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operator==, !=, <, <=, >, >=, <=>(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.

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

#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