operator==, !=, <, <=, >, >=, <=>(std::variant) (original) (raw)
| Defined in header | ||
|---|---|---|
| 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) |
If I < sizeof...(Types) is false, the program is ill-formed.
If I == v.index() is false, the behavior is undefined.
[edit] Parameters
| lhs,rhs | - | variants to compare |
|---|
[edit] 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<=>:
- 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.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
| __cpp_lib_constrained_equality | 202403L | (C++26) | constrained comparison operators for std::variant |
[edit] Example
#include #include #include 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