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

 

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External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (C++20) C-style bit manipulation (C++26) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::expected

Member functions

expected::expected

expected::~expected

expected::operator=

Observers

expected::operator->expected::operator*

expected::operator boolexpected::has_value

expected::value

expected::error

expected::value_or

expected::error_or

Monadic operations

expected::and_then

expected::or_else

expected::transform

expected::transform_error

Modifiers

expected::emplace

expected::swap

Non-member functions

operator==(std::expected)

swap(std::expected)

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

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(other.unex));
    try
    {
        std::construct_at(std::addressof(other.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(other.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(other.unex));
        std::construct_at(std::addressof(other.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:

• std::is_swappable_v<T>
• std::is_swappable_v<E>
• std::is_move_constructible_v<T> && std::is_move_constructible_v<E>
• std::is_nothrow_move_constructible_v<T> || std::is_nothrow_move_constructible_v<E>

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(rhs.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_view>
 
using Ex = std::expected<std::string, int>;
 
void show(const Ex& ex1, const Ex& ex2, std::string_view term = "\n")
{
    for (int i{}; i != 2; ++i)
    {
        std::cout << (i ? "ex2" : "ex1");
        if (const Ex& ex = (i ? ex2 : ex1); ex.has_value())
            std::cout << ".value() = " << *ex << "  ";
        else
            std::cout << ".error() = " << ex.error() << "  ";
    }
    std::cout << term;
}
 
int main()
{
    Ex ex1("\N{CAT FACE}");
    Ex ex2{"\N{GREEN HEART}"};
    show(ex1, ex2, "after ex1.swap(ex2):\n");
    ex1.swap(ex2);
    show(ex1, ex2, "\n\n");
 
    ex2 = std::unexpected(13);
    show(ex1, ex2, "after ex1.swap(ex2):\n");
    ex1.swap(ex2);
    show(ex1, ex2, "\n\n");
 
    ex2 = std::unexpected(37);
    show(ex1, ex2, "after ex1.swap(ex2):\n");
    ex1.swap(ex2);
    show(ex1, ex2);
}

Output:

ex1.value() = 🐱  ex2.value() = 💚  after ex1.swap(ex2):
ex1.value() = 💚  ex2.value() = 🐱 
 
ex1.value() = 💚  ex2.error() = 13  after ex1.swap(ex2):
ex1.error() = 13  ex2.value() = 💚 
 
ex1.error() = 13  ex2.error() = 37  after ex1.swap(ex2):
ex1.error() = 37  ex2.error() = 13

See also

swap(std::expected) (C++23)

specializes the std::swap algorithm (function)