std::type_identity

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries