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std::identity(3)	       C++ Standard Libary		std::identity(3)

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 ar-
     gument
	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 algo-
     rithms. 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 pro-
     jection
      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)

http://cppreference.com 	   2024.06.10			std::identity(3)
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