std::type_identity
From cppreference.com
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.
Contents |
[edit]Member types
| Name | Definition |
type | T |
[edit]Helper types
template<class T > using type_identity_t = type_identity<T>::type; | (since C++20) | |
[edit]Possible implementation
template<class T>struct type_identity {using type = T;}; |
[edit] 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 |
[edit]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
[edit]See also
(C++20) | function object that returns its argument unchanged (class) |
