight));
...

bool IsVisibleInQuadrant1(bool visible, int x, int y) {
  return visible && x >= 0 && y >= 0;
}
...

  EXPECT_CALL(mock, Foo(_, _, _, _, _, _, _))
      .WillOnce(Invoke(IsVisibleInQuadrant1));  // Uh, won't compile. :-(
```

To please the compiler God, you can to define an "adaptor" that has
the same signature as `Foo()` and calls the custom action with the
right arguments:

```
using ::testing::_;
using ::testing::Invoke;

bool MyIsVisibleInQuadrant1(bool visible, const string& name, int x, int y,
                            const map<pair<int, int>, double>& weight,
                            double min_weight, double max_wight) {
  return IsVisibleInQuadrant1(visible, x, y);
}
...

  EXPECT_CALL(mock, Foo(_, _, _, _, _, _, _))
      .WillOnce(Invoke(MyIsVisibleInQuadrant1));  // Now it works.
```

But isn't this awkward?

Google Mock provides a generic _action adaptor_, so you can spend your
time minding more important business than writing your own
adaptors. Here's the syntax:

```
  WithArgs<N1, N2, ..., Nk>(action)
```

creates an action that passes the arguments of the mock function at
the given indices (0-based) to the inner `action` and performs
it. Using `WithArgs`, our original example can be written as:

```
using ::testing::_;
using ::testing::Invoke;
using ::testing::WithArgs;
...
  EXPECT_CALL(mock, Foo(_, _, _, _, _, _, _))
      .WillOnce(WithArgs<0, 2, 3>(Invoke(IsVisibleInQuadrant1)));
      // No need to define your own adaptor.
```

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,
given

```
  MOCK_METHOD3(Foo, double(const string& label, double x, double y));
  MOCK_METHOD3(Bar, double(int index, double x, double y));
```

instead of

```
using ::testing::_;
using ::testing::Invoke;

double DistanceToOriginWithLabel(const string& label, double x, double y) {
  return sqrt(x*x + y*y);
}

double DistanceToOriginWithIndex(int index, double x, double y) {
  return sqrt(x*x + y*y);
}
...

  EXEPCT_CALL(mock, Foo("abc", _, _))
      .WillOnce(Invoke(DistanceToOriginWithLabel));
  EXEPCT_CALL(mock, Bar(5, _, _))
      .WillOnce(Invoke(DistanceToOriginWithIndex));
```

you could write

```
using ::testing::_;
using ::testing::Invoke;
using ::testing::Unused;

double DistanceToOrigin(Unused, double x, double y) {
  return sqrt(x*x + y*y);
}
...

  EXEPCT_CALL(mock, Foo("abc", _, _))
      .WillOnce(Invoke(DistanceToOrigin));
  EXEPCT_CALL(mock, Bar(5, _, _))
      .WillOnce(Invoke(DistanceToOrigin));
```

## Sharing Actions ##

Just like matchers, a Google Mock action object consists of a pointer
to a ref-counted implementation object. Therefore copying actions is
also allowed and very efficient. When the last action that references
the implementation object dies, the implementation object will be
deleted.

If you have some complex action that you want to use again and again,
you may not have to build it from scratch everytime. If the action
doesn't have an internal state (i.e. if it always does the same thing
no matter how many times it has been called), you can assign it to an
action variable and use that variable repeatedly. For example:

```
  Action<bool(int*)> set_flag = DoAll(SetArgPointee<0>(5),
                                      Return(true));
  ... use set_flag in .WillOnce() and .WillRepeatedly() ...
```

However, if the action has its own state, you may be surprised if you
share the action object. Suppose you have an action factory
`IncrementCounter(init)` which creates an action that increments and
returns a counter whose initial value is `init`, using two actions
created from the same expression and using a shared action will
exihibit different behaviors. Example:

```
  EXPECT_CALL(foo, DoThis())
      .WillRepeatedly(IncrementCounter(0));
  EXPECT_CALL(foo, DoThat())
      .WillRepeatedly(IncrementCounter(0));
  foo.DoThis();  // Returns 1.
  foo.DoThis();  // Returns 2.
  foo.DoThat();  // Returns 1 - Blah() uses a different
                 // counter than Bar()'s.
```

versus

```
  Action<int()> increment = IncrementCounter(0);

  EXPECT_CALL(foo, DoThis())
      .WillRepeatedly(increment);
  EXPECT_CALL(foo, DoThat())
      .WillRepeatedly(increment);
  foo.DoThis();  // Returns 1.
  foo.DoThis();  // Returns 2.
  foo.DoThat();  // Returns 3 - the counter is shared.
```

# Misc Recipes on Using Google Mock #

## Mocking Methods That Use Move-Only Types ##

C++11 introduced <em>move-only types</em>.  A move-only-typed value can be moved from one object to another, but cannot be copied.  `std::unique_ptr<T>` is probably the most commonly used move-only type.

Mocking a method that takes and/or returns move-only types presents some challenges, but nothing insurmountable.  This recipe shows you how you can do it.

Let’s say we are working on a fictional project that lets one post and share snippets called “buzzes”.  Your code uses these types:

```
enum class AccessLevel { kInternal, kPublic };

class Buzz {
 public:
  explicit Buzz(AccessLevel access) { … }
  ...
};

class Buzzer {
 public:
  virtual ~Buzzer() {}
  virtual std::unique_ptr<Buzz> MakeBuzz(const std::string& text) = 0;
  virtual bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) = 0;
  ...
};
```

A `Buzz` object represents a snippet being posted.  A class that implements the `Buzzer` interface is capable of creating and sharing `Buzz`.  Methods in `Buzzer` may return a `unique_ptr<Buzz>` or take a `unique_ptr<Buzz>`.  Now we need to mock `Buzzer` in our tests.

To mock a method that returns a move-only type, you just use the familiar `MOCK_METHOD` syntax as usual:

```
class MockBuzzer : public Buzzer {
 public:
  MOCK_METHOD1(MakeBuzz, std::unique_ptr<Buzz>(const std::string& text));
  …
};
```

However, if you attempt to use the same `MOCK_METHOD` pattern to mock a method that takes a move-only parameter, you’ll get a compiler error currently:

