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

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< cpp‎ | utility‎ | variant
 
 
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operator==operator!=operator<operator<=operator>operator>=operator<=>
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Defined in header <variant>
template< class... Types >

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

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

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

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

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

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

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

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

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

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

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

                           const std::variant<Types...>& w );
(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...>& v,

                 const std::variant<Types...>& w );
(7) (since C++20)
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).

Contents

[edit] Parameters

v,w - variants to compare

[edit] Return value

The result of the comparison as described above.

[edit] 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

[edit] See also

(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
compares optional objects
(function template) [edit]