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Difference between revisions of "cpp/utility/compare/weak order"

From cppreference.com
< cpp‎ | utility
m (Cubbi moved page cpp/compare/weak order to cpp/utility/compare/weak order without leaving a redirect: not creating a "cpp/compare" top-level library)
(Replaced {{cpp/expr-eq}}.)
 
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{{dcl begin}}
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{{dcl header | compare}}
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{{dcl header|compare}}
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{{dcl |since=c++20 |
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{{dcl|since=c++20|1=
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template< class T >
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inline namespace /* unspecified */ {
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constexpr std::weak_ordering weak_order(const T& a, const T& b);
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    inline constexpr /* unspecified */ weak_order = /* unspecified */;
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}
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}}
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{{dcl h|Call signature}}
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{{dcl|1=
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template< class T, class U >
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    requires /* see below */
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constexpr std::weak_ordering weak_order(T&& t, U&& u) noexcept(/* see below */);
 
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Compares two values using 3-way comparison and produces a result of type {{lc|std::weak_ordering}}
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Compares two values using 3-way comparison and produces a result of type {{rlpt|weak_ordering|std::weak_ordering}}.
  
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Specifically,
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Let {{c|t}} and {{c|u}} be expressions and {{tt|T}} and {{tt|U}} denote {{c/core|decltype((t))}} and {{c/core|decltype((u))}} respectively, {{c|std::weak_order(t, u)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to:
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* If the expression {{c|a <{{=}}> b}} is well-formed and its result is convertible to {{lc|std::weak_ordering}}, returns that result.
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* If {{c|std::is_same_v<std::decay_t<T>, std::decay_t<U>>}} is {{c|true}}:
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* Otherwise, if the expression {{c|a <{{=}}> b}} is well-formed, but its result is not convertible to {{lc|std::weak_ordering}}, then the function is defined as deleted.
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** {{c|std::weak_ordering(weak_order(t, u))}}, if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of {{tt|std::weak_order}},
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* Otherwise, if the expression {{c|a <{{=}}> b}} is ill-formed, but the expressions {{c|a {{==}} b}} and {{c|a < b}} are both well-formed and convertible to {{c|bool}},
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** otherwise, if {{tt|T}} is a floating-point type:
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:* if {{c|a {{==}} b}} is {{c|true}}, returns {{lc|std::weak_ordering::equivalent}}
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*** if {{c|std::numeric_limits<T>::is_iec559}} is {{c|true}}, performs the weak ordering comparison of floating-point values (see below) and returns that result as a value of type {{rlpt|weak_ordering|std::weak_ordering}},
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:* otherwise, if {{c|a < b}} is {{c|true}}, returns {{lc|std::weak_ordering::less}}
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*** otherwise, yields a value of type {{rlpt|weak_ordering|std::weak_ordering}} that is consistent with the ordering observed by {{tt|T}}'s comparison operators,
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:* otherwise, returns {{lc|std::weak_ordering::greater}}
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** otherwise, {{c|std::weak_ordering(std::compare_three_way()(t, u))}}, if it is well-formed,
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* Otherwise, the function is defined as deleted.
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** otherwise, {{c|std::weak_ordering(std::strong_order(t, u))}}, if it is well-formed.
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* In all other cases, the expression is ill-formed, which can result in [[cpp/language/sfinae|substitution failure]] when it appears in the immediate context of a template instantiation.
  
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===Parameters===
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{{cpp/ranges/cpo|std}}
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{{par begin}}
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{{par | a, b | values to compare}}
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{{par end}}
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===Return value===
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====Strict weak order of IEEE floating-point types====
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A value of type {{lc|std::weak_ordering}}, as described above.
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Let {{c|x}} and {{c|y}} be values of same IEEE floating-point type, and {{c|weak_order_less(x, y)}} be the boolean result indicating if {{c|x}} precedes {{c|y}} in the strict weak order defined by the C++ standard.
  
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===Notes===
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* If neither {{c|x}} nor {{c|y}} is NaN, then {{c|1=weak_order_less(x, y) == true}} if and only if {{c|x < y}}, i.e. all representations of equal floating-point value are equivalent;
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* If {{c|x}} is negative NaN and {{c|y}} is not negative NaN, then {{c|1=weak_order_less(x, y) == true}};
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* If {{c|x}} is not positive NaN and {{c|y}} is positive NaN, then {{c|1=weak_order_less(x, y) == true}};
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* If both {{c|x}} and {{c|y}} are NaNs with the same sign, then {{c|1=(weak_order_less(x, y) {{!!}} weak_order_less(y, x)) == false}}, i.e. all NaNs with the same sign are equivalent.
  
 
===Example===
 
===Example===
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===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/utility/compare/dsc weak_ordering}}
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{{dsc inc|cpp/utility/compare/dsc weak_ordering}}
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{{dsc inc | cpp/utility/compare/dsc strong_order}}
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{{dsc inc|cpp/utility/compare/dsc strong_order}}
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{{dsc inc | cpp/utility/compare/dsc partial_order}}
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{{dsc inc|cpp/utility/compare/dsc partial_order}}
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{{dsc inc | cpp/utility/compare/dsc weak_equal}}
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{{dsc inc|cpp/utility/compare/dsc compare_weak_order_fallback}}
 
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{{dsc end}}
  
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{{langlinks|zh}}
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{{langlinks|de|es|ja|ru|zh}}

Latest revision as of 22:39, 22 April 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Defined in header <compare>
inline namespace /* unspecified */ {

    inline constexpr /* unspecified */ weak_order = /* unspecified */;

}
 (since C++20)
Call signature
template< class T, class U >

    requires /* see below */

constexpr std::weak_ordering weak_order(T&& t, U&& u) noexcept(/* see below */);

Compares two values using 3-way comparison and produces a result of type std::weak_ordering.

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

Contents

Customization point objects

The name std::weak_order 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 __weak_order_fn.

All instances of __weak_order_fn are equal. The effects of invoking different instances of type __weak_order_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::weak_order 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::weak_order above, __weak_order_fn models

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

[edit] Strict weak order of IEEE floating-point types

Let x and y be values of same IEEE floating-point type, and weak_order_less(x, y) be the boolean result indicating if x precedes y in the strict weak order defined by the C++ standard.

  • If neither x nor y is NaN, then weak_order_less(x, y) == true if and only if x < y, i.e. all representations of equal floating-point value are equivalent;
  • If x is negative NaN and y is not negative NaN, then weak_order_less(x, y) == true;
  • If x is not positive NaN and y is positive NaN, then weak_order_less(x, y) == true;
  • If both x and y are NaNs with the same sign, then (weak_order_less(x, y) || weak_order_less(y, x)) == false, i.e. all NaNs with the same sign are equivalent.

[edit] Example

This section is incomplete
Reason: no example

[edit] See also

(C++20)
 the result type of 3-way comparison that supports all 6 operators and is not substitutable
(class) [edit]
(C++20)
 performs 3-way comparison and produces a result of type std::strong_ordering
(customization point object)[edit]
(C++20)
 performs 3-way comparison and produces a result of type std::partial_ordering
(customization point object)[edit]
(C++20)
 performs 3-way comparison and produces a result of type std::weak_ordering, even if operator<=> is unavailable
(customization point object)[edit]
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