std::compare_three_way - cppreference.com
From cppreference.com
| Defined in header |
||
| Defined in header |
||
|
|
(since C++20) | |
Function object for performing comparisons. Deduces the parameter types and the return type of the function call operator.
Nested types
Member functions
| obtains the result of three-way comparison on both arguments (public member function) |
std::compare_three_way::operator()
|
|
||
Given the expression std::forward<T>(t) <=> std::forward<U>(u) as expr:
- If
exprresults in a call to built-inoperator<=>comparing pointers, given the composite pointer type oftanduasP:
- Compares the two converted pointers (of type
P) in the implementation-defined strict total order over pointers:
- If
tprecedesu, returns std::strong_ordering::less. - If
uprecedest, returns std::strong_ordering::greater. - Otherwise, returns std::strong_ordering::equal.
- If
- If the conversion sequence from
TtoPor the conversion sequence fromUtoPis not equality-preserving, the behavior is undefined.
- Compares the two converted pointers (of type
- Otherwise:
- Returns the result of
expr. - If
std::three_way_comparable_with<T, U>is not modeled, the behavior is undefined.
- Returns the result of
This overload participates in overload resolution only if std::three_way_comparable_with<T, U> is satisfied.
Example
#include <compare> #include <iostream> struct Rational { int num; int den; // > 0 // Although the comparison X <=> Y will work, a direct call // to std::compare_three_way{}(X, Y) requires the operator== // be defined, to satisfy the std::three_way_comparable_with. constexpr bool operator==(Rational const&) const = default; }; constexpr std::weak_ordering operator<=>(Rational lhs, Rational rhs) { return lhs.num * rhs.den <=> rhs.num * lhs.den; } void print(std::weak_ordering value) { value < 0 ? std::cout << "less\n" : value > 0 ? std::cout << "greater\n" : std::cout << "equal\n"; } int main() { Rational a{6, 5}; Rational b{8, 7}; print(a <=> b); print(std::compare_three_way{}(a, b)); }
Output:
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3530 | C++20 | syntactic checks were relaxed while comparing pointers | only semantic requirements are relaxed |
See also
(C++20) |
constrained function object implementing x == y (class) [edit] |
(C++20) |
constrained function object implementing x != y (class) [edit] |
(C++20) |
constrained function object implementing x < y (class) [edit] |
(C++20) |
constrained function object implementing x > y (class) [edit] |
(C++20) |
constrained function object implementing x <= y (class) [edit] |
(C++20) |
constrained function object implementing x >= y (class) [edit] |