std::compare_three_way
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compare_three_way
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compare_three_way
(C++20)
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(until C++20*)
Defined in header
<compare>
Defined in header
<functional>
struct compare_three_way;
(since C++20)
Function object for performing comparisons. Deduces the parameter types and the return type of the function call operator.
Contents
[edit] Nested types
Nested type
Definition
is_transparent
unspecified
[edit] Member functions
operator()
(public member function)
std::compare_three_way::operator()
template< class T, class U >
constexpr auto operator()( T&& t, U&& u ) const;
constexpr auto operator()( T&& t, U&& u ) const;
Given the expression std::forward <T>(t) <=> std::forward <U>(u) as expr:
- If expr results in a call to built-in operator<=> comparing pointers, given the composite pointer type of t and u as
P:
- Compares the two converted pointers (of type
P) in the implementation-defined strict total order over pointers:
- If t precedes u, returns std::strong_ordering::less.
- If u precedes t, returns std::strong_ordering::greater.
- Otherwise, returns std::strong_ordering::equal.
- 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.
This overload participates in overload resolution only if std::three_way_comparable_with <T, U> is satisfied.
[edit] Example
Run this code
#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:
greater greater
[edit] 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 |