CodeHealth: Enabling clang-tidy google-explicit-constructor (#3250)

* Adding google-explicit-constructor to .clang-tidy

* clang-tidy explicit attr.h (all automatic)

* clang-tidy explicit cast.h (all automatic)

* clang-tidy detail/init.h (1 NOLINT)

* clang-tidy detail/type_caster_base.h (2 NOLINT)

* clang-tidy pybind11.h (7 NOLINT)

* clang-tidy detail/common.h (3 NOLINT)

* clang-tidy detail/descr.h (2 NOLINT)

* clang-tidy pytypes.h (23 NOLINT, only 1 explicit)

* clang-tidy eigen.h (7 NOLINT, 0 explicit)

* Adding 2 explicit in functional.h

* Adding 4 explicit in iostream.h

* clang-tidy numpy.h (1 NOLINT, 1 explicit)

* clang-tidy embed.h (0 NOLINT, 1 explicit)

* clang-tidy tests/local_bindings.h (0 NOLINT, 4 explicit)

* clang-tidy tests/pybind11_cross_module_tests.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/pybind11_tests.h (0 NOLINT, 2 explicit)

* clang-tidy tests/test_buffers.cpp (0 NOLINT, 2 explicit)

* clang-tidy tests/test_builtin_casters.cpp (0 NOLINT, 4 explicit)

* clang-tidy tests/test_class.cpp (0 NOLINT, 6 explicit)

* clang-tidy tests/test_copy_move.cpp (0 NOLINT, 7 explicit)

* clang-tidy tests/test_embed/external_module.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/test_embed/test_interpreter.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/object.h (0 NOLINT, 2 explicit)

* clang-tidy batch of fully automatic fixes.

* Workaround for MSVC 19.16.27045.0 C++17 Python 2 C++ syntax error.

tests/test_class.cpp

@@ -27,7 +27,7 @@
 
 // test_brace_initialization
 struct NoBraceInitialization {
-    NoBraceInitialization(std::vector<int> v) : vec{std::move(v)} {}
+    explicit NoBraceInitialization(std::vector<int> v) : vec{std::move(v)} {}
     template <typename T>
     NoBraceInitialization(std::initializer_list<T> l) : vec(l) {}
 
@@ -65,18 +65,18 @@ TEST_SUBMODULE(class_, m) {
 
     class Dog : public Pet {
     public:
-        Dog(const std::string &name) : Pet(name, "dog") {}
+        explicit Dog(const std::string &name) : Pet(name, "dog") {}
         std::string bark() const { return "Woof!"; }
     };
 
     class Rabbit : public Pet {
     public:
-        Rabbit(const std::string &name) : Pet(name, "parrot") {}
+        explicit Rabbit(const std::string &name) : Pet(name, "parrot") {}
     };
 
     class Hamster : public Pet {
     public:
-        Hamster(const std::string &name) : Pet(name, "rodent") {}
+        explicit Hamster(const std::string &name) : Pet(name, "rodent") {}
     };
 
     class Chimera : public Pet {
@@ -208,7 +208,7 @@ TEST_SUBMODULE(class_, m) {
     struct ConvertibleFromUserType {
         int i;
 
-        ConvertibleFromUserType(UserType u) : i(u.value()) { }
+        explicit ConvertibleFromUserType(UserType u) : i(u.value()) {}
     };
 
     py::class_<ConvertibleFromUserType>(m, "AcceptsUserType")
@@ -263,7 +263,7 @@ TEST_SUBMODULE(class_, m) {
     };
     struct PyAliasedHasOpNewDelSize : AliasedHasOpNewDelSize {
         PyAliasedHasOpNewDelSize() = default;
-        PyAliasedHasOpNewDelSize(int) { }
+        explicit PyAliasedHasOpNewDelSize(int) {}
         std::uint64_t j;
     };
     struct HasOpNewDelBoth {

