1.. title:: clang-tidy - bugprone-forwarding-reference-overload 2 3bugprone-forwarding-reference-overload 4====================================== 5 6The check looks for perfect forwarding constructors that can hide copy or move 7constructors. If a non const lvalue reference is passed to the constructor, the 8forwarding reference parameter will be a better match than the const reference 9parameter of the copy constructor, so the perfect forwarding constructor will be 10called, which can be confusing. 11For detailed description of this issue see: Scott Meyers, Effective Modern C++, 12Item 26. 13 14Consider the following example: 15 16.. code-block:: c++ 17 18 class Person { 19 public: 20 // C1: perfect forwarding ctor 21 template<typename T> 22 explicit Person(T&& n) {} 23 24 // C2: perfect forwarding ctor with parameter default value 25 template<typename T> 26 explicit Person(T&& n, int x = 1) {} 27 28 // C3: perfect forwarding ctor guarded with enable_if 29 template<typename T, typename X = enable_if_t<is_special<T>, void>> 30 explicit Person(T&& n) {} 31 32 // C4: variadic perfect forwarding ctor guarded with enable_if 33 template<typename... A, 34 enable_if_t<is_constructible_v<tuple<string, int>, A&&...>, int> = 0> 35 explicit Person(A&&... a) {} 36 37 // C5: perfect forwarding ctor guarded with requires expression 38 template<typename T> 39 requires requires { is_special<T>; } 40 explicit Person(T&& n) {} 41 42 // C6: perfect forwarding ctor guarded with concept requirement 43 template<Special T> 44 explicit Person(T&& n) {} 45 46 // (possibly compiler generated) copy ctor 47 Person(const Person& rhs); 48 }; 49 50The check warns for constructors C1 and C2, because those can hide copy and move 51constructors. We suppress warnings if the copy and the move constructors are both 52disabled (deleted or private), because there is nothing the perfect forwarding 53constructor could hide in this case. We also suppress warnings for constructors 54like C3-C6 that are guarded with an ``enable_if`` or a concept, assuming the 55programmer was aware of the possible hiding. 56 57Background 58---------- 59 60For deciding whether a constructor is guarded with enable_if, we consider the 61types of the constructor parameters, the default values of template type parameters 62and the types of non-type template parameters with a default literal value. If any 63part of these types is ``std::enable_if`` or ``std::enable_if_t``, we assume the 64constructor is guarded. 65 66