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std::decay - cppreference.com

From cppreference.com

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.

If the program adds specializations for std::decay, the behavior 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>;

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)>
);

int main() {}

See also