std::decay (3) - Linux Manuals

std::decay: std::decay

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.

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 <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