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std::swap(std::optional)

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< cpp‎ | utility‎ | optional
 
 
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Defined in header <optional>
template< class T >

void swap( std::optional <T>& lhs,

           std::optional <T>& rhs ) noexcept(/* see below */);
(since C++17)
(constexpr since C++20)

Overloads the std::swap algorithm for std::optional . Exchanges the state of lhs with that of rhs. Effectively calls lhs.swap(rhs).

This overload participates in overload resolution only if std::is_move_constructible_v <T> and std::is_swappable_v <T> are both true.

[edit] Parameters

lhs, rhs - optional objects whose states to swap

[edit] Return value

(none)

[edit] Exceptions

noexcept specification:  
noexcept(noexcept(lhs.swap(rhs)))

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_optional 202106L (C++20)
(DR20)
Fully constexpr

[edit] Example

Run this code
#include <iostream>
#include <optional>
#include <string>
 
int main()
{
 std::optional <std::string > a{"██████"}, b{"▒▒▒▒▒▒"};
 
 auto print = [&](auto const& s)
 {
 std::cout << s << "\t"
 "a = " << a.value_or("(null)") << " "
 "b = " << b.value_or("(null)") << '\n';
 };
 
 print("Initially:");
 std::swap (a, b);
 print("swap(a, b):");
 a.reset();
 print("\n""a.reset():");
 std::swap (a, b);
 print("swap(a, b):");
}

Output:

Initially: a = ██████ b = ▒▒▒▒▒▒
swap(a, b): a = ▒▒▒▒▒▒ b = ██████
 
a.reset(): a = (null) b = ██████
swap(a, b): a = ██████ b = (null)

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2231R1 C++20 swap was not constexpr while the required operations can be constexpr in C++20 made constexpr

[edit] See also

exchanges the contents
(public member function) [edit]

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