operator==, !=, <, <=, >, >=, <=>(std::variant)
From cppreference.com
| Defined in header <variant>
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| template< class... Types > constexpr bool operator==( const std::variant<Types...>& lhs, |
(1) | (since C++17) |
| template< class... Types > constexpr bool operator!=( const std::variant<Types...>& lhs, |
(2) | (since C++17) |
| template< class... Types > constexpr bool operator<( const std::variant<Types...>& lhs, |
(3) | (since C++17) |
| template< class... Types > constexpr bool operator>( const std::variant<Types...>& lhs, |
(4) | (since C++17) |
| template< class... Types > constexpr bool operator<=( const std::variant<Types...>& lhs, |
(5) | (since C++17) |
| template< class... Types > constexpr bool operator>=( const std::variant<Types...>& lhs, |
(6) | (since C++17) |
| template< class... Types > constexpr std::common_comparison_category_t |
(7) | (since C++20) |
| Helper function template |
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template< std::size_t I, class... Types > constexpr const std::variant_alternative_t<I, std::variant<Types...>>& |
(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:
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If, for some values of I, the corresponding expression |
(until C++26) |
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This overload participates in overload resolution only if for all values of I, the corresponding expression |
(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.
Contents |
[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)
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false | lhs_empty && rhs_empty |
| != | GET <I>(lhs) != GET <I>(rhs)
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true | lhs_empty != rhs_empty |
| < | GET <I>(lhs) < GET <I>(rhs)
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lhs.index() < rhs.index() | lhs_empty && !rhs_empty |
| > | GET <I>(lhs) > GET <I>(rhs)
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lhs.index() > rhs.index() | !lhs_empty && rhs_empty |
| <= | GET <I>(lhs) <= GET <I>(rhs)
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lhs.index() < rhs.index() | lhs_empty |
| >= | GET <I>(lhs) >= GET <I>(rhs)
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lhs.index() > rhs.index() | rhs_empty |
| <=> | GET <I>(lhs) <=> GET <I>(rhs)
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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
Run this code
#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) |
