std::compare_three_way - cppreference.com (original) (raw)

| Defined in header | | | | -------------------------------------------------------------------------------------- | | ------------- | | Defined in header | | | | 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

• 1 Nested types
• 2 Member functions
• 3 std::compare_three_way::operator()
  • 3.1 Example
  • 3.2 Defect reports
  • 3.3 See also

[edit] Nested types

[edit] Member functions

| | obtains the result of three-way comparison on both arguments (public member function) | | ---------------------------------------------------------------------------------------- |

std::compare_three_way::operator()

| template< class T, class U > 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 T to P or the conversion sequence from U to P is not equality-preserving, the behavior is undefined.

• 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

#include #include   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:

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also