std::swap(std::optional)
From cppreference.com
| Defined in header <optional>
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template< class T >
void swap( std::optional<T>& lhs,
std::optional<T>& rhs ) noexcept(/* see below */);
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(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.
Parameters
| lhs, rhs | - | optional objects whose states to swap
|
Return value
(none)
Exceptions
noexcept specification:
noexcept(noexcept(lhs.swap(rhs)))Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_optional |
202106L |
(C++20) (DR20) |
Fully constexpr
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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)
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
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