NAME¶

std::identity - std::identity

Synopsis¶

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

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

Member functions¶

operator() returns the argument unchanged
(public member function)

std::identity::operator()

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

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;
friend std::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 projection
void print(std::string_view const rem, R&& range, Projection projection = {})
{
std::cout << rem << '{';
std::ranges::for_each(
range,
[O = 0](const auto& o) mutable { std::cout << (O++ ? ", " : "") << o; },
projection
);
std::cout << "}\n";
}

int main()
{
const auto 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) { return std::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 returns the type argument unchanged
(C++20) (class template)