std:: type_identity

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type_identity
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Compile-time rational arithmetic
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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).

If the program adds specializations for std::type_identity , the behavior is undefined.

Member types

Name Definition
type T

Helper types

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

Possible implementation 

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

Notes

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

Feature-test macro Value Std Feature
__cpp_lib_type_identity 201806L (C++20) std::type_identity

Example

Run this code 
#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

See also

(C++20)
function object that returns its argument unchanged
(class)
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