std::compare_weak_order_fallback

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

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 */);

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

Let t and u be expressions and T and U denote decltype((t)) and decltype((u)) respectively, std::compare_weak_order_fallback(t, u) is expression-equivalent to:

• If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
  • 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 t == u and t < u are both well-formed and convertible to bool, 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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

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. For exposition purposes, the cv-unqualified version of its type is denoted as __compare_weak_order_fallback_fn.

All instances of __compare_weak_order_fallback_fn are equal. The effects of invoking different instances of type __compare_weak_order_fallback_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, std::compare_weak_order_fallback can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to std::compare_weak_order_fallback above, __compare_weak_order_fallback_fn models

• std::invocable<__compare_weak_order_fallback_fn, Args...>,
• std::invocable<const __compare_weak_order_fallback_fn, Args...>,
• std::invocable<__compare_weak_order_fallback_fn&, Args...>, and
• std::invocable<const __compare_weak_order_fallback_fn&, Args...>.
  

Otherwise, no function call operator of __compare_weak_order_fallback_fn participates in overload resolution.

Example

Run this code

#include <iostream>
#include <compare>
 
// 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
 
    bool operator==(Rational_2 const&) const = default;
};
 
inline 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_1 a{1, 2};
    Rational_1 b{3, 4};
//  print(a <=> b);                // doesn't work
    print(std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==
 
    Rational_2 c{6, 5};
    Rational_2 d{8, 7};
    print(c <=> d);                // works
    print(std::compare_weak_order_fallback(c, d)); // works
}

Output:

less
greater
greater

See also

weak_order (C++20)

performs 3-way comparison and produces a result of type std::weak_ordering (customization point object)