```
  // Does NOT compile!
  MOCK_METHOD2(ShareBuzz, bool(std::unique_ptr<Buzz> buzz, Time timestamp));
```

While it’s highly desirable to make this syntax just work, it’s not trivial and the work hasn’t been done yet.  Fortunately, there is a trick you can apply today to get something that works nearly as well as this.

The trick, is to delegate the `ShareBuzz()` method to a mock method (let’s call it `DoShareBuzz()`) that does not take move-only parameters:

```
class MockBuzzer : public Buzzer {
 public:
  MOCK_METHOD1(MakeBuzz, std::unique_ptr<Buzz>(const std::string& text));
  MOCK_METHOD2(DoShareBuzz, bool(Buzz* buzz, Time timestamp));
  bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) {
    return DoShareBuzz(buzz.get(), timestamp);
  }
};
```

Note that there's no need to define or declare `DoShareBuzz()` in a base class.  You only need to define it as a `MOCK_METHOD` in the mock class.

Now that we have the mock class defined, we can use it in tests.  In the following code examples, we assume that we have defined a `MockBuzzer` object named `mock_buzzer_`:

```
  MockBuzzer mock_buzzer_;
```

First let’s see how we can set expectations on the `MakeBuzz()` method, which returns a `unique_ptr<Buzz>`.

As usual, if you set an expectation without an action (i.e. the `.WillOnce()` or `.WillRepeated()` clause), when that expectation fires, the default action for that method will be taken.  Since `unique_ptr<>` has a default constructor that returns a null `unique_ptr`, that’s what you’ll get if you don’t specify an action:

```
  // Use the default action.
  EXPECT_CALL(mock_buzzer_, MakeBuzz("hello"));

  // Triggers the previous EXPECT_CALL.
  EXPECT_EQ(nullptr, mock_buzzer_.MakeBuzz("hello"));
```

If you are not happy with the default action, you can tweak it.  Depending on what you need, you may either tweak the default action for a specific (mock object, mock method) combination using `ON_CALL()`, or you may tweak the default action for all mock methods that return a specific type.  The usage of `ON_CALL()` is similar to `EXPECT_CALL()`, so we’ll skip it and just explain how to do the latter (tweaking the default action for a specific return type).  You do this via the `DefaultValue<>::SetFactory()` and `DefaultValue<>::Clear()` API:

```
  // Sets the default action for return type std::unique_ptr<Buzz> to
  // creating a new Buzz every time.
  DefaultValue<std::unique_ptr<Buzz>>::SetFactory(
      [] { return MakeUnique<Buzz>(AccessLevel::kInternal); });

  // When this fires, the default action of MakeBuzz() will run, which
  // will return a new Buzz object.
  EXPECT_CALL(mock_buzzer_, MakeBuzz("hello")).Times(AnyNumber());

  auto buzz1 = mock_buzzer_.MakeBuzz("hello");
  auto buzz2 = mock_buzzer_.MakeBuzz("hello");
  EXPECT_NE(nullptr, buzz1);
  EXPECT_NE(nullptr, buzz2);
  EXPECT_NE(buzz1, buzz2);

  // Resets the default action for return type std::unique_ptr<Buzz>,
  // to avoid interfere with other tests.
  DefaultValue<std::unique_ptr<Buzz>>::Clear();
```

What if you want the method to do something other than the default action?  If you just need to return a pre-defined move-only value, you can use the `Return(ByMove(...))` action:

```
  // When this fires, the unique_ptr<> specified by ByMove(...) will
  // be returned.
  EXPECT_CALL(mock_buzzer_, MakeBuzz("world"))
      .WillOnce(Return(ByMove(MakeUnique<Buzz>(AccessLevel::kInternal))));

  EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("world"));
```

Note that `ByMove()` is essential here - if you drop it, the code won’t compile.

Quiz time!  What do you think will happen if a `Return(ByMove(...))` action is performed more than once (e.g. you write `….WillRepeatedly(Return(ByMove(...)));`)?  Come think of it, after the first time the action runs, the source value will be consumed (since it’s a move-only value), so the next time around, there’s no value to move from -- you’ll get a run-time error that `Return(ByMove(...))` can only be run once.

If you need your mock method to do more than just moving a pre-defined value, remember that you can always use `Invoke()` to call a lambda or a callable object, which can do pretty much anything you want:

```
  EXPECT_CALL(mock_buzzer_, MakeBuzz("x"))
      .WillRepeatedly(Invoke([](const std::string& text) {
        return std::make_unique<Buzz>(AccessLevel::kInternal);
      }));

  EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
  EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
```

Every time this `EXPECT_CALL` fires, a new `unique_ptr<Buzz>` will be created and returned.  You cannot do this with `Return(ByMove(...))`.

Now there’s one topic we haven’t covered: how do you set expectations on `ShareBuzz()`, which takes a move-only-typed parameter?  The answer is you don’t.  Instead, you set expectations on the `DoShareBuzz()` mock method (remember that we defined a `MOCK_METHOD` for `DoShareBuzz()`, not `ShareBuzz()`):

```
  EXPECT_CALL(mock_buzzer_, DoShareBuzz(NotNull(), _));

  // When one calls ShareBuzz() on the MockBuzzer like this, the call is
  // forwarded to DoShareBuzz(), which is mocked.  Therefore this statement
  // will trigger the above EXPECT_CALL.
  mock_buzzer_.ShareBuzz(MakeUnique&lt;Buzz&gt;(AccessLevel::kInternal),
                         ::base::Now());
```

Some of you may have spotted one problem with this approach: the `DoShareBuzz()` mock method differs from the real `ShareBuzz()` method in that it cannot take ownership of the buzz parameter - `ShareBuzz()` will always delete buzz after `DoShareBuzz()` returns.  What if you need to save the buzz object somewhere for later use when `ShareBuzz()` is called?  Indeed, you'd be stuck.

Another problem with the `DoShareBuzz()` we had is that it can surprise people reading or maintaining the test, as one would expect that `DoShareBuzz()` has (logically) the same contract as `ShareBuzz()`.

Fortunately, these problems can be fixed with a bit more code.  Let's try to get it right this time:

