swap(std::expected)

 

C++

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Debugging support is_debugger_present (C++26) breakpoint_if_debugging (C++26) breakpoint (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 Date and time Function objects Formatting library (C++20) 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) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

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

 

friend constexpr void swap( expected& lhs, expected& rhs ) noexcept(/*see below*/);		(since C++23)

Overloads the std::swap algorithm for std::expected. Exchanges the state of lhs with that of rhs. Effectively calls lhs.swap(rhs).

This overload participates in overload resolution only if lhs.swap(rhs) is valid.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::expected<T, E> is an associated class of the arguments.

Parameters

lhs, rhs

-

expected objects whose states to swap

Return value

(none)

Exceptions

noexcept specification:  

noexcept(noexcept(lhs.swap(rhs)))

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{DOG FACE}");
    Ex ex2{"\N{BONE}"};
    show(ex1, ex2);
    swap(ex1, ex2);
    std::cout << "swap(ex1, ex2);\n";
    show(ex1, ex2);
    std::cout << '\n';
 
    ex2 = std::unexpected(13);
    show(ex1, ex2);
    swap(ex1, ex2);
    std::cout << "swap(ex1, ex2);\n";
    show(ex1, ex2);
    std::cout << '\n';
 
    ex2 = std::unexpected(19937);
    show(ex1, ex2);
    swap(ex1, ex2);
    std::cout << "swap(ex1, ex2);\n";
    show(ex1, ex2);
    std::cout << '\n';
}

Output:

ex1.has_value() = 🐶
ex2.has_value() = 🦴
swap(ex1, ex2);
ex1.has_value() = 🦴
ex2.has_value() = 🐶
 
ex1.has_value() = 🦴
ex2.error() = 13
swap(ex1, ex2);
ex1.error() = 13
ex2.has_value() = 🦴
 
ex1.error() = 13
ex2.error() = 19937
swap(ex1, ex2);
ex1.error() = 19937
ex2.error() = 13

See also

swap	exchanges the contents (public member function)