std::decay

template< class T >
struct decay;

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

Helper types

template< class T >
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::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

#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