tests/test_copy_move.cpp

@@ -37,7 +37,7 @@ template <> lacking_move_ctor empty<lacking_move_ctor>::instance_ = {};
 class MoveOnlyInt {
 public:
     MoveOnlyInt() { print_default_created(this); }
-    MoveOnlyInt(int v) : value{v} { print_created(this, value); }
+    explicit MoveOnlyInt(int v) : value{v} { print_created(this, value); }
     MoveOnlyInt(MoveOnlyInt &&m) noexcept {
         print_move_created(this, m.value);
         std::swap(value, m.value);
@@ -56,7 +56,7 @@ public:
 class MoveOrCopyInt {
 public:
     MoveOrCopyInt() { print_default_created(this); }
-    MoveOrCopyInt(int v) : value{v} { print_created(this, value); }
+    explicit MoveOrCopyInt(int v) : value{v} { print_created(this, value); }
     MoveOrCopyInt(MoveOrCopyInt &&m) noexcept {
         print_move_created(this, m.value);
         std::swap(value, m.value);
@@ -75,7 +75,7 @@ public:
 class CopyOnlyInt {
 public:
     CopyOnlyInt() { print_default_created(this); }
-    CopyOnlyInt(int v) : value{v} { print_created(this, value); }
+    explicit CopyOnlyInt(int v) : value{v} { print_created(this, value); }
     CopyOnlyInt(const CopyOnlyInt &c) { print_copy_created(this, c.value); value = c.value; }
     CopyOnlyInt &operator=(const CopyOnlyInt &c) { print_copy_assigned(this, c.value); value = c.value; return *this; }
     ~CopyOnlyInt() { print_destroyed(this); }
@@ -107,8 +107,8 @@ public:
         if (!src) return none().release();
         return cast(*src, policy, parent);
     }
-    operator CopyOnlyInt*() { return &value; }
-    operator CopyOnlyInt&() { return value; }
+    explicit operator CopyOnlyInt *() { return &value; }
+    explicit operator CopyOnlyInt &() { return value; }
     template <typename T> using cast_op_type = pybind11::detail::cast_op_type<T>;
 };
 PYBIND11_NAMESPACE_END(detail)
@@ -219,7 +219,7 @@ TEST_SUBMODULE(copy_move_policies, m) {
     // #389: rvp::move should fall-through to copy on non-movable objects
     struct MoveIssue1 {
         int v;
-        MoveIssue1(int v) : v{v} {}
+        explicit MoveIssue1(int v) : v{v} {}
         MoveIssue1(const MoveIssue1 &c) = default;
         MoveIssue1(MoveIssue1 &&) = delete;
     };
@@ -227,7 +227,7 @@ TEST_SUBMODULE(copy_move_policies, m) {
 
     struct MoveIssue2 {
         int v;
-        MoveIssue2(int v) : v{v} {}
+        explicit MoveIssue2(int v) : v{v} {}
         MoveIssue2(MoveIssue2 &&) = default;
     };
     py::class_<MoveIssue2>(m, "MoveIssue2").def(py::init<int>()).def_readwrite("value", &MoveIssue2::v);

tests/test_embed/external_module.cpp

@@ -9,7 +9,7 @@ namespace py = pybind11;
 PYBIND11_MODULE(external_module, m) {
     class A {
     public:
-        A(int value) : v{value} {};
+        explicit A(int value) : v{value} {};
         int v;
     };
 

tests/test_embed/test_interpreter.cpp

@@ -18,7 +18,7 @@ using namespace py::literals;
 
 class Widget {
 public:
-    Widget(std::string message) : message(std::move(message)) {}
+    explicit Widget(std::string message) : message(std::move(message)) {}
     virtual ~Widget() = default;
 
     std::string the_message() const { return message; }

tests/test_exceptions.cpp

@@ -81,7 +81,7 @@ private:
 
 
 struct PythonCallInDestructor {
-    PythonCallInDestructor(const py::dict &d) : d(d) {}
+    explicit PythonCallInDestructor(const py::dict &d) : d(d) {}
     ~PythonCallInDestructor() { d["good"] = true; }
 
     py::dict d;
@@ -90,7 +90,7 @@ struct PythonCallInDestructor {
 
 
 struct PythonAlreadySetInDestructor {
-    PythonAlreadySetInDestructor(const py::str &s) : s(s) {}
+    explicit PythonAlreadySetInDestructor(const py::str &s) : s(s) {}
     ~PythonAlreadySetInDestructor() {
         py::dict foo;
         try {

