std::decay(3) C++ Standard Libary std::decay(3) NAME std::decay - std::decay Synopsis Defined in header <type_traits> template< class T > (since C++11) struct decay; Performs the type conversions equivalent to the ones performed when passing function arguments by value. Formally: * If T is "array of U" or reference to it, the member typedef type is U*. * Otherwise, if T is a function type F or reference to one, the member typedef type is std::add_pointer<F>::type. * Otherwise, the member typedef type is std::remove_cv<std::remove_reference<T>::type>::type. If the program adds specializations for std::decay, the behavior is un- defined. Member types Name Definition type the result of applying the decay type conversions to T Helper types template< class T > (since C++14) using decay_t = typename decay<T>::type; Possible implementation template<class T> struct decay { private: typedef typename std::remove_reference<T>::type U; public: typedef typename std::conditional< std::is_array<U>::value, typename std::add_pointer<typename std::remove_ex- tent<U>::type>::type, typename std::conditional< std::is_function<U>::value, typename std::add_pointer<U>::type, typename std::remove_cv<U>::type >::type >::type type; }; Example // Run this code #include <type_traits> template<typename T, typename U> constexpr bool is_decay_equ = std::is_same_v<std::decay_t<T>, U>; int main() { static_assert ( is_decay_equ<int, int> && ! is_decay_equ<int, float> && is_decay_equ<int&, int> && is_decay_equ<int&&, int> && is_decay_equ<const int&, int> && is_decay_equ<int[2], int*> && ! is_decay_equ<int[4][2], int*> && ! is_decay_equ<int[4][2], int**> && is_decay_equ<int[4][2], int(*)[2]> && is_decay_equ<int(int), int(*)(int)> ); } See also remove_cvref combines std::remove_cv and std::remove_reference (C++20) (class template) implicit conversion array-to-pointer, function-to-pointer, lvalue-to- rvalue conversions http://cppreference.com 2024.06.10 std::decay(3)
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