operator==, !=, <, <=, >, >=, <=>(std::variant)
Defined in header <variant> | ||
|---|---|---|
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.
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, -
lhsis considered equal torhsif, and only if, bothlhsandrhsdo not contain a value. -
lhsis considered less thanrhsif, and only if, eitherrhscontains a value andlhsdoes not, orlhs.index()is less thanrhs.index().
1-6) Let
@ denote the corresponding comparison operator, for each of these functions: | If, for some values of | (until C++26) |
| This overload participates in overload resolution only if for all values of | (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.Parameters
| lhs,rhs | - | variants to compare |
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
lhsis valueless, returnsstd::strong_ordering::less. - If only
rhsis valueless, returnsstd::strong_ordering::greater. - If both
lhsandrhsare valueless, returnsstd::strong_ordering::equal.
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_constrained_equality | 202403L | (C++26) | constrained comparison operators for std::variant |
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
See also
|
(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20) | compares optional objects (function template) |
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