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std::decay

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

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.

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::add_pointer<typename std::remove_extent<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

#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)>
    );
}

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

std::decay

Member types

Helper types

Possible implementation

Example

See also