tests/test_factory_constructors.cpp

@@ -19,8 +19,9 @@
 class TestFactory1 {
     friend class TestFactoryHelper;
     TestFactory1() : value("(empty)") { print_default_created(this); }
-    TestFactory1(int v) : value(std::to_string(v)) { print_created(this, value); }
-    TestFactory1(std::string v) : value(std::move(v)) { print_created(this, value); }
+    explicit TestFactory1(int v) : value(std::to_string(v)) { print_created(this, value); }
+    explicit TestFactory1(std::string v) : value(std::move(v)) { print_created(this, value); }
+
 public:
     std::string value;
     TestFactory1(TestFactory1 &&) = delete;
@@ -33,8 +34,9 @@ public:
 class TestFactory2 {
     friend class TestFactoryHelper;
     TestFactory2() : value("(empty2)") { print_default_created(this); }
-    TestFactory2(int v) : value(std::to_string(v)) { print_created(this, value); }
-    TestFactory2(std::string v) : value(std::move(v)) { print_created(this, value); }
+    explicit TestFactory2(int v) : value(std::to_string(v)) { print_created(this, value); }
+    explicit TestFactory2(std::string v) : value(std::move(v)) { print_created(this, value); }
+
 public:
     TestFactory2(TestFactory2 &&m) noexcept {
         value = std::move(m.value);
@@ -53,9 +55,10 @@ class TestFactory3 {
 protected:
     friend class TestFactoryHelper;
     TestFactory3() : value("(empty3)") { print_default_created(this); }
-    TestFactory3(int v) : value(std::to_string(v)) { print_created(this, value); }
+    explicit TestFactory3(int v) : value(std::to_string(v)) { print_created(this, value); }
+
 public:
-    TestFactory3(std::string v) : value(std::move(v)) { print_created(this, value); }
+    explicit TestFactory3(std::string v) : value(std::move(v)) { print_created(this, value); }
     TestFactory3(TestFactory3 &&m) noexcept {
         value = std::move(m.value);
         print_move_created(this);
@@ -72,13 +75,13 @@ public:
 class TestFactory4 : public TestFactory3 {
 public:
     TestFactory4() : TestFactory3() { print_default_created(this); }
-    TestFactory4(int v) : TestFactory3(v) { print_created(this, v); }
+    explicit TestFactory4(int v) : TestFactory3(v) { print_created(this, v); }
     ~TestFactory4() override { print_destroyed(this); }
 };
 // Another class for an invalid downcast test
 class TestFactory5 : public TestFactory3 {
 public:
-    TestFactory5(int i) : TestFactory3(i) { print_created(this, i); }
+    explicit TestFactory5(int i) : TestFactory3(i) { print_created(this, i); }
     ~TestFactory5() override { print_destroyed(this); }
 };
 
