std::identity

 

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unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*)   mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (until C++20*)

 

Defined in header <functional>
struct identity;		(since C++20)

std::identity is a function object type whose operator() returns its argument unchanged.

Member types

Type	Definition
is_transparent	unspecified

Member functions

operator()

returns the argument unchanged (public member function)

std::identity::operator()

template<class T > constexpr T&& operator()( T&& t )constnoexcept;

Returns std::forward<T>(t).

Parameters

t

-

argument to return

Return value

std::forward<T>(t).

Notes

std::identity serves as the default projection in constrained algorithms. Its direct usage is usually not needed.

Example

Run this code

#include <algorithm>#include <functional>#include <iostream>#include <ranges>#include <string>   struct Pair {int n;std::string s;friendstd::ostream& operator<<(std::ostream& os, const Pair& p){return os <<'{'<< p.n<<", "<< p.s<<'}';}};   // A range-printer that can print projected (modified) elements of a range.template<std::ranges::input_range R, typename Projection = std::identity>//<- Notice the default projectionvoid print(std::string_viewconst rem, R&& range, Projection projection ={}){std::cout<< rem <<'{'; std::ranges::for_each( range, [O =0](constauto& o) mutable {std::cout<<(O++?", ":"")<< o;}, projection );std::cout<<"}\n";}   int main(){constauto v ={Pair{1, "one"}, {2, "two"}, {3, "three"}};   print("Print using std::identity as a projection: ", v); print("Project the Pair::n: ", v, &Pair::n); print("Project the Pair::s: ", v, &Pair::s); print("Print using custom closure as a projection: ", v, [](Pair const& p){returnstd::to_string(p.n)+':'+ p.s;});}

Output:

Print using std::identity as a projection: {{1, one}, {2, two}, {3, three}} Project the Pair::n: {1, 2, 3} Project the Pair::s: {one, two, three} Print using custom closure as a projection: {1:one, 2:two, 3:three}

See also

type_identity (C++20)

returns the type argument unchanged (class template)