std::optional<T>:: swap

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 .

Parameters

Return value

(none)

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.

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");
    };
 
    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);
}

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.'

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also