std::compare_three_way

 

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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.

Member types

Member type	Definition
is_transparent	/* unspecified */

Member functions

operator()

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

std::compare_three_way::operator()

template<class T, class U>     requires std::three_way_comparable_with<T, U> ||              /* std::declval<T>() <=> std::declval<U>() resolves to                 a built-in operator comparing pointers */ constexpr auto operator()(T&& t, U&& u) const;

Compares t and u, equivalent to return std::forward<T>(t) <=> std::forward<U>(u);, except when that expression resolves to a call to a builtin operator<=> comparing pointers.

When a call would invoke a built-in operator comparing pointers of type P, the result is instead determined as follows:

• Returns std::strong_ordering::less if the (possibly converted) value of the first argument precedes the (possibly converted) value of the second argument in the implementation-defined strict total ordering over all pointer values of type P. This strict total ordering is consistent with the partial order imposed by the builtin operators <, >, <=, and >=.
• Otherwise, returns std::strong_ordering::greater if (possibly converted) value of the second argument precedes the (possibly converted) value of the first argument in the same strict total ordering.
• Otherwise, returns std::strong_ordering::equal.

The behavior is undefined unless the the conversion sequences from both T and U to P are equality-preserving (see below).

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Example

Run this code

#include <iostream>
#include <compare>
 
struct Rational_2 {
    int num;
    int den; // > 0
};
 
constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
{
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
}
 
void print(std::weak_ordering value)
{
    if (value == 0)
        std::cout << "equal\n";
    else if (value < 0)
        std::cout << "less\n";
    else
        std::cout << "greater\n";
}
 
int main()
{
    Rational_2 c{6,5};
    Rational_2 d{8,7};
    print(c <=> d);
    print(std::compare_three_way{}(c,d));
}

Output:

greater
greater

See also

ranges::equal_to (C++20)	function object implementing x == y (class)
ranges::not_equal_to (C++20)	function object implementing x != y (class)
ranges::less (C++20)	function object implementing x < y (class)
ranges::greater (C++20)	function object implementing x > y (class)
ranges::less_equal (C++20)	function object implementing x <= y (class)
ranges::greater_equal (C++20)	function object implementing x >= y (class)