```
class MockBuzzer : public Buzzer {
 public:
  MockBuzzer() {
    // Since DoShareBuzz(buzz, time) is supposed to take ownership of
    // buzz, define a default behavior for DoShareBuzz(buzz, time) to
    // delete buzz.
    ON_CALL(*this, DoShareBuzz(_, _))
        .WillByDefault(Invoke([](Buzz* buzz, Time timestamp) {
          delete buzz;
          return true;
        }));
  }

  MOCK_METHOD1(MakeBuzz, std::unique_ptr<Buzz>(const std::string& text));

  // Takes ownership of buzz.
  MOCK_METHOD2(DoShareBuzz, bool(Buzz* buzz, Time timestamp));
  bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) {
    return DoShareBuzz(buzz.release(), timestamp);
  }
};
```

Now, the mock `DoShareBuzz()` method is free to save the buzz argument for later use if this is what you want:

```
  std::unique_ptr<Buzz> intercepted_buzz;
  EXPECT_CALL(mock_buzzer_, DoShareBuzz(NotNull(), _))
      .WillOnce(Invoke([&amp;intercepted_buzz](Buzz* buzz, Time timestamp) {
        // Save buzz in intercepted_buzz for analysis later.
        intercepted_buzz.reset(buzz);
        return false;
      }));

  mock_buzzer_.ShareBuzz(std::make_unique<Buzz>(AccessLevel::kInternal),
                         Now());
  EXPECT_NE(nullptr, intercepted_buzz);
```

Using the tricks covered in this recipe, you are now able to mock methods that take and/or return move-only types.  Put your newly-acquired power to good use - when you design a new API, you can now feel comfortable using `unique_ptrs` as appropriate, without fearing that doing so will compromise your tests.

## Making the Compilation Faster ##

Believe it or not, the _vast majority_ of the time spent on compiling
a mock class is in generating its constructor and destructor, as they
perform non-trivial tasks (e.g. verification of the
expectations). What's more, mock methods with different signatures
have different types and thus their constructors/destructors need to
be generated by the compiler separately. As a result, if you mock many
different types of methods, compiling your mock class can get really
slow.

If you are experiencing slow compilation, you can move the definition
of your mock class' constructor and destructor out of the class body
and into a `.cpp` file. This way, even if you `#include` your mock
class in N files, the compiler only needs to generate its constructor
and destructor once, resulting in a much faster compilation.

Let's illustrate the idea using an example. Here's the definition of a
mock class before applying this recipe:

```
// File mock_foo.h.
...
class MockFoo : public Foo {
 public:
  // Since we don't declare the constructor or the destructor,
  // the compiler will generate them in every translation unit
  // where this mock class is used.

  MOCK_METHOD0(DoThis, int());
  MOCK_METHOD1(DoThat, bool(const char* str));
  ... more mock methods ...
};
```

After the change, it would look like:

```
// File mock_foo.h.
...
class MockFoo : public Foo {
 public:
  // The constructor and destructor are declared, but not defined, here.
  MockFoo();
  virtual ~MockFoo();

  MOCK_METHOD0(DoThis, int());
  MOCK_METHOD1(DoThat, bool(const char* str));
  ... more mock methods ...
};
```
and
```
// File mock_foo.cpp.
#include "path/to/mock_foo.h"

// The definitions may appear trivial, but the functions actually do a
// lot of things through the constructors/destructors of the member
// variables used to implement the mock methods.
MockFoo::MockFoo() {}
MockFoo::~MockFoo() {}
```

## Forcing a Verification ##

When it's being destoyed, your friendly mock object will automatically
verify that all expectations on it have been satisfied, and will
generate [Google Test](http://code.google.com/p/googletest/) failures
if not. This is convenient as it leaves you with one less thing to
worry about. That is, unless you are not sure if your mock object will
be destoyed.

How could it be that your mock object won't eventually be destroyed?
Well, it might be created on the heap and owned by the code you are
testing. Suppose there's a bug in that code and it doesn't delete the
mock object properly - you could end up with a passing test when
there's actually a bug.

Using a heap checker is a good idea and can alleviate the concern, but
its implementation may not be 100% reliable. So, sometimes you do want
to _force_ Google Mock to verify a mock object before it is
(hopefully) destructed. You can do this with
`Mock::VerifyAndClearExpectations(&mock_object)`:

```
TEST(MyServerTest, ProcessesRequest) {
  using ::testing::Mock;

  MockFoo* const foo = new MockFoo;
  EXPECT_CALL(*foo, ...)...;
  // ... other expectations ...

  // server now owns foo.
  MyServer server(foo);
  server.ProcessRequest(...);

  // In case that server's destructor will forget to delete foo,
  // this will verify the expectations anyway.
  Mock::VerifyAndClearExpectations(foo);
}  // server is destroyed when it goes out of scope here.
```

**Tip:** The `Mock::VerifyAndClearExpectations()` function returns a
`bool` to indicate whether the verification was successful (`true` for
yes), so you can wrap that function call inside a `ASSERT_TRUE()` if
there is no point going further when the verification has failed.

## Using Check Points ##

Sometimes you may want to "reset" a mock object at various check
points in your test: at each check point, you verify that all existing
expectations on the mock object have been satisfied, and then you set
some new expectations on it as if it's newly created. This allows you
to work with a mock object in "phases" whose sizes are each
manageable.

One such scenario is that in your test's `SetUp()` function, you may
want to put the object you are testing into a certain state, with the
help from a mock object. Once in the desired state, you want to clear
all expectations on the mock, such that in the `TEST_F` body you can
set fresh expectations on it.

As you may have figured out, the `Mock::VerifyAndClearExpectations()`
function we saw in the previous recipe can help you here. Or, if you
are using `ON_CALL()` to set default actions on the mock object and
want to clear the default actions as well, use
`Mock::VerifyAndClear(&mock_object)` instead. This function does what
`Mock::VerifyAndClearExpectations(&mock_object)` does and returns the
same `bool`, **plus** it clears the `ON_CALL()` statements on
`mock_object` too.

Another trick you can use to achieve the same effect is to put the
expectations in sequences and insert calls to a dummy "check-point"
function at specific places. Then you can verify that the mock
function calls do happen at the right time. For example, if you are
exercising code:

