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    std::compare_weak_order_fallback

    From cppreference.com
     
    C++
     
    Utilities library
    General utilities
    Relational operators (deprecated in C++20)
     
    Defined in header <compare> 
    inline namespace /* unspecified */ {
    inline constexpr /* unspecified */
        compare_weak_order_fallback = /* unspecified */;
}
    		(since C++20)
    Call signature
    		 
    template< class T, class U >
    requires /* see below */
constexpr std::weak_ordering
    compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see below */);
    		(since C++20)

    Performs three-way comparison on subexpressions t and u and produces a result of type std::weak_ordering, even if the operator <=> is unavailable.

    If std::decay_t<T> and std::decay_t<U> are the same type, std::compare_weak_order_fallback(t, u) is expression-equivalent to:

    • std::weak_order(t, u), if it is a well-formed expression; otherwise,
    • t == u ? std::weak_ordering::equivalent :
      t < u ? std::weak_ordering::less :
               std::weak_ordering::greater      , if the expressions t == u and t < u are both well-formed and each of decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u are evaluated only once.

    In all other cases, std::compare_weak_order_fallback(t, u) is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.

    Customization point objects

    The name std::compare_weak_order_fallback denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

    Example

    Run this code
    #include <compare>
#include <iostream>

// does not support <=>
struct Rational_1
{
    int num;
    int den; // > 0
};

inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs)
{
    return lhs.num * rhs.den < rhs.num * lhs.den;
}

inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs)
{
    return lhs.num * rhs.den == rhs.num * lhs.den;
}

// supports <=>
struct Rational_2
{
    int num;
    int den; // > 0
};

inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
{
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
}

inline constexpr bool operator==(Rational_2 lhs, Rational_2 rhs)
{
    return lhs <=> rhs == 0;
}

void print(int id, std::weak_ordering value)
{
    std::cout << id << ") ";
    if (value == 0)
        std::cout << "equal\n";
    else if (value < 0)
        std::cout << "less\n";
    else
        std::cout << "greater\n";
}

int main()
{
    Rational_1 a{1, 2}, b{3, 4};
//  print(0, a <=> b); // does not work
    print(1, std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==

    Rational_2 c{6, 5}, d{8, 7};
    print(2, c <=> d); // works
    print(3, std::compare_weak_order_fallback(c, d)); // works

    Rational_2 e{2, 3}, f{4, 6};
    print(4, e <=> f); // works
    print(5, std::compare_weak_order_fallback(e, f)); // works
}

    Output: 

    1) less
2) greater
3) greater
4) equal
5) equal

    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 2114
    (P2167R3)     		C++20 the fallback mechanism only required
    return types to be convertible to bool     		constraints strengthened

    See also

    (C++20)
    		 performs 3-way comparison and produces a result of type std::weak_ordering
    (customization point object)[edit]