@@ -87,7 +90,7 @@ protected:
     int value;
     bool alias = false;
 public:
-    TestFactory6(int i) : value{i} { print_created(this, i); }
+    explicit TestFactory6(int i) : value{i} { print_created(this, i); }
     TestFactory6(TestFactory6 &&f) noexcept {
         print_move_created(this);
         value = f.value;
@@ -102,11 +105,20 @@ class PyTF6 : public TestFactory6 {
 public:
     // Special constructor that allows the factory to construct a PyTF6 from a TestFactory6 only
     // when an alias is needed:
-    PyTF6(TestFactory6 &&base) : TestFactory6(std::move(base)) { alias = true; print_created(this, "move", value); }
-    PyTF6(int i) : TestFactory6(i) { alias = true; print_created(this, i); }
+    explicit PyTF6(TestFactory6 &&base) : TestFactory6(std::move(base)) {
+        alias = true;
+        print_created(this, "move", value);
+    }
+    explicit PyTF6(int i) : TestFactory6(i) {
+        alias = true;
+        print_created(this, i);
+    }
     PyTF6(PyTF6 &&f) noexcept : TestFactory6(std::move(f)) { print_move_created(this); }
     PyTF6(const PyTF6 &f) : TestFactory6(f) { print_copy_created(this); }
-    PyTF6(std::string s) : TestFactory6((int) s.size()) { alias = true; print_created(this, s); }
+    explicit PyTF6(std::string s) : TestFactory6((int) s.size()) {
+        alias = true;
+        print_created(this, s);
+    }
     ~PyTF6() override { print_destroyed(this); }
     int get() override { PYBIND11_OVERRIDE(int, TestFactory6, get, /*no args*/); }
 };
@@ -116,7 +128,7 @@ protected:
     int value;
     bool alias = false;
 public:
-    TestFactory7(int i) : value{i} { print_created(this, i); }
+    explicit TestFactory7(int i) : value{i} { print_created(this, i); }
     TestFactory7(TestFactory7 &&f) noexcept {
         print_move_created(this);
         value = f.value;
@@ -129,7 +141,10 @@ public:
 };
 class PyTF7 : public TestFactory7 {
 public:
-    PyTF7(int i) : TestFactory7(i) { alias = true; print_created(this, i); }
+    explicit PyTF7(int i) : TestFactory7(i) {
+        alias = true;
+        print_created(this, i);
+    }
     PyTF7(PyTF7 &&f) noexcept : TestFactory7(std::move(f)) { print_move_created(this); }
     PyTF7(const PyTF7 &f) : TestFactory7(f) { print_copy_created(this); }
     ~PyTF7() override { print_destroyed(this); }
@@ -300,7 +315,7 @@ TEST_SUBMODULE(factory_constructors, m) {
     // Class with a custom new operator but *without* a placement new operator (issue #948)
     class NoPlacementNew {
     public:
-        NoPlacementNew(int i) : i(i) { }
+        explicit NoPlacementNew(int i) : i(i) {}
         static void *operator new(std::size_t s) {
             auto *p = ::operator new(s);
             py::print("operator new called, returning", reinterpret_cast<uintptr_t>(p));
@@ -324,8 +339,8 @@ TEST_SUBMODULE(factory_constructors, m) {
     // Class that has verbose operator_new/operator_delete calls
     struct NoisyAlloc {
         NoisyAlloc(const NoisyAlloc &) = default;
-        NoisyAlloc(int i) { py::print(py::str("NoisyAlloc(int {})").format(i)); }
-        NoisyAlloc(double d) { py::print(py::str("NoisyAlloc(double {})").format(d)); }
+        explicit NoisyAlloc(int i) { py::print(py::str("NoisyAlloc(int {})").format(i)); }
+        explicit NoisyAlloc(double d) { py::print(py::str("NoisyAlloc(double {})").format(d)); }
         ~NoisyAlloc() { py::print("~NoisyAlloc()"); }
 
         static void *operator new(size_t s) { py::print("noisy new"); return ::operator new(s); }

tests/test_local_bindings.cpp

@@ -91,7 +91,7 @@ TEST_SUBMODULE(local_bindings, m) {
 
     class Cat : public pets::Pet {
     public:
-        Cat(std::string name) : Pet(std::move(name)) {}
+        explicit Cat(std::string name) : Pet(std::move(name)) {}
     };
     py::class_<pets::Pet>(m, "Pet", py::module_local())
         .def("get_name", &pets::Pet::name);

tests/test_methods_and_attributes.cpp

@@ -19,9 +19,9 @@ using overload_cast_ = pybind11::detail::overload_cast_impl<Args...>;
 class ExampleMandA {
 public:
     ExampleMandA() { print_default_created(this); }
-    ExampleMandA(int value) : value(value) { print_created(this, value); }
+    explicit ExampleMandA(int value) : value(value) { print_created(this, value); }
     ExampleMandA(const ExampleMandA &e) : value(e.value) { print_copy_created(this); }
-    ExampleMandA(std::string&&) {}
+    explicit ExampleMandA(std::string &&) {}
     ExampleMandA(ExampleMandA &&e) noexcept : value(e.value) { print_move_created(this); }
     ~ExampleMandA() { print_destroyed(this); }
 
@@ -124,14 +124,14 @@ class NoneCastTester {
 public:
     int answer = -1;
     NoneCastTester() = default;
-    NoneCastTester(int v) : answer(v) {}
+    explicit NoneCastTester(int v) : answer(v) {}
 };
 
 struct StrIssue {
     int val = -1;
 
