std::identity
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Defined in header
<functional>
struct identity;
(since C++20)
std::identity is a function object type whose operator() returns its argument unchanged.
Contents
[edit] Member types
Type
Definition
is_transparent
unspecified
[edit] Member functions
operator()
(public member function)
std::identity::operator()
template< class T >
constexpr T&& operator()( T&& t ) const noexcept;
constexpr T&& operator()( T&& t ) const noexcept;
Returns std::forward <T>(t).
Parameters
t
-
argument to return
Return value
std::forward <T>(t).
[edit] Notes
std::identity serves as the default projection in constrained algorithms. Its direct usage is usually not needed.
[edit] 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}