NAME

std::decay - std::decay

Synopsis

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

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::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 combines std::remove_cv and std::remove_reference
(C++20) (class template)
implicit conversion array-to-pointer, function-to-pointer, lvalue-to-rvalue
conversions