table of contents
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 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¶
t - 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¶
// Run this code
#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 }
See also¶
type_identity returns the type argument unchanged
(C++20) (class template)
2022.07.31 | http://cppreference.com |