std::optional<T>::swap
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std::optional
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optional::swap
void swap( optional& other ) noexcept(/* see below */);
(since C++17) (constexpr since C++20)
Swaps the contents with those of other.
- If neither *this nor other contain a value, the function has no effect.
- If only one of *this and other contains a value (let's call this object
inand the otherun), the contained value ofunis direct-initialized from std::move(*in), followed by destruction of the contained value ofinas if by in->T::~T(). After this call,indoes not contain a value;uncontains a value.
- If both *this and other contain values, the contained values are exchanged by calling using std::swap ; swap(**this, *other).
The program is ill-formed unless type T is Swappable and std::is_move_constructible_v <T> is true.
[edit] Parameters
other
-
the
optional object to exchange the contents with
[edit] Return value
(none)
[edit] Exceptions
noexcept specification:
noexcept(std::is_nothrow_move_constructible_v <T> &&
std::is_nothrow_swappable_v <T>)
std::is_nothrow_swappable_v <T>)
In the case of thrown exception, the states of the contained values of *this and other are determined by the exception safety guarantees of swap of type T or T's move constructor, whichever is called. For both *this and other, if the object contained a value, it is left containing a value, and the other way round.
| 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 > opt1("First example text"); std::optional <std::string > opt2("2nd text"); enum Swap { Before, After }; auto print_opts = [&](Swap e) { std::cout << (e == Before ? "Before swap:\n" : "After swap:\n"); std::cout << "opt1 contains '" << opt1.value_or("") << "'\n"; std::cout << "opt2 contains '" << opt2.value_or("") << "'\n"; std::cout << (e == Before ? "---SWAP---\n": "\n"); }; print_opts(Before); opt1.swap(opt2); print_opts(After); // Swap with only 1 set opt1 = "Lorem ipsum dolor sit amet, consectetur tincidunt."; opt2.reset(); print_opts(Before); opt1.swap(opt2); print_opts(After); }
Output:
Before swap: opt1 contains 'First example text' opt2 contains '2nd text' ---SWAP--- After swap: opt1 contains '2nd text' opt2 contains 'First example text' Before swap: opt1 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.' opt2 contains '' ---SWAP--- After swap: opt1 contains '' opt2 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.'
[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 |