std::type_identity

< cpp‎ | types

Defined in header `<type_traits>`
template< class T > struct type_identity;		(since C++20)

Provides the member typedef type that names T (i.e., the identity transformation).

The behavior of a program that adds specializations for type_identity is undefined.

Member types

Name	Definition
`type`	`T`

Helper types

template< class T > using type_identity_t = typename type_identity<T>::type;		(since C++20)

Possible implementation

template< class T >
struct type_identity {
    using type = T;
};

Notes

type_identity can be used to establish non-deduced contexts in template argument deduction.

Example

#include <iostream>
#include <type_traits>
 
template <class T>
T foo(T a, T b) {
  return a + b;
}
 
template <class T>
T bar(T a, std::type_identity_t<T> b) {
  return a + b;
}
 
int main() {
  // foo(4.2, 1); // error, deduced conflicting types for 'T'
  std::cout << bar(4.2, 1) << '\n';  // OK, calls bar<double>
}

Output:

5.2