     StrIssue() = default;
-    StrIssue(int i) : val{i} {}
+    explicit StrIssue(int i) : val{i} {}
 };
 
 // Issues #854, #910: incompatible function args when member function/pointer is in unregistered base class

tests/test_modules.cpp

@@ -20,7 +20,7 @@ TEST_SUBMODULE(modules, m) {
     // test_reference_internal
     class A {
     public:
-        A(int v) : v(v) { print_created(this, v); }
+        explicit A(int v) : v(v) { print_created(this, v); }
         ~A() { print_destroyed(this); }
         A(const A&) { print_copy_created(this); }
         A& operator=(const A &copy) { print_copy_assigned(this); v = copy.v; return *this; }

tests/test_multiple_inheritance.cpp

@@ -16,7 +16,7 @@ namespace {
 // Many bases for testing that multiple inheritance from many classes (i.e. requiring extra
 // space for holder constructed flags) works.
 template <int N> struct BaseN {
-    BaseN(int i) : i(i) { }
+    explicit BaseN(int i) : i(i) {}
     int i;
 };
 
@@ -47,12 +47,12 @@ int VanillaStaticMix2::static_value = 12;
 
 // test_multiple_inheritance_virtbase
 struct Base1a {
-    Base1a(int i) : i(i) { }
+    explicit Base1a(int i) : i(i) {}
     int foo() const { return i; }
     int i;
 };
 struct Base2a {
-    Base2a(int i) : i(i) { }
+    explicit Base2a(int i) : i(i) {}
     int bar() const { return i; }
     int i;
 };
@@ -77,7 +77,7 @@ TEST_SUBMODULE(multiple_inheritance, m) {
     // test_multiple_inheritance_mix1
     // test_multiple_inheritance_mix2
     struct Base1 {
-        Base1(int i) : i(i) { }
+        explicit Base1(int i) : i(i) {}
         int foo() const { return i; }
         int i;
     };
@@ -86,7 +86,7 @@ TEST_SUBMODULE(multiple_inheritance, m) {
       .def("foo", &Base1::foo);
 
     struct Base2 {
-        Base2(int i) : i(i) { }
+        explicit Base2(int i) : i(i) {}
         int bar() const { return i; }
         int i;
     };

tests/test_numpy_vectorize.cpp

@@ -52,7 +52,7 @@ TEST_SUBMODULE(numpy_vectorize, m) {
     // Passthrough test: references and non-pod types should be automatically passed through (in the
     // function definition below, only `b`, `d`, and `g` are vectorized):
     struct NonPODClass {
-        NonPODClass(int v) : value{v} {}
+        explicit NonPODClass(int v) : value{v} {}
         int value;
     };
     py::class_<NonPODClass>(m, "NonPODClass")
@@ -71,7 +71,7 @@ TEST_SUBMODULE(numpy_vectorize, m) {
 
     // test_method_vectorization
     struct VectorizeTestClass {
-        VectorizeTestClass(int v) : value{v} {};
+        explicit VectorizeTestClass(int v) : value{v} {};
         float method(int x, float y) const { return y + (float) (x + value); }
         int value = 0;
     };

tests/test_pickling.cpp

@@ -67,7 +67,7 @@ TEST_SUBMODULE(pickling, m) {
     // test_roundtrip
     class Pickleable {
     public:
-        Pickleable(const std::string &value) : m_value(value) { }
+        explicit Pickleable(const std::string &value) : m_value(value) { }
         const std::string &value() const { return m_value; }
 
         void setExtra1(int extra1) { m_extra1 = extra1; }
@@ -132,7 +132,7 @@ TEST_SUBMODULE(pickling, m) {
     // test_roundtrip_with_dict
     class PickleableWithDict {
     public:
-        PickleableWithDict(const std::string &value) : value(value) { }
+        explicit PickleableWithDict(const std::string &value) : value(value) { }
 
         std::string value;
         int extra;