```
Foo(1);
Foo(2);
Foo(3);
```

and want to verify that `Foo(1)` and `Foo(3)` both invoke
`mock.Bar("a")`, but `Foo(2)` doesn't invoke anything. You can write:

```
using ::testing::MockFunction;

TEST(FooTest, InvokesBarCorrectly) {
  MyMock mock;
  // Class MockFunction<F> has exactly one mock method.  It is named
  // Call() and has type F.
  MockFunction<void(string check_point_name)> check;
  {
    InSequence s;

    EXPECT_CALL(mock, Bar("a"));
    EXPECT_CALL(check, Call("1"));
    EXPECT_CALL(check, Call("2"));
    EXPECT_CALL(mock, Bar("a"));
  }
  Foo(1);
  check.Call("1");
  Foo(2);
  check.Call("2");
  Foo(3);
}
```

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 together:

  * 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!

## Gaining Super Vision into Mock Calls ##

You have a test using Google Mock. It fails: Google Mock tells you
that some expectations aren't satisfied. However, you aren't sure why:
Is there a typo somewhere in the matchers? Did you mess up the order
of the `EXPECT_CALL`s? Or is the code under test doing something
wrong?  How can you find out the cause?

Won't it be nice if you have X-ray vision and can actually see the
trace of all `EXPECT_CALL`s and mock method calls as they are made?
For each call, would you like to see its actual argument values and
which `EXPECT_CALL` Google Mock thinks it matches?

You can unlock this power by running your test with the
`--gmock_verbose=info` flag. For example, given the test program:

```
using testing::_;
using testing::HasSubstr;
using testing::Return;

class MockFoo {
 public:
  MOCK_METHOD2(F, void(const string& x, const string& y));
};

TEST(Foo, Bar) {
  MockFoo mock;
  EXPECT_CALL(mock, F(_, _)).WillRepeatedly(Return());
  EXPECT_CALL(mock, F("a", "b"));
  EXPECT_CALL(mock, F("c", HasSubstr("d")));

  mock.F("a", "good");
  mock.F("a", "b");
}
```

if you run it with `--gmock_verbose=info`, you will see this output:

```
[ RUN      ] Foo.Bar

foo_test.cc:14: EXPECT_CALL(mock, F(_, _)) invoked
foo_test.cc:15: EXPECT_CALL(mock, F("a", "b")) invoked
foo_test.cc:16: EXPECT_CALL(mock, F("c", HasSubstr("d"))) invoked
foo_test.cc:14: Mock function call matches EXPECT_CALL(mock, F(_, _))...
    Function call: F(@0x7fff7c8dad40"a", @0x7fff7c8dad10"good")
foo_test.cc:15: Mock function call matches EXPECT_CALL(mock, F("a", "b"))...
    Function call: F(@0x7fff7c8dada0"a", @0x7fff7c8dad70"b")
foo_test.cc:16: Failure
Actual function call count doesn't match EXPECT_CALL(mock, F("c", HasSubstr("d")))...
         Expected: to be called once
           Actual: never called - unsatisfied and active
[  FAILED  ] Foo.Bar
```

Suppose the bug is that the `"c"` in the third `EXPECT_CALL` is a typo
and should actually be `"a"`. With the above message, you should see
that the actual `F("a", "good")` call is matched by the first
`EXPECT_CALL`, not the third as you thought. From that it should be
obvious that the third `EXPECT_CALL` is written wrong. Case solved.

## 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
`~/.emacs` file:

```
(global-set-key "\M-m"   'compile)  ; m is for make
(global-set-key [M-down] 'next-error)
(global-set-key [M-up]   '(lambda () (interactive) (next-error -1)))
```

Then you can type `M-m` to start a build, or `M-up`/`M-down` to move
back and forth between errors.

## Fusing Google Mock Source Files ##

Google Mock's implementation consists of dozens of files (excluding
its own tests).  Sometimes you may want them to be packaged up in
fewer files instead, such that you can easily copy them to a new
machine and start hacking there.  For this we provide an experimental
Python script `fuse_gmock_files.py` in the `scripts/` directory
(starting with release 1.2.0).  Assuming you have Python 2.4 or above
installed on your machine, just go to that directory and run
```
python fuse_gmock_files.py OUTPUT_DIR
```

and you should see an `OUTPUT_DIR` directory being created with files
`gtest/gtest.h`, `gmock/gmock.h`, and `gmock-gtest-all.cc` in it.
These three files contain everything you need to use Google Mock (and
Google Test).  Just copy them to anywhere you want and you are ready
to write tests and use mocks.  You can use the
[scrpts/test/Makefile](http://code.google.com/p/googlemock/source/browse/trunk/scripts/test/Makefile) file as an example on how to compile your tests
against them.

# Extending Google Mock #

## Writing New Matchers Quickly ##

The `MATCHER*` family of macros can be used to define custom matchers
easily.  The syntax:

