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

From cppreference.net
C++
Utilities library
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

Parameters

lhs,rhs - variants to compare

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 <=> :

Notes

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

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';
    };
    std::variant<int, std::string> v1, v2;
    std::cout << "operator==\n";
    {
        cmp = "==";
        // by default v1 = 0, v2 = 0;
        result = v1 == v2; // true
        std::visit(print2, v1, v2);
        v1 = v2 = 1;
        result = v1 == v2; // true
        std::visit(print2, v1, v2);
        v2 = 2;
        result = v1 == v2; // false
        std::visit(print2, v1, v2);
        v1 = "A";
        result = v1 == v2; // false: v1.index == 1, v2.index == 0
        std::visit(print2, v1, v2);
        v2 = "B";
        result = v1 == v2; // false
        std::visit(print2, v1, v2);
        v2 = "A";
        result = v1 == v2; // true
        std::visit(print2, v1, v2);
    }
    std::cout << "operator<\n";
    {
        cmp = "<";
        v1 = v2 = 1;
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
        v2 = 2;
        result = v1 < v2; // true
        std::visit(print2, v1, v2);
        v1 = 3;
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
        v1 = "A"; v2 = 1;
        result = v1 < v2; // false: v1.index == 1, v2.index == 0
        std::visit(print2, v1, v2);
        v1 = 1; v2 = "A";
        result = v1 < v2; // true: v1.index == 0, v2.index == 1
        std::visit(print2, v1, v2);
        v1 = v2 = "A";
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
        v2 = "B";
        result = v1 < v2; // true
        std::visit(print2, v1, v2);
        v1 = "C";
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
    }
    {
        std::variant<int, std::string> v1;
        std::variant<std::string, int> v2;
    //  v1 == v2; // Compilation error: no known conversion
    }
    // TODO: C++20 three-way comparison operator <=> for variants
}

Output: 

operator==
0 == 0 : true
1 == 1 : true
1 == 2 : false
A == 2 : false
A == B : false
A == A : true
operator<
1 < 1 : false
1 < 2 : true
3 < 2 : false
A < 1 : false
1 < A : true
A < A : false
A < B : true
C < B : false

See also

(C++17) (C++17) (C++17) (C++17) (C++17) (C++17) (C++20)
compares optional objects
(function template)