operator==(std::expected)

 

C++

Compiler support

Freestanding and hosted

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Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

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) 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

 

Primary template
template< class T2, class E2 >     requires (!std::is_void_v<T2>) friend constexpr bool operator==( const expected& x,                                   const std::expected<T2, E2>& y );	(1)	(since C++23)
template< class T2 > friend constexpr bool operator==( const expected& x, const T2& val );	(2)	(since C++23)
template< class E2 > friend constexpr bool operator==( const expected& x,                                   const std::unexpected<E2>& e );	(3)	(since C++23)
void partial specialization
template< class T2, class E2 >   requires std::is_void_v<T2> friend constexpr bool operator==( const expected& x,                                   const std::expected<T2, E2>& y );	(4)	(since C++23)
template< class E2 > friend constexpr bool operator==( const expected& x,                                   const std::unexpected<E2>& e );	(5)	(since C++23)

Performs comparison operations on expected objects.

1) Compares two expected objects. The objects compare equal if and only if both x and y contain expected values that are equal, or both contain unexpected values that are equal.

If any of the following expressions is ill-formed, or its result is not convertible to bool, the program is ill-formed:

• *x == *y
• x.error() == y.error()

2) Compares expected object with an expected value. The objects compare equal if and only if x contains an expected value, and the expected value is equal to val.

If the expression *x == val is not well-formed, or its result is not convertible to bool, the program is ill-formed.

3) Compares expected object with an unexpected value. The objects compare equal if and only if x contains an unexpected value, and the unexpected value is equal to e.error().

If the expression x.error() == e.error() is not well-formed, or its result is not convertible to bool, the program is ill-formed.

4) Compares two expected objects. The objects compare equal if and only if x and y both represent expected values, or both contain unexpected values that are equal.

If the expression x.error() == y.error() is not well-formed, or its result is not convertible to bool, the program is ill-formed.

5) Compares expected object with an unexpected value. The objects compare equal if and only if x contains an unexpected value, and the unexpected value is equal to e.error().

If the expression x.error() == e.error() is not well-formed, or its result is not convertible to bool, the program is ill-formed.

These functions are 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.

The != operator is synthesized from operator==.

Parameters

x, y	-	expected object to compare
val	-	value to compare to the expected value contained in x
e	-	value to compare to the unexpected value contained in x

Return value

1) x.has_value() ? (y.has_value() && *x == *y) : (!y.has_value() && x.error() == y.error())

2) x.has_value() && static_cast<bool>(*x == val)

3) !x.has_value() && static_cast<bool>(x.error() == e.error())

4) x.has_value() ? y.has_value() : (!y.has_value() && x.error() == y.error())

5) !x.has_value() && static_cast<bool>(x.error() == e.error())

Exceptions

Throws when and what the comparison throws.

Example

Run this code

#include <expected>
#include <iostream>
#include <string_view>
using namespace std::string_view_literals;
 
int main()
{
    auto x1{"\N{GREEN HEART}"sv};
    auto x2{"\N{CROSS MARK}"sv};
    std::expected<std::string_view, int> e1{x1}, e2{x1}, e3{x2};
    std::unexpected u1{13};
 
    std::cout << "Overload (1):\n"
              << e1.value() << (e1 == e2 ? " == " : " != ") << *e2 << '\n'
              << e1.value() << (e1 != e3 ? " != " : " == ") << *e3 << "\n\n";
 
    std::cout << "Overload (2):\n"
              << *e1 << (e1 == x1 ? " == " : " != ") << x1 << '\n'
              << *e1 << (e1 != x2 ? " != " : " == ") << x2 << "\n\n";
 
    std::cout << "Overload (3):\n"
              << e1.value() << (e1 == u1 ? " == " : " != ") << u1.error() << '\n';
    e1 = std::unexpected{13};
    std::cout << e1.error() << (e1 == u1 ? " == " : " != ") << u1.error() << '\n';
    e1 = std::unexpected{31};
    std::cout << e1.error() << (e1 != u1 ? " != " : " == ") << u1.error() << '\n';
}

Output:

Overload (1):
💚 == 💚
💚 != ❌
 
Overload (2):
💚 == 💚
💚 != ❌
 
Overload (3):
💚 != 13
13 == 13
31 != 13

See also

unexpected (C++23)

represented as an unexpected value (class template)