std::compare_strong_order_fallback

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

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) is_within_lifetime (C++26) 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_strong_order_fallback = /* unspecified */; }		(since C++20)
Call signature
template< class T, class U >     requires /* see below */ constexpr std::strong_ordering     compare_strong_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::strong_ordering, even if the operator <=> is unavailable.

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

• std::strong_order(t, u), if it is a well-formed expression;
• otherwise, t == u ? std::strong_ordering::equal :
  t < u  ? std::strong_ordering::less :
           std::strong_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_strong_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_strong_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_strong_order_fallback_fn.

All instances of __compare_strong_order_fallback_fn are equal. The effects of invoking different instances of type __compare_strong_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_strong_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_strong_order_fallback above, __compare_strong_order_fallback_fn models

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

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

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

strong_order (C++20)

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