```
MATCHER(name, description_string_expression) { statements; }
```

will define a matcher with the given name that executes the
statements, which must return a `bool` to indicate if the match
succeeds.  Inside the statements, you can refer to the value being
matched by `arg`, and refer to its type by `arg_type`.

The description string is a `string`-typed expression that documents
what the matcher does, and is used to generate the failure message
when the match fails.  It can (and should) reference the special
`bool` variable `negation`, and should evaluate to the description of
the matcher when `negation` is `false`, or that of the matcher's
negation when `negation` is `true`.

For convenience, we allow the description string to be empty (`""`),
in which case Google Mock will use the sequence of words in the
matcher name as the description.

For example:
```
MATCHER(IsDivisibleBy7, "") { return (arg % 7) == 0; }
```
allows you to write
```
  // Expects mock_foo.Bar(n) to be called where n is divisible by 7.
  EXPECT_CALL(mock_foo, Bar(IsDivisibleBy7()));
```
or,
```
using ::testing::Not;
...
  EXPECT_THAT(some_expression, IsDivisibleBy7());
  EXPECT_THAT(some_other_expression, Not(IsDivisibleBy7()));
```
If the above assertions fail, they will print something like:
```
  Value of: some_expression
  Expected: is divisible by 7
    Actual: 27
...
  Value of: some_other_expression
  Expected: not (is divisible by 7)
    Actual: 21
```
where the descriptions `"is divisible by 7"` and `"not (is divisible
by 7)"` are automatically calculated from the matcher name
`IsDivisibleBy7`.

As you may have noticed, the auto-generated descriptions (especially
those for the negation) may not be so great. You can always override
them with a string expression of your own:
```
MATCHER(IsDivisibleBy7, std::string(negation ? "isn't" : "is") +
                        " divisible by 7") {
  return (arg % 7) == 0;
}
```

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
can use the macro:
```
MATCHER_P(name, param_name, description_string) { statements; }
```
where the description string can be either `""` or a string expression
that references `negation` and `param_name`.

For example:
```
MATCHER_P(HasAbsoluteValue, value, "") { return abs(arg) == value; }
```
will allow you to write:
```
  EXPECT_THAT(Blah("a"), HasAbsoluteValue(n));
```
which may lead to this message (assuming `n` is 10):
```
  Value of: Blah("a")
  Expected: has absolute value 10
    Actual: -9
```

Note that both the matcher description and its parameter are
printed, making the message human-friendly.

In the matcher definition body, you can write `foo_type` to
reference the type of a parameter named `foo`.  For example, in the
body of `MATCHER_P(HasAbsoluteValue, value)` above, you can write
`value_type` to refer to the type of `value`.

Google Mock also provides `MATCHER_P2`, `MATCHER_P3`, ..., up to
`MATCHER_P10` to support multi-parameter matchers:
```
MATCHER_Pk(name, param_1, ..., param_k, description_string) { statements; }
```

Please note that the custom description string is for a particular
**instance** of the matcher, where the parameters have been bound to
actual values.  Therefore usually you'll want the parameter values to
be part of the description.  Google Mock lets you do that by
referencing the matcher parameters in the description string
expression.

For example,
```
  using ::testing::PrintToString;
  MATCHER_P2(InClosedRange, low, hi,
             std::string(negation ? "isn't" : "is") + " in range [" +
             PrintToString(low) + ", " + PrintToString(hi) + "]") {
    return low <= arg && arg <= hi;
  }
  ...
  EXPECT_THAT(3, InClosedRange(4, 6));
```
would generate a failure that contains the message:
```
  Expected: is in range [4, 6]
```

If you specify `""` as the description, the failure message will
contain the sequence of words in the matcher name followed by the
parameter values printed as a tuple.  For example,
```
  MATCHER_P2(InClosedRange, low, hi, "") { ... }
  ...
  EXPECT_THAT(3, InClosedRange(4, 6));
```
would generate a failure that contains the text:
```
  Expected: in closed range (4, 6)
```

For the purpose of typing, you can view
```
MATCHER_Pk(Foo, p1, ..., pk, description_string) { ... }
```
as shorthand for
```
template <typename p1_type, ..., typename pk_type>
FooMatcherPk<p1_type, ..., pk_type>
Foo(p1_type p1, ..., pk_type pk) { ... }
```

When you write `Foo(v1, ..., vk)`, the compiler infers the types of
the parameters `v1`, ..., and `vk` for you.  If you are not happy with
the result of the type inference, you can specify the types by
explicitly instantiating the template, as in `Foo<long, bool>(5, false)`.
As said earlier, you don't get to (or need to) specify
`arg_type` as that's determined by the context in which the matcher
is used.

You can assign the result of expression `Foo(p1, ..., pk)` to a
variable of type `FooMatcherPk<p1_type, ..., pk_type>`.  This can be
useful when composing matchers.  Matchers that don't have a parameter
or have only one parameter have special types: you can assign `Foo()`
to a `FooMatcher`-typed variable, and assign `Foo(p)` to a
`FooMatcherP<p_type>`-typed variable.

While you can instantiate a matcher template with reference types,
passing the parameters by pointer usually makes your code more
readable.  If, however, you still want to pass a parameter by
reference, be aware that in the failure message generated by the
matcher you will see the value of the referenced object but not its
address.

You can overload matchers with different numbers of parameters:
```
MATCHER_P(Blah, a, description_string_1) { ... }
MATCHER_P2(Blah, a, b, description_string_2) { ... }
```

While it's tempting to always use the `MATCHER*` macros when defining
a new matcher, you should also consider implementing
`MatcherInterface` or using `MakePolymorphicMatcher()` instead (see
the recipes that follow), especially if you need to use the matcher a
lot.  While these approaches require more work, they give you more
control on the types of the value being matched and the matcher
parameters, which in general leads to better compiler error messages
that pay off in the long run.  They also allow overloading matchers
based on parameter types (as opposed to just based on the number of
parameters).

## Writing New Monomorphic Matchers ##

A matcher of argument type `T` implements
`::testing::MatcherInterface<T>` and does two things: it tests whether a
value of type `T` matches the matcher, and can describe what kind of
values it matches. The latter ability is used for generating readable
error messages when expectations are violated.

The interface looks like this:

```
class MatchResultListener {
 public:
  ...
  // Streams x to the underlying ostream; does nothing if the ostream
  // is NULL.
  template <typename T>
  MatchResultListener& operator<<(const T& x);

  // Returns the underlying ostream.
  ::std::ostream* stream();
};

template <typename T>
class MatcherInterface {
 public:
  virtual ~MatcherInterface();

  // Returns true iff the matcher matches x; also explains the match
  // result to 'listener'.
  virtual bool MatchAndExplain(T x, MatchResultListener* listener) const = 0;

  // Describes this matcher to an ostream.
  virtual void DescribeTo(::std::ostream* os) const = 0;

  // Describes the negation of this matcher to an ostream.
  virtual void DescribeNegationTo(::std::ostream* os) const;
};
```

If you need a custom matcher but `Truly()` is not a good option (for
example, you may not be happy with the way `Truly(predicate)`
describes itself, or you may want your matcher to be polymorphic as
`Eq(value)` is), you can define a matcher to do whatever you want in
two steps: first implement the matcher interface, and then define a
factory function to create a matcher instance. The second step is not
strictly needed but it makes the syntax of using the matcher nicer.

For example, you can define a matcher to test whether an `int` is
divisible by 7 and then use it like this:
```
using ::testing::MakeMatcher;
using ::testing::Matcher;
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;

class DivisibleBy7Matcher : public MatcherInterface<int> {
 public:
  virtual bool MatchAndExplain(int n, MatchResultListener* listener) const {
    return (n % 7) == 0;
  }

  virtual void DescribeTo(::std::ostream* os) const {
    *os << "is divisible by 7";
  }

  virtual void DescribeNegationTo(::std::ostream* os) const {
    *os << "is not divisible by 7";
  }
};

inline Matcher<int> DivisibleBy7() {
  return MakeMatcher(new DivisibleBy7Matcher);
}
...

