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

Defined in header <variant>.

Declarations

Since C++17

// 1)
template< class... Types >
constexpr bool operator==( const std::variant<Types...>& v,
                           const std::variant<Types...>& w );
// 2)
template< class... Types >
constexpr bool operator!=( const std::variant<Types...>& v,
                           const std::variant<Types...>& w );
// 3)
template< class... Types >
constexpr bool operator<( const std::variant<Types...>& v,
                          const std::variant<Types...>& w );
// 4)
template< class... Types >
constexpr bool operator>( const std::variant<Types...>& v,
                          const std::variant<Types...>& w );
// 5)
template< class... Types >
constexpr bool operator<=( const std::variant<Types...>& v,
                           const std::variant<Types...>& w );
// 6)
template< class... Types >
constexpr bool operator>=( const std::variant<Types...>& v,
                           const std::variant<Types...>& w );

Since C++20

// 7)
template< class... Types >
constexpr std::common_comparison_category_t<
    std::compare_three_way_result_t<Types>...>
    operator<=>( const std::variant<Types...>& v,
                 const std::variant<Types...>& w );

1. 
   

Equality operator for variants:

• If v.index() != w.index(), returns false;
• otherwise if v.valueless_by_exception(), returns true;
• otherwise returns std::get<v.index()>(v) == std::get<v.index()>(w).
  The behavior is undefined (until C++20) / The program is ill-formed (since C++20) if std::get<i>(v) == std::get<i>(w) is not a valid expression returning a type convertible to bool, for any i.

2. 
   

Inequality operator for variants:

• If v.index() != w.index(), returns true;
• otherwise if v.valueless_by_exception(), returns false;
• otherwise returns std::get<v.index()>(v) != std::get<v.index()>(w).
  The behavior is undefined (until C++20) / The program is ill-formed (since C++20) if std::get<i>(v) != std::get<i>(w) is not a valid expression returning a type convertible to bool, for any i.

3. 
   

Less-than operator for variants:

• If w.valueless_by_exception(), returns false;
• otherwise if v.valueless_by_exception(), returns true;
• otherwise if v.index() < w.index(), returns true;
• otherwise if v.index() > w.index(), returns false;
• otherwise returns std::get<v.index()>(v) < std::get<v.index()>(w).
  The behavior is undefined (until C++20) / The program is ill-formed (since C++20) if std::get<i>(v) < std::get<i>(w) is not a valid expression returning a type convertible to bool, for any i.

4. 
   

Greater-than operator for variants:

• If v.valueless_by_exception(), returns false;
• otherwise if w.valueless_by_exception(), returns true;
• otherwise if v.index() > w.index(), returns true;
• otherwise if v.index() < w.index(), returns false;
• otherwise returns std::get<v.index()>(v) > std::get<v.index()>(w).
  The behavior is undefined (until C++20) / The program is ill-formed (since C++20) if std::get<i>(v) > std::get<i>(w) is not a valid expression returning a type convertible to bool, for any i.

5. 
   

Less-equal operator for variants:

• If v.valueless_by_exception(), returns true;
• otherwise if w.valueless_by_exception(), returns false;
• otherwise if v.index() < w.index(), returns true;
• otherwise if v.index() > w.index(), returns false;
  
• otherwise returns std::get<v.index()>(v) <= std::get<v.index()>(w). The behavior is undefined (until C++20) / The program is ill-formed (since C++20) if std::get<i>(v) <= std::get<i>(w) is not a valid expression returning a type convertible to bool, for any i.

6. 
   

Greater-equal operator for variants:

• If w.valueless_by_exception(), returns true;
• otherwise if v.valueless_by_exception(), returns false;
• otherwise if v.index() > w.index(), returns true;
• otherwise if v.index() < w.index(), returns false;
• otherwise std::get<v.index()>(v) >= std::get<v.index()>(w).
  The behavior is undefined (until C++20)The program is ill-formed (since C++20) if std::get<i>(v) >= std::get<i>(w) is not a valid expression returning a type convertible to bool, for any i.

7. 
   

Three-way comparison operator for variants:

• If both v.valueless_by_exception() and w.valueless_by_exception() are true, returns std::strong_ordering::equal;
• otherwise if v.valueless_by_exception() is true, returns std::strong_ordering::less;
• otherwise if w.valueless_by_exception() is true, returns std::strong_ordering::greater;
• otherwise if v.index() != w.index(), returns v.index() <=> w.index();
• otherwise equivalent to std::get<v.index()>(v) <=> std::get<v.index()>(w).

Parameters

v,w - variants to compare

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
}

Result

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