tests/test_sequences_and_iterators.cpp

@@ -20,7 +20,7 @@ template<typename T>
 class NonZeroIterator {
     const T* ptr_;
 public:
-    NonZeroIterator(const T* ptr) : ptr_(ptr) {}
+    explicit NonZeroIterator(const T *ptr) : ptr_(ptr) {}
     const T& operator*() const { return *ptr_; }
     NonZeroIterator& operator++() { ++ptr_; return *this; }
 };
@@ -77,9 +77,9 @@ TEST_SUBMODULE(sequences_and_iterators, m) {
     // test_sliceable
     class Sliceable{
     public:
-      Sliceable(int n): size(n) {}
-      int start,stop,step;
-      int size;
+        explicit Sliceable(int n) : size(n) {}
+        int start, stop, step;
+        int size;
     };
     py::class_<Sliceable>(m, "Sliceable")
         .def(py::init<int>())
@@ -96,12 +96,12 @@ TEST_SUBMODULE(sequences_and_iterators, m) {
     // test_sequence
     class Sequence {
     public:
-        Sequence(size_t size) : m_size(size) {
+        explicit Sequence(size_t size) : m_size(size) {
             print_created(this, "of size", m_size);
             m_data = new float[size];
             memset(m_data, 0, sizeof(float) * size);
         }
-        Sequence(const std::vector<float> &value) : m_size(value.size()) {
+        explicit Sequence(const std::vector<float> &value) : m_size(value.size()) {
             print_created(this, "of size", m_size, "from std::vector");
             m_data = new float[m_size];
             memcpy(m_data, &value[0], sizeof(float) * m_size);
@@ -239,7 +239,7 @@ TEST_SUBMODULE(sequences_and_iterators, m) {
     class StringMap {
     public:
         StringMap() = default;
-        StringMap(std::unordered_map<std::string, std::string> init)
+        explicit StringMap(std::unordered_map<std::string, std::string> init)
             : map(std::move(init)) {}
 
         void set(const std::string &key, std::string val) { map[key] = std::move(val); }
@@ -276,7 +276,7 @@ TEST_SUBMODULE(sequences_and_iterators, m) {
     // test_generalized_iterators
     class IntPairs {
     public:
-        IntPairs(std::vector<std::pair<int, int>> data) : data_(std::move(data)) {}
+        explicit IntPairs(std::vector<std::pair<int, int>> data) : data_(std::move(data)) {}
         const std::pair<int, int>* begin() const { return data_.data(); }
     private:
         std::vector<std::pair<int, int>> data_;

tests/test_smart_ptr.cpp

@@ -24,7 +24,7 @@ template <typename T> class huge_unique_ptr {
     std::unique_ptr<T> ptr;
     uint64_t padding[10];
 public:
-    huge_unique_ptr(T *p) : ptr(p) {}
+    explicit huge_unique_ptr(T *p) : ptr(p) {}
     T *get() { return ptr.get(); }
 };
 