  EXPECT_CALL(foo, Bar(DivisibleBy7()));
```

You may improve the matcher message by streaming additional
information to the `listener` argument in `MatchAndExplain()`:

```
class DivisibleBy7Matcher : public MatcherInterface<int> {
 public:
  virtual bool MatchAndExplain(int n,
                               MatchResultListener* listener) const {
    const int remainder = n % 7;
    if (remainder != 0) {
      *listener << "the remainder is " << remainder;
    }
    return remainder == 0;
  }
  ...
};
```

Then, `EXPECT_THAT(x, DivisibleBy7());` may general a message like this:
```
Value of: x
Expected: is divisible by 7
  Actual: 23 (the remainder is 2)
```

## Writing New Polymorphic Matchers ##

You've learned how to write your own matchers in the previous
recipe. Just one problem: a matcher created using `MakeMatcher()` only
works for one particular type of arguments. If you want a
_polymorphic_ matcher that works with arguments of several types (for
instance, `Eq(x)` can be used to match a `value` as long as `value` ==
`x` compiles -- `value` and `x` don't have to share the same type),
you can learn the trick from `"gmock/gmock-matchers.h"` but it's a bit
involved.

Fortunately, most of the time you can define a polymorphic matcher
easily with the help of `MakePolymorphicMatcher()`. Here's how you can
define `NotNull()` as an example:

```
using ::testing::MakePolymorphicMatcher;
using ::testing::MatchResultListener;
using ::testing::NotNull;
using ::testing::PolymorphicMatcher;

class NotNullMatcher {
 public:
  // To implement a polymorphic matcher, first define a COPYABLE class
  // that has three members MatchAndExplain(), DescribeTo(), and
  // DescribeNegationTo(), like the following.

  // In this example, we want to use NotNull() with any pointer, so
  // MatchAndExplain() accepts a pointer of any type as its first argument.
  // In general, you can define MatchAndExplain() as an ordinary method or
  // a method template, or even overload it.
  template <typename T>
  bool MatchAndExplain(T* p,
                       MatchResultListener* /* listener */) const {
    return p != NULL;
  }

  // Describes the property of a value matching this matcher.
  void DescribeTo(::std::ostream* os) const { *os << "is not NULL"; }

  // Describes the property of a value NOT matching this matcher.
  void DescribeNegationTo(::std::ostream* os) const { *os << "is NULL"; }
};

// To construct a polymorphic matcher, pass an instance of the class
// to MakePolymorphicMatcher().  Note the return type.
inline PolymorphicMatcher<NotNullMatcher> NotNull() {
  return MakePolymorphicMatcher(NotNullMatcher());
}
...

  EXPECT_CALL(foo, Bar(NotNull()));  // The argument must be a non-NULL pointer.
```

**Note:** Your polymorphic matcher class does **not** need to inherit from
`MatcherInterface` or any other class, and its methods do **not** need
to be virtual.

Like in a monomorphic matcher, you may explain the match result by
streaming additional information to the `listener` argument in
`MatchAndExplain()`.

## Writing New Cardinalities ##

A cardinality is used in `Times()` to tell Google Mock how many times
you expect a call to occur. It doesn't have to be exact. For example,
you can say `AtLeast(5)` or `Between(2, 4)`.

If the built-in set of cardinalities doesn't suit you, you are free to
define your own by implementing the following interface (in namespace
`testing`):

```
class CardinalityInterface {
 public:
  virtual ~CardinalityInterface();

  // Returns true iff call_count calls will satisfy this cardinality.
  virtual bool IsSatisfiedByCallCount(int call_count) const = 0;

  // Returns true iff call_count calls will saturate this cardinality.
  virtual bool IsSaturatedByCallCount(int call_count) const = 0;

  // Describes self to an ostream.
  virtual void DescribeTo(::std::ostream* os) const = 0;
};
```

For example, to specify that a call must occur even number of times,
you can write

```
using ::testing::Cardinality;
using ::testing::CardinalityInterface;
using ::testing::MakeCardinality;

class EvenNumberCardinality : public CardinalityInterface {
 public:
  virtual bool IsSatisfiedByCallCount(int call_count) const {
    return (call_count % 2) == 0;
  }

  virtual bool IsSaturatedByCallCount(int call_count) const {
    return false;
  }

  virtual void DescribeTo(::std::ostream* os) const {
    *os << "called even number of times";
  }
};

Cardinality EvenNumber() {
  return MakeCardinality(new EvenNumberCardinality);
}
...

