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 undefined.

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_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¶

// 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