@@ -33,7 +33,7 @@ template <typename T>
 class custom_unique_ptr {
     std::unique_ptr<T> impl;
 public:
-    custom_unique_ptr(T* p) : impl(p) { }
+    explicit custom_unique_ptr(T *p) : impl(p) {}
     T* get() const { return impl.get(); }
     T* release_ptr() { return impl.release(); }
 };
@@ -46,7 +46,7 @@ class shared_ptr_with_addressof_operator {
     std::shared_ptr<T> impl;
 public:
     shared_ptr_with_addressof_operator( ) = default;
-    shared_ptr_with_addressof_operator(T* p) : impl(p) { }
+    explicit shared_ptr_with_addressof_operator(T *p) : impl(p) {}
     T* get() const { return impl.get(); }
     T** operator&() { throw std::logic_error("Call of overloaded operator& is not expected"); }
 };
@@ -59,7 +59,7 @@ class unique_ptr_with_addressof_operator {
     std::unique_ptr<T> impl;
 public:
     unique_ptr_with_addressof_operator() = default;
-    unique_ptr_with_addressof_operator(T* p) : impl(p) { }
+    explicit unique_ptr_with_addressof_operator(T *p) : impl(p) {}
     T* get() const { return impl.get(); }
     T* release_ptr() { return impl.release(); }
     T** operator&() { throw std::logic_error("Call of overloaded operator& is not expected"); }
@@ -68,7 +68,7 @@ public:
 // Custom object with builtin reference counting (see 'object.h' for the implementation)
 class MyObject1 : public Object {
 public:
-    MyObject1(int value) : value(value) { print_created(this, toString()); }
+    explicit MyObject1(int value) : value(value) { print_created(this, toString()); }
     std::string toString() const override { return "MyObject1[" + std::to_string(value) + "]"; }
 protected:
     ~MyObject1() override { print_destroyed(this); }
@@ -80,7 +80,7 @@ private:
 class MyObject2 {
 public:
     MyObject2(const MyObject2 &) = default;
-    MyObject2(int value) : value(value) { print_created(this, toString()); }
+    explicit MyObject2(int value) : value(value) { print_created(this, toString()); }
     std::string toString() const { return "MyObject2[" + std::to_string(value) + "]"; }
     virtual ~MyObject2() { print_destroyed(this); }
 private:
@@ -91,7 +91,7 @@ private:
 class MyObject3 : public std::enable_shared_from_this<MyObject3> {
 public:
     MyObject3(const MyObject3 &) = default;
-    MyObject3(int value) : value(value) { print_created(this, toString()); }
+    explicit MyObject3(int value) : value(value) { print_created(this, toString()); }
     std::string toString() const { return "MyObject3[" + std::to_string(value) + "]"; }
     virtual ~MyObject3() { print_destroyed(this); }
 private:
@@ -104,7 +104,7 @@ class MyObject4;
 std::unordered_set<MyObject4 *> myobject4_instances;
 class MyObject4 {
 public:
-    MyObject4(int value) : value{value} {
+    explicit MyObject4(int value) : value{value} {
         print_created(this);
         myobject4_instances.insert(this);
     }
@@ -130,7 +130,7 @@ class MyObject4a;
 std::unordered_set<MyObject4a *> myobject4a_instances;
 class MyObject4a {
 public:
-    MyObject4a(int i) {
+    explicit MyObject4a(int i) {
         value = i;
         print_created(this);
         myobject4a_instances.insert(this);
@@ -153,14 +153,14 @@ protected:
 // Object derived but with public destructor and no Deleter in default holder
 class MyObject4b : public MyObject4a {
 public:
-    MyObject4b(int i) : MyObject4a(i) { print_created(this); }
+    explicit MyObject4b(int i) : MyObject4a(i) { print_created(this); }
     ~MyObject4b() override { print_destroyed(this); }
 };
 
 // test_large_holder
 class MyObject5 { // managed by huge_unique_ptr
 public:
-    MyObject5(int value) : value{value} { print_created(this); }
+    explicit MyObject5(int value) : value{value} { print_created(this); }
     ~MyObject5() { print_destroyed(this); }
     int value;
 };
@@ -222,7 +222,7 @@ struct TypeForHolderWithAddressOf {
 
 // test_move_only_holder_with_addressof_operator
 struct TypeForMoveOnlyHolderWithAddressOf {
-    TypeForMoveOnlyHolderWithAddressOf(int value) : value{value} { print_created(this); }
+    explicit TypeForMoveOnlyHolderWithAddressOf(int value) : value{value} { print_created(this); }
     ~TypeForMoveOnlyHolderWithAddressOf() { print_destroyed(this); }
     std::string toString() const {
         return "MoveOnlyHolderWithAddressOf[" + std::to_string(value) + "]";
@@ -242,7 +242,7 @@ struct ElementBase {
 };
 
 struct ElementA : ElementBase {
-    ElementA(int v) : v(v) { }
+    explicit ElementA(int v) : v(v) {}
     int value() const { return v; }
     int v;
 };
@@ -291,9 +291,9 @@ TEST_SUBMODULE(smart_ptr, m) {
     py::implicitly_convertible<py::int_, MyObject1>();
 
