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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 in and the other un), the contained value of un is direct-initialized from std::move(*in), followed by destruction of the contained value of in as if by in->T::~T(). After this call, in does not contain a value; un contains a value.
  • If both *this and other contain values, the contained values are exchanged by calling using std::swap; swap(**this, *other). T lvalues must satisfy Swappable.

The program is ill-formed if std::is_move_constructible_v<T> is false.


[edit] Parameters

other - the optional object to exchange the contents with

[edit] Return value


[edit] Exceptions

noexcept specification:  

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)
Fully constexpr

[edit] Example

#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");
    // Swap with only 1 set
    opt1 = "Lorem ipsum dolor sit amet, consectetur tincidunt.";


Before swap:
opt1 contains 'First example text'
opt2 contains '2nd text'
After swap:
opt1 contains '2nd text'
opt2 contains 'First example text'
Before swap:
opt1 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.'
opt2 contains ''
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

[edit] See also

specializes the std::swap algorithm
(function template) [edit]