std::identity
From cppreference.com
| Defined in header <functional>
|
||
struct identity;
|
(since C++20) | |
std::identity is a function object type whose operator() returns its argument unchanged.
Member types
| Type | Definition |
is_transparent
|
unspecified |
Member functions
| 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
(C++20) |
returns the type argument unchanged (class template) |
