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operator==(std::expected) - cppreference.com

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Primary template

template< class T2, class E2 > requires (!std::is_void_v<T2>) friend constexpr bool operator==( const expected& lhs, const std::expected<T2, E2>& rhs );

 (1) (since C++23)

template< class E2 > friend constexpr bool operator==( const expected& lhs, const std::unexpected<E2>& unex );

 (2) (since C++23)

template< class T2 > friend constexpr bool operator==( const expected& lhs, const T2& val );

 (3) (since C++23)

void partial specialization

template< class T2, class E2 > requires std::is_void_v<T2> friend constexpr bool operator==( const expected& lhs, const std::expected<T2, E2>& rhs );

 (4) (since C++23)

template< class E2 > friend constexpr bool operator==( const expected& lhs, const std::unexpected<E2>& unex );

 (5) (since C++23)

Performs comparison operations on std::expected objects.

1) Compares two std::expected objects. The objects compare equal if and only if both lhs and rhs 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:

(until C++26)

This overload participates in overload resolution only if all following expressions is well-formed, and their results are convertible to bool:

(since C++26)
  • *lhs == *rhs
  • lhs.error() == rhs.error()

2) Compares std::expected object with an std::unexpected object. The objects compare equal if and only if lhs contains an unexpected value that is equal to unex.error().

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

(until C++26)

This overload participates in overload resolution only if the expression lhs.error() == unex.error() is well-formed, and its result is convertible to bool.

(since C++26)

3) Compares std::expected object with an expected value. The objects compare equal if and only if lhs contains an expected value that is equal to val.

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

(until C++26)

This overload participates in overload resolution only if all following conditions are satisfied:

  • T2 is not a specialization of std::expected.
  • The expression *lhs == val is well-formed, and its result is convertible to bool.
 (since C++26)

4) Compares two std::expected objects. The objects compare equal if and only if lhs and rhs both represent expected values, or both contain unexpected values that are equal.

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

(until C++26)

This overload participates in overload resolution only if the expression lhs.error() == rhs.error() is well-formed, and its result is convertible to bool.

(since C++26)

5) Compares std::expected object with an std::unexpected object. The objects compare equal if and only if lhs contains an unexpected value that is equal to unex.error().

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

(until C++26)

This overload participates in overload resolution only if the expression lhs.error() == unex.error() is well-formed, and its result is convertible to bool.

(since C++26)

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

lhs, rhs - std::expected object(s) to compare
unex - std::unexpected value to compare to lhs
val - value to compare to the expected value contained in lhs

Return value

1) lhs.has_value() != rhs.has_value() ? false :
(lhs.has_value() ? *lhs == *rhs : lhs.error() == rhs.error())

2) !lhs.has_value() && static_cast<bool>(lhs.error() == unex.error())

3) lhs.has_value() && static_cast<bool>(*lhs == val)

4) lhs.has_value() != rhs.has_value() ? false :
lhs.has_value() || static_cast<bool>(lhs.error() == rhs.error())

5) !lhs.has_value() && static_cast<bool>(lhs.error() == unex.error())

Exceptions

Throws when and what the comparison throws.

Notes

Feature-test macro Value Std Feature
__cpp_lib_constrained_equality 202411L (C++26) constrained comparison operators for std::expected

Example

#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.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';
    
    std::cout << "Overload (3):\n"
              << *e1 << (e1 == x1 ? " == " : " != ") << x1 << '\n'
              << *e1 << (e1 != x2 ? " != " : " == ") << x2 << "\n\n";
}

Output: 

Overload (1):
💚 == 💚
💚 != ❌

Overload (2):
💚 != 13
13 == 13
31 != 13

Overload (3):
💚 == 💚
💚 != ❌

See also