std::swap(std::optional)
From cppreference.com
Defined in header <optional>
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template< class T > void swap( std::optional<T>& lhs, |
(since C++17) (constexpr since C++20) |
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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.
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[edit] Parameters
lhs, rhs | - | optional objects whose states to swap
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[edit] Return value
(none)
[edit] Exceptions
noexcept specification:
noexcept(noexcept(lhs.swap(rhs)))
[edit] Notes
Feature-test macro | Value | Std | Feature |
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__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 |
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P2231R1 | C++20 | swap was not constexpr while the required operations can be constexpr in C++20
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made constexpr |
[edit] See also
exchanges the contents (public member function) |