std::optional::swap - cppreference.com (original) (raw)

| 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).

The program is ill-formed unless type T is Swappable and std::is_move_constructible_v<T> is true.

[edit] Parameters

[edit] Return value

(none)

[edit] Exceptions

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.

[edit] Example

#include #include #include   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.

[edit] See also