  EXPECT_CALL(foo, Bar(3))
      .Times(EvenNumber());
```

## Writing New Actions Quickly ##

If the built-in actions don't work for you, and you find it
inconvenient to use `Invoke()`, you can use a macro from the `ACTION*`
family to quickly define a new action that can be used in your code as
if it's a built-in action.

By writing
```
ACTION(name) { statements; }
```
in a namespace scope (i.e. not inside a class or function), you will
define an action with the given name that executes the statements.
The value returned by `statements` will be used as the return value of
the action.  Inside the statements, you can refer to the K-th
(0-based) argument of the mock function as `argK`.  For example:
```
ACTION(IncrementArg1) { return ++(*arg1); }
```
allows you to write
```
... WillOnce(IncrementArg1());
```

Note that you don't need to specify the types of the mock function
arguments.  Rest assured that your code is type-safe though:
you'll get a compiler error if `*arg1` doesn't support the `++`
operator, or if the type of `++(*arg1)` isn't compatible with the mock
function's return type.

Another example:
```
ACTION(Foo) {
  (*arg2)(5);
  Blah();
  *arg1 = 0;
  return arg0;
}
```
defines an action `Foo()` that invokes argument #2 (a function pointer)
with 5, calls function `Blah()`, sets the value pointed to by argument
#1 to 0, and returns argument #0.

For more convenience and flexibility, you can also use the following
pre-defined symbols in the body of `ACTION`:

| `argK_type` | The type of the K-th (0-based) argument of the mock function |
|:------------|:-------------------------------------------------------------|
| `args`      | All arguments of the mock function as a tuple                |
| `args_type` | The type of all arguments of the mock function as a tuple    |
| `return_type` | The return type of the mock function                         |
| `function_type` | The type of the mock function                                |

For example, when using an `ACTION` as a stub action for mock function:
```
int DoSomething(bool flag, int* ptr);
```
we have:
| **Pre-defined Symbol** | **Is Bound To** |
|:-----------------------|:----------------|
| `arg0`                 | the value of `flag` |
| `arg0_type`            | the type `bool` |
| `arg1`                 | the value of `ptr` |
| `arg1_type`            | the type `int*` |
| `args`                 | the tuple `(flag, ptr)` |
| `args_type`            | the type `::testing::tuple<bool, int*>` |
| `return_type`          | the type `int`  |
| `function_type`        | the type `int(bool, int*)` |

## Writing New Parameterized Actions Quickly ##

Sometimes you'll want to parameterize an action you define.  For that
we have another macro
```
ACTION_P(name, param) { statements; }
```

For example,
```
ACTION_P(Add, n) { return arg0 + n; }
```
will allow you to write
```
// Returns argument #0 + 5.
... WillOnce(Add(5));
```

For convenience, we use the term _arguments_ for the values used to
invoke the mock function, and the term _parameters_ for the values
used to instantiate an action.

Note that you don't need to provide the type of the parameter either.
Suppose the parameter is named `param`, you can also use the
Google-Mock-defined symbol `param_type` to refer to the type of the
parameter as inferred by the compiler.  For example, in the body of
`ACTION_P(Add, n)` above, you can write `n_type` for the type of `n`.

Google Mock also provides `ACTION_P2`, `ACTION_P3`, and etc to support
multi-parameter actions.  For example,
```
ACTION_P2(ReturnDistanceTo, x, y) {
  double dx = arg0 - x;
  double dy = arg1 - y;
  return sqrt(dx*dx + dy*dy);
}
```
lets you write
```
... WillOnce(ReturnDistanceTo(5.0, 26.5));
```

You can view `ACTION` as a degenerated parameterized action where the
number of parameters is 0.

You can also easily define actions overloaded on the number of parameters:
```
ACTION_P(Plus, a) { ... }
ACTION_P2(Plus, a, b) { ... }
```

## Restricting the Type of an Argument or Parameter in an ACTION ##

For maximum brevity and reusability, the `ACTION*` macros don't ask
you to provide the types of the mock function arguments and the action
parameters.  Instead, we let the compiler infer the types for us.

Sometimes, however, we may want to be more explicit about the types.
There are several tricks to do that.  For example:
```
ACTION(Foo) {
  // Makes sure arg0 can be converted to int.
  int n = arg0;
  ... use n instead of arg0 here ...
}

ACTION_P(Bar, param) {
  // Makes sure the type of arg1 is const char*.
  ::testing::StaticAssertTypeEq<const char*, arg1_type>();

  // Makes sure param can be converted to bool.
  bool flag = param;
}
```
where `StaticAssertTypeEq` is a compile-time assertion in Google Test
that verifies two types are the same.

## Writing New Action Templates Quickly ##

Sometimes you want to give an action explicit template parameters that
cannot be inferred from its value parameters.  `ACTION_TEMPLATE()`
supports that and can be viewed as an extension to `ACTION()` and
`ACTION_P*()`.

The syntax:
```
ACTION_TEMPLATE(ActionName,
                HAS_m_TEMPLATE_PARAMS(kind1, name1, ..., kind_m, name_m),
                AND_n_VALUE_PARAMS(p1, ..., p_n)) { statements; }
```

defines an action template that takes _m_ explicit template parameters
and _n_ value parameters, where _m_ is between 1 and 10, and _n_ is
between 0 and 10.  `name_i` is the name of the i-th template
parameter, and `kind_i` specifies whether it's a `typename`, an
integral constant, or a template.  `p_i` is the name of the i-th value
parameter.

Example:
```
// DuplicateArg<k, T>(output) converts the k-th argument of the mock
// function to type T and copies it to *output.
ACTION_TEMPLATE(DuplicateArg,
                // Note the comma between int and k:
                HAS_2_TEMPLATE_PARAMS(int, k, typename, T),
                AND_1_VALUE_PARAMS(output)) {
  *output = T(::testing::get<k>(args));
}
```

To create an instance of an action template, write:
```
  ActionName<t1, ..., t_m>(v1, ..., v_n)
```
where the `t`s are the template arguments and the
`v`s are the value arguments.  The value argument
types are inferred by the compiler.  For example:
```
using ::testing::_;
...
  int n;
  EXPECT_CALL(mock, Foo(_, _))
      .WillOnce(DuplicateArg<1, unsigned char>(&n));
```

If you want to explicitly specify the value argument types, you can
provide additional template arguments:
```
  ActionName<t1, ..., t_m, u1, ..., u_k>(v1, ..., v_n)
```
where `u_i` is the desired type of `v_i`.

`ACTION_TEMPLATE` and `ACTION`/`ACTION_P*` can be overloaded on the
number of value parameters, but not on the number of template
parameters.  Without the restriction, the meaning of the following is
unclear:

```
  OverloadedAction<int, bool>(x);
```

Are we using a single-template-parameter action where `bool` refers to
the type of `x`, or a two-template-parameter action where the compiler
is asked to infer the type of `x`?

## Using the ACTION Object's Type ##

If you are writing a function that returns an `ACTION` object, you'll
need to know its type.  The type depends on the macro used to define
the action and the parameter types.  The rule is relatively simple:
| **Given Definition** | **Expression** | **Has Type** |
|:---------------------|:---------------|:-------------|
| `ACTION(Foo)`        | `Foo()`        | `FooAction`  |
| `ACTION_TEMPLATE(Foo, HAS_m_TEMPLATE_PARAMS(...), AND_0_VALUE_PARAMS())` |	`Foo<t1, ..., t_m>()` | `FooAction<t1, ..., t_m>` |
| `ACTION_P(Bar, param)` | `Bar(int_value)` | `BarActionP<int>` |
| `ACTION_TEMPLATE(Bar, HAS_m_TEMPLATE_PARAMS(...), AND_1_VALUE_PARAMS(p1))` | `Bar<t1, ..., t_m>(int_value)` | `FooActionP<t1, ..., t_m, int>` |
| `ACTION_P2(Baz, p1, p2)` | `Baz(bool_value, int_value)` | `BazActionP2<bool, int>` |
| `ACTION_TEMPLATE(Baz, HAS_m_TEMPLATE_PARAMS(...), AND_2_VALUE_PARAMS(p1, p2))` | `Baz<t1, ..., t_m>(bool_value, int_value)` | `FooActionP2<t1, ..., t_m, bool, int>` |
| ...                  | ...            | ...          |

Note that we have to pick different suffixes (`Action`, `ActionP`,
`ActionP2`, and etc) for actions with different numbers of value
parameters, or the action definitions cannot be overloaded on the
number of them.

## Writing New Monomorphic Actions ##

While the `ACTION*` macros are very convenient, sometimes they are
inappropriate.  For example, despite the tricks shown in the previous
recipes, they don't let you directly specify the types of the mock
function arguments and the action parameters, which in general leads
to unoptimized compiler error messages that can baffle unfamiliar
users.  They also don't allow overloading actions based on parameter
types without jumping through some hoops.

An alternative to the `ACTION*` macros is to implement
`::testing::ActionInterface<F>`, where `F` is the type of the mock
function in which the action will be used. For example:

```
template <typename F>class ActionInterface {
 public:
  virtual ~ActionInterface();

  // Performs the action.  Result is the return type of function type
  // F, and ArgumentTuple is the tuple of arguments of F.
  //
  // For example, if F is int(bool, const string&), then Result would
  // be int, and ArgumentTuple would be ::testing::tuple<bool, const string&>.
  virtual Result Perform(const ArgumentTuple& args) = 0;
};

using ::testing::_;
using ::testing::Action;
using ::testing::ActionInterface;
using ::testing::MakeAction;

typedef int IncrementMethod(int*);

class IncrementArgumentAction : public ActionInterface<IncrementMethod> {
 public:
  virtual int Perform(const ::testing::tuple<int*>& args) {
    int* p = ::testing::get<0>(args);  // Grabs the first argument.
    return *p++;
  }
};

Action<IncrementMethod> IncrementArgument() {
  return MakeAction(new IncrementArgumentAction);
}
...

  EXPECT_CALL(foo, Baz(_))
      .WillOnce(IncrementArgument());

  int n = 5;
  foo.Baz(&n);  // Should return 5 and change n to 6.
```

## Writing New Polymorphic Actions ##

The previous recipe showed you how to define your own action. This is
all good, except that you need to know the type of the function in
which the action will be used. Sometimes that can be a problem. For
example, if you want to use the action in functions with _different_
types (e.g. like `Return()` and `SetArgPointee()`).

If an action can be used in several types of mock functions, we say
it's _polymorphic_. The `MakePolymorphicAction()` function template
makes it easy to define such an action:

```
namespace testing {

template <typename Impl>
PolymorphicAction<Impl> MakePolymorphicAction(const Impl& impl);

}  // namespace testing
```

As an example, let's define an action that returns the second argument
in the mock function's argument list. The first step is to define an
implementation class:

```
class ReturnSecondArgumentAction {
 public:
  template <typename Result, typename ArgumentTuple>
  Result Perform(const ArgumentTuple& args) const {
    // To get the i-th (0-based) argument, use ::testing::get<i>(args).
    return ::testing::get<1>(args);
  }
};
```

This implementation class does _not_ need to inherit from any
particular class. What matters is that it must have a `Perform()`
method template. This method template takes the mock function's
arguments as a tuple in a **single** argument, and returns the result of
the action. It can be either `const` or not, but must be invokable
with exactly one template argument, which is the result type. In other
words, you must be able to call `Perform<R>(args)` where `R` is the
mock function's return type and `args` is its arguments in a tuple.

Next, we use `MakePolymorphicAction()` to turn an instance of the
implementation class into the polymorphic action we need. It will be
convenient to have a wrapper for this:

```
using ::testing::MakePolymorphicAction;
using ::testing::PolymorphicAction;

PolymorphicAction<ReturnSecondArgumentAction> ReturnSecondArgument() {
  return MakePolymorphicAction(ReturnSecondArgumentAction());
}
```

Now, you can use this polymorphic action the same way you use the
built-in ones:

```
using ::testing::_;

class MockFoo : public Foo {
 public:
  MOCK_METHOD2(DoThis, int(bool flag, int n));
  MOCK_METHOD3(DoThat, string(int x, const char* str1, const char* str2));
};
...

  MockFoo foo;
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(ReturnSecondArgument());
  EXPECT_CALL(foo, DoThat(_, _, _))
      .WillOnce(ReturnSecondArgument());
  ...
  foo.DoThis(true, 5);         // Will return 5.
  foo.DoThat(1, "Hi", "Bye");  // Will return "Hi".
```

## Teaching Google Mock How to Print Your Values ##

When an uninteresting or unexpected call occurs, Google Mock prints the
argument values and the stack trace to help you debug.  Assertion
macros like `EXPECT_THAT` and `EXPECT_EQ` also print the values in
question when the assertion fails.  Google Mock and Google Test do this using
Google Test's user-extensible value printer.

This printer knows how to print built-in C++ types, native arrays, STL
containers, and any type that supports the `<<` operator.  For other
types, it prints the raw bytes in the value and hopes that you the
user can figure it out.
[Google Test's advanced guide](http://code.google.com/p/googletest/wiki/AdvancedGuide#Teaching_Google_Test_How_to_Print_Your_Values)
explains how to extend the printer to do a better job at
printing your particular type than to dump the bytes.