std::swap(std::optional)
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swap(std::optional)
Defined in header
<optional>
template< class T >
(since C++17) void swap( std::optional <T>& lhs,
(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 |