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std::identity

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

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

Member types

Member 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 ) const noexcept;

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

Parameters

argument to return

Return value

std::forward<T>(t).

Notes

The member type is_transparent indicates to the caller that this function object is a transparent function object: it accepts arguments of arbitrary types and uses perfect forwarding, which avoids unnecessary copying and conversion when the function object is used in heterogeneous context, or with rvalue arguments. In particular, template functions such as std::set::find and std::set::lower_bound make use of this member type on their Compare types.

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

Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <vector>
 
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&& r, Projection proj = {})
{
    std::cout << rem << "{ ";
    std::ranges::for_each(r, [](const auto& o) { std::cout << o << ' '; }, proj);
    std::cout << "}\n";
}
 
int main()
{
    const std::vector<Pair> v{ {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 }