Difference between revisions of "cpp/utility/functional/identity"
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< cpp | utility | functional
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{{tt|std::identity}} is a function object type whose {{c|operator()}} returns its argument unchanged. | {{tt|std::identity}} is a function object type whose {{c|operator()}} returns its argument unchanged. | ||
| − | === | + | ===Member types=== |
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc hitem| | + | {{dsc hitem|Type|Definition}} |
{{dsc|{{tt|is_transparent}}|[[cpp/utility/functional#Transparent function objects|unspecified]]}} | {{dsc|{{tt|is_transparent}}|[[cpp/utility/functional#Transparent function objects|unspecified]]}} | ||
{{dsc end}} | {{dsc end}} | ||
| Line 45: | Line 45: | ||
#include <ranges> | #include <ranges> | ||
#include <string> | #include <string> | ||
| − | |||
struct Pair | struct Pair | ||
| Line 53: | Line 52: | ||
friend std::ostream& operator<<(std::ostream& os, const Pair& p) | friend std::ostream& operator<<(std::ostream& os, const Pair& p) | ||
{ | { | ||
| − | return os << | + | return os << '{' << p.n << ", " << p.s << '}'; |
} | } | ||
}; | }; | ||
| Line 60: | Line 59: | ||
template<std::ranges::input_range R, | template<std::ranges::input_range R, | ||
typename Projection = std::identity> //<- Notice the default projection | typename Projection = std::identity> //<- Notice the default projection | ||
| − | void print(std::string_view const rem, R&& | + | void print(std::string_view const rem, R&& range, Projection projection = {}) |
{ | { | ||
| − | std::cout << rem << | + | std::cout << rem << '{'; |
| − | std::ranges::for_each( | + | std::ranges::for_each( |
| + | range, | ||
| + | [O = 0](const auto& o) mutable { std::cout << (O++ ? ", " : "") << o; }, | ||
| + | projection | ||
| + | ); | ||
std::cout << "}\n"; | std::cout << "}\n"; | ||
} | } | ||
| Line 69: | Line 72: | ||
int main() | int main() | ||
{ | { | ||
| − | const | + | const auto v = {Pair{1, "one"}, {2, "two"}, {3, "three"}<!---->}; |
print("Print using std::identity as a projection: ", v); | print("Print using std::identity as a projection: ", v); | ||
| Line 78: | Line 81: | ||
} | } | ||
|output= | |output= | ||
| − | Print using std::identity as a projection: { { 1, one } { 2, two } { 3, three } } | + | Print using std::identity as a projection: {<!---->{1, one}, {2, two}, {3, three}<!---->} |
| − | Project the Pair::n: { 1 2 3 } | + | Project the Pair::n: {1, 2, 3} |
| − | Project the Pair::s: { one two three } | + | Project the Pair::s: {one, two, three} |
| − | Print using custom closure as a projection: { 1:one 2:two 3:three } | + | Print using custom closure as a projection: {1:one, 2:two, 3:three} |
}} | }} | ||
Latest revision as of 20:39, 13 July 2024
| Defined in header <functional>
|
||
| struct identity; |
(since C++20) | |
std::identity is a function object type whose operator() returns its argument unchanged.
Contents |
[edit] Member types
| Type | Definition |
is_transparent
|
unspecified |
[edit] 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).
[edit] Notes
std::identity serves as the default projection in constrained algorithms. Its direct usage is usually not needed.
[edit] 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}[edit] See also
| (C++20) |
returns the type argument unchanged (class template) |
