std::decay

 

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Compile-time rational arithmetic
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integer_sequence (C++14)

 

Defined in header <type_traits>
template< class T > struct decay;		(since C++11)

Applies lvalue-to-rvalue, array-to-pointer, and function-to-pointer implicit conversions to the type T, removes cv-qualifiers, and defines the resulting type as the member typedef type. Formally:

• If T names the type "array of U" or "reference to array of U", the member typedef type is U*.

• Otherwise, if T is a function type F or a reference thereto, 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.

These conversions model the type conversion applied to all function arguments when passed by value.

The behavior of a program that adds specializations for decay is undefined.

Member types

Name	Definition
type	the result of applying the decay type conversions to T

Helper types

template< class T > using decay_t = typename decay<T>::type;		(since C++14)

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::remove_extent<U>::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>
struct decay_equiv : 
    std::is_same<typename std::decay<T>::type, U>::type 
{};
template <typename T, typename U>
constexpr bool is_decay_equ = decay_equiv<T, U>::value;
 
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 (C++20)	combines std::remove_cv and std::remove_reference (class template)
implicit conversion	array-to-pointer, function-to-pointer, lvalue-to-rvalue conversions