Std::decay (3) Linux Manual Page

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.

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::value,
 typename std::remove_extent::type*,
 typename std::conditional<
 std::is_function::value,
 typename std::add_pointer::type,
 typename std::remove_cv::type
 >::type
 >::type type;
 };

Example

// Run this code

#include <iostream>

#include <type_traits>
template <typename T, typename U>

struct decay_equiv : std::is_same<typename std::decay<T>::type, U>::type

{
};
int main()

{
    std::cout << std::boolalpha
              << decay_equiv<int, int>::value << '\n'
              << decay_equiv<int &, int>::value << '\n'
              << decay_equiv<int &&, int>::value << '\n'
              << decay_equiv<const int &, int>::value << '\n'
              << decay_equiv<int[2], int *>::value << '\n'
              << decay_equiv<int(int), int (*)(int)>::value << '\n';
}

Output:

  true
  true
  true
  true
  true
  true

std::decay (3) Linux Manual Page