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std::decay

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

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 >
using decay_t = typename decay<T>::type;
(since C++14)

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 <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

See also

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

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