std::identity - cppreference.com (original) (raw)
| | | | | ---------------- | | ------------- | | struct identity; | | (since C++20) |
std::identity is a function object type whose operator() returns its argument unchanged.
Contents
[edit] Member types
[edit] Member functions
| | returns the argument unchanged (public member function) | | ---------------------------------------------------------- |
std:🆔:operator()
| template< class T > constexpr T&& operator()( T&& t ) const noexcept; | | | | ---------------------------------------------------------------------- | | |
Returns std::forward<T>(t).
Parameters
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
#include #include #include #include #include  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}