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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../utility/variant/operator_cmp.html below:

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

Performs comparison operations on std::variant objects.

Compares two

objects

and

. The contained values are compared (using the corresponding operator of

) only if both

and

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().

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.

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.

If I < sizeof...(Types) is false, the program is ill-formed.

If I == v.index() is false, the behavior is undefined.

For operator<=>:

• If only lhs is valueless, returns std::strong_ordering::less.
• If only rhs is valueless, returns std::strong_ordering::greater.
• If both lhs and rhs are valueless, returns std::strong_ordering::equal.

#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

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