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struct
identity
;
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(since C++20)
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std::identity
is a function object type whose
operator
(
)
returns its argument unchanged.
Member types
Member functions
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returns the argument unchanged
(public member function)
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std::identity::
operator()
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template
<
class
T
>
constexpr
T
&&
operator
(
)
(
T
&&
t
)
const
noexcept
;
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Returns
std::
forward
<
T
>
(
t
)
.
Parameters
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
#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
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returns the type argument unchanged
(class template)
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