std::compare_partial_order_fallback

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Utilities library

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Library feature-test macros(C++20)

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initializer_list

(C++11)

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(C++20)

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(C++26)

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(C++20)

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Three-way comparison

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)

(C++20)

(C++20)

(C++20)

(C++20)

compare_strong_order_fallback

(C++20)

compare_weak_order_fallback

(C++20)

compare_partial_order_fallback

(C++20)

is_eqis_ltis_lteq

(C++20)(C++20)(C++20)

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(C++20)(C++20)(C++20)

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cmp_equalcmp_lesscmp_less_than

(C++20)(C++20)(C++20)

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(C++20)(C++20)(C++20)

in_range

(C++20)

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tuple_size (C++11)
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expected (C++23)

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Defined in header`<compare>`
inlinenamespace/* unspecified /{ inlineconstexpr/ unspecified / compare_partial_order_fallback=/ unspecified */; }		(since C++20)
Call signature
template<class T,class U> requires/* see below / constexprstd::partial_ordering compare_partial_order_fallback( T&& t, U&& u)noexcept(/ see below */);		(since C++20)

Performs three-way comparison onsubexpressionst andu and produces a result of typestd::partial_ordering, even if the operator<=> is unavailable.

Ifstd::decay_t<T> andstd::decay_t<U>> are the same type,std::compare_partial_order_fallback(t, u) isexpression-equivalent to:

std::partial_order(t, u), if it is a well-formed expression;
otherwise,t== u? std::partial_ordering::equivalent:
t< u ? std::partial_ordering::less:
u< t ? std::partial_ordering::greater:
         std::partial_ordering::unordered, if the expressionst== u,t< u andu< t are all well-formed and each ofdecltype(t== u),decltype(t< u) anddecltype(u< t) modelsboolean-testable, except thatt andu are evaluated only once.

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

Customization point objects

The namestd::compare_partial_order_fallback denotes acustomization point object, which is a constfunction object of aliteral semiregular class type. SeeCustomizationPointObject for details.

[edit]Example

This section is incomplete
Reason: no example

[edit]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 tobool	constraints strengthened
LWG 3465	C++20	the fallback mechanism did not requireu< t to be well-formed	required
LWG 4157	C++20	P2167R3 overwrote the resolution ofLWG issue 3465	restored the resolution