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operator==, !=, <, <=, >, >=, <=>(std::variant)

From cppreference.com
< cpp‎ | utility‎ | variant
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::variant
 
Defined in header <variant>
template< class... Types >

constexpr bool operator==( const std::variant<Types...>& lhs,

                           const std::variant<Types...>& rhs );
 (1) (since C++17)
template< class... Types >

constexpr bool operator!=( const std::variant<Types...>& lhs,

                           const std::variant<Types...>& rhs );
 (2) (since C++17)
template< class... Types >

constexpr bool operator<( const std::variant<Types...>& lhs,

                          const std::variant<Types...>& rhs );
 (3) (since C++17)
template< class... Types >

constexpr bool operator>( const std::variant<Types...>& lhs,

                          const std::variant<Types...>& rhs );
 (4) (since C++17)
template< class... Types >

constexpr bool operator<=( const std::variant<Types...>& lhs,

                           const std::variant<Types...>& rhs );
 (5) (since C++17)
template< class... Types >

constexpr bool operator>=( const std::variant<Types...>& lhs,

                           const std::variant<Types...>& rhs );
 (6) (since C++17)
template< class... Types >

constexpr std::common_comparison_category_t
              <std::compare_three_way_result_t<Types>...>
    operator<=>( const std::variant<Types...>& lhs,

                 const std::variant<Types...>& rhs );
 (7) (since C++20)
Helper function template
template< std::size_t I, class... Types >

constexpr const std::variant_alternative_t<I, std::variant<Types...>>&

    GET( const variant<Types...>& v );
 (8) (exposition only*)

Performs comparison operations on std::variant objects.

1-7) Compares two std::variant objects lhs and rhs. The contained values are compared (using the corresponding operator of T) only if both lhs and rhs contain values corresponding to the same index. Otherwise,
  • lhs is considered equal to rhs if, and only if, both lhs and rhs do not contain a value.
  • lhs is considered less than rhs if, and only if, either rhs contains a value and lhs does not, or lhs.index() is less than rhs.index().
1-6) Let @ denote the corresponding comparison operator, for each of these functions:

If, for some values of I, the corresponding expression GET <I>(lhs) @ GET <I>(rhs) 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 for all values of I, the corresponding expression GET <I>(lhs) @ GET <I>(rhs) is well-formed and its result is convertible to bool.

(since C++26)
8) The exposition-only function template GET behaves like std::get(std::variant), except that std::bad_variant_access is never thrown.
If I < sizeof...(Types) is false, the program is ill-formed.
If I == v.index() is false, the behavior is undefined.

Contents

[edit] Parameters

lhs,rhs - variants to compare

[edit] Return value

 Operator  Both operands contains a value
(let I be lhs.index() and J be rhs.index()) 		lhs or rhs is valueless
(let lhs_empty be lhs.valueless_by_exception() and rhs_empty be rhs.valueless_by_exception())
I and J are equal I and J are unequal
== GET <I>(lhs) == GET <I>(rhs) false lhs_empty && rhs_empty
!= GET <I>(lhs) != GET <I>(rhs) true lhs_empty != rhs_empty
< GET <I>(lhs) < GET <I>(rhs) lhs.index() < rhs.index() lhs_empty && !rhs_empty
> GET <I>(lhs) > GET <I>(rhs) lhs.index() > rhs.index() !lhs_empty && rhs_empty
<= GET <I>(lhs) <= GET <I>(rhs) lhs.index() < rhs.index() lhs_empty
>= GET <I>(lhs) >= GET <I>(rhs) lhs.index() > rhs.index() rhs_empty
<=> GET <I>(lhs) <=> GET <I>(rhs) lhs.index() <=> rhs.index() see below

For operator<=>:

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_constrained_equality 202403L (C++26) constrained comparison operators for std::variant

[edit] Example

Run this code
#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::cout << std::boolalpha;
    std::string cmp;
    bool result;
 
    auto print2 = [&cmp, &result](const auto& lhs, const auto& rhs)
    {
        std::cout << lhs << ' ' << cmp << ' ' << rhs << " : " << result << '\n';
    };