std::expected<T,E>::swap

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< cpp‎ | utility‎ | expected
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)
(C++20)
Swap and type operations
(C++20)
(C++14)
(C++11)
(C++23)
(C++11)
(C++23)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++11)
(C++17)
(C++17)
(C++23)



std::expected
Primary template
constexpr void swap( expected& other ) noexcept ( /* see below */ ) ;
(1) (since C++23)
void partial specialization
constexpr void swap( expected& other ) noexcept ( /* see below */ ) ;
(2) (since C++23)

Swaps the contents with those of other.

1) The contained values are swapped as follows:
Value of
 has_value()  		Value of other.has_value()
true false
true using std::swap;
swap( val , rhs. val ); 		see below
false other.swap(*this); using std::swap;
swap( unex , rhs. unex );
If has_value() is true and other.has_value() is false, equivalent to:

// Case 1: the move constructions of unexpected values are non-throwing:
// “other.unex” will be restored if the construction of “other.val” fails
if constexpr ( std::is_nothrow_move_constructible_v <E> )
{
E temp(std::move(other. unex ));
std::destroy_at ( std::addressof ( unex ));
try
{
std::construct_at ( std::addressof ( val ), std::move( val )); // may throw
std::destroy_at ( std::addressof ( val ));
std::construct_at ( std::addressof ( unex ), std::move(temp));
}
catch(...)
{
std::construct_at ( std::addressof ( unex ), std::move(temp));
throw;
}
}
// Case 2: the move constructions of expected values are non-throwing:
// “this->val” will be restored if the construction of “this->unex” fails
else
{
T temp(std::move( val ));
std::destroy_at ( std::addressof ( val ));
try
{
std::construct_at ( std::addressof ( unex ), std::move(other. unex )); // may throw
std::destroy_at ( std::addressof ( unex ));
std::construct_at ( std::addressof ( val ), std::move(temp));
}
catch(...)
{
std::construct_at ( std::addressof ( val ), std::move(temp));
throw;
}
}
has_val = false;
rhs. has_val = true;

This overload participates in overload resolution only if all following values are true:
2) The unexpected values are swapped as follows:
Value of
 has_value()  		Value of other.has_value()
true false
true using std::swap;
swap( val , rhs. val ); 		std::construct_at ( std::addressof ( unex ),
std::move(rhs. unex ));
std::destroy_at ( std::addressof ( unex ));
has_val = false;
rhs. has_val = true;
false other.swap(*this); using std::swap;
swap( unex , rhs. unex );
This overload participates in overload resolution only if std::is_swappable_v<E> and std::is_move_constructible_v<E> are both true

Parameters

other - the expected object to exchange the contents with

Exceptions

1)
noexcept specification:  
noexcept (

std::is_nothrow_move_constructible_v <T> && std::is_nothrow_swappable_v <T> &&
std::is_nothrow_move_constructible_v <E> && std::is_nothrow_swappable_v <E>

)
2)
noexcept specification:  
noexcept (

std::is_nothrow_move_constructible_v <E> && std::is_nothrow_swappable_v <E>

)

Example

Run this code
#include <expected>
#include <iostream>
#include <string>
 
using Ex = std::expected<std::string, int>;
 
void show(const Ex& ex1, const Ex& ex2)
{
    for (int i{}; i < 2; ++i)
    {
        std::cout << (i ? "ex2" : "ex1");
        if (const Ex& ex = (i ? ex2 : ex1); ex.has_value())
            std::cout << ".has_value() = " << *ex << '\n';
        else
            std::cout << ".error() = " << ex.error() << '\n';
    }
}
 
int main()
{
    Ex ex1("\N{CAT FACE}");
    Ex ex2{"\N{GREEN HEART}"};
    show(ex1, ex2);
    ex1.swap(ex2);
    std::cout << "ex1.swap(ex2);\n";
    show(ex1, ex2);
    std::cout << '\n';
 
    ex2 = std::unexpected(13);
    show(ex1, ex2);
    std::cout << "ex1.swap(ex2);\n";
    ex1.swap(ex2);
    show(ex1, ex2);
    std::cout << '\n';
 
    ex2 = std::unexpected(19937);
    show(ex1, ex2);
    std::cout << "ex1.swap(ex2);\n";
    ex1.swap(ex2);
    show(ex1, ex2);
}

Output: 

ex1.has_value() = 🐱
ex2.has_value() = 💚
ex1.swap(ex2);
ex1.has_value() = 💚
ex2.has_value() = 🐱
 
ex1.has_value() = 💚
ex2.error() = 13
ex1.swap(ex2);
ex1.error() = 13
ex2.has_value() = 💚
 
ex1.error() = 13
ex2.error() = 19937
ex1.swap(ex2);
ex1.error() = 19937
ex2.error() = 13

See also

(C++23)
 specializes the std::swap algorithm
(function)
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