     m.def("make_object_1", []() -> Object * { return new MyObject1(1); });
-    m.def("make_object_2", []() -> ref<Object> { return new MyObject1(2); });
+    m.def("make_object_2", []() -> ref<Object> { return ref<Object>(new MyObject1(2)); });
     m.def("make_myobject1_1", []() -> MyObject1 * { return new MyObject1(4); });
-    m.def("make_myobject1_2", []() -> ref<MyObject1> { return new MyObject1(5); });
+    m.def("make_myobject1_2", []() -> ref<MyObject1> { return ref<MyObject1>(new MyObject1(5)); });
     m.def("print_object_1", [](const Object *obj) { py::print(obj->toString()); });
     m.def("print_object_2", [](ref<Object> obj) { py::print(obj->toString()); });
     m.def("print_object_3", [](const ref<Object> &obj) { py::print(obj->toString()); });
@@ -328,7 +328,7 @@ TEST_SUBMODULE(smart_ptr, m) {
 
     // test_smart_ptr_refcounting
     m.def("test_object1_refcounting", []() {
-        ref<MyObject1> o = new MyObject1(0);
+        auto o = ref<MyObject1>(new MyObject1(0));
         bool good = o->getRefCount() == 1;
         py::object o2 = py::cast(o, py::return_value_policy::reference);
         // always request (partial) ownership for objects with intrusive

tests/test_stl.cpp

@@ -45,7 +45,8 @@ PYBIND11_MAKE_OPAQUE(std::vector<std::string, std::allocator<std::string>>);
 
 /// Issue #528: templated constructor
 struct TplCtorClass {
-    template <typename T> TplCtorClass(const T &) { }
+    template <typename T>
+    explicit TplCtorClass(const T &) {}
     bool operator==(const TplCtorClass &) const { return true; }
 };
 

tests/test_stl_binders.cpp

@@ -18,7 +18,7 @@
 class El {
 public:
     El() = delete;
-    El(int v) : a(v) { }
+    explicit El(int v) : a(v) {}
 
     int a;
 };

tests/test_tagbased_polymorphic.cpp

@@ -37,33 +37,35 @@ struct Animal
 
 struct Dog : Animal
 {
-    Dog(const std::string& _name, Kind _kind = Kind::Dog) : Animal(_name, _kind) {}
+    explicit Dog(const std::string &_name, Kind _kind = Kind::Dog) : Animal(_name, _kind) {}
     std::string bark() const { return name_of_kind(kind) + " " + name + " goes " + sound; }
     std::string sound = "WOOF!";
 };
 
 struct Labrador : Dog
 {
-    Labrador(const std::string& _name, int _excitement = 9001)
+    explicit Labrador(const std::string &_name, int _excitement = 9001)
         : Dog(_name, Kind::Labrador), excitement(_excitement) {}
     int excitement;
 };
 
 struct Chihuahua : Dog
 {
-    Chihuahua(const std::string& _name) : Dog(_name, Kind::Chihuahua) { sound = "iyiyiyiyiyi"; }
+    explicit Chihuahua(const std::string &_name) : Dog(_name, Kind::Chihuahua) {
+        sound = "iyiyiyiyiyi";
+    }
     std::string bark() const { return Dog::bark() + " and runs in circles"; }
 };
 
 struct Cat : Animal
 {
-    Cat(const std::string& _name, Kind _kind = Kind::Cat) : Animal(_name, _kind) {}
+    explicit Cat(const std::string &_name, Kind _kind = Kind::Cat) : Animal(_name, _kind) {}
     std::string purr() const { return "mrowr"; }
 };
 
 struct Panther : Cat
 {
-    Panther(const std::string& _name) : Cat(_name, Kind::Panther) {}
+    explicit Panther(const std::string &_name) : Cat(_name, Kind::Panther) {}
     std::string purr() const { return "mrrrRRRRRR"; }
 };
 

tests/test_virtual_functions.cpp

@@ -15,7 +15,7 @@
 /* This is an example class that we'll want to be able to extend from Python */
 class ExampleVirt  {
 public:
-    ExampleVirt(int state) : state(state) { print_created(this, state); }
+    explicit ExampleVirt(int state) : state(state) { print_created(this, state); }
     ExampleVirt(const ExampleVirt &e) : state(e.state) { print_copy_created(this); }
     ExampleVirt(ExampleVirt &&e) noexcept : state(e.state) {
         print_move_created(this);