NAME

std::strong_order - std::strong_order

Synopsis

Defined in header <compare>
inline namespace /* unspecified */ {

inline constexpr /* unspecified */ strong_order = /* unspecified */; (since C++20)

}
Call signature
template< class T, class U >

requires /* see below */

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

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

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

* If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:

* std::strong_ordering(strong_order(t, u)), if it is a well-formed expression
with overload resolution performed in a context that does not include a
declaration of std::strong_order,
* otherwise, if T is a floating-point type:

* if std::numeric_limits<T>::is_iec559 is true, performs the
ISO/IEC/IEEE 60559 totalOrder comparison of floating-point values and
returns that result as a value of type std::strong_ordering (note:
this comparison can distinguish between the positive and negative zero
and between the NaNs with different representations),
* otherwise, yields a value of type std::strong_ordering that is
consistent with the ordering observed by T's comparison operators,

* otherwise, std::strong_ordering(std::compare_three_way()(t, u)) if it is
well-formed.

* In all other cases, the expression 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::strong_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 __strong_order_fn.

All instances of __strong_order_fn are equal. The effects of invoking different
instances of type __strong_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::strong_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::strong_order above, __strong_order_fn models

* std::invocable<__strong_order_fn, Args...>,
* std::invocable<const __strong_order_fn, Args...>,
* std::invocable<__strong_order_fn&, Args...>, and
* std::invocable<const __strong_order_fn&, Args...>.

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

Notes

Strict total order of IEEE floating-point types

Let x and y be values of same IEEE floating-point type, and total_order_less(x, y)
be the boolean result indicating if x precedes y in the strict total order defined
by totalOrder in ISO/IEC/IEEE 60559.

(total_order_less(x, y) || total_order_less(y, x)) == false if and only if x and y
have the same bit pattern.

* if neither x nor y is NaN:

* if x < y, then total_order_less(x, y) == true;
* if x > y, then total_order_less(x, y) == false;
* if x == y,

* if x is negative zero and y is positive zero, total_order_less(x, y)
== true,
* if x is not zero and x's exponent field is less than y's, then
total_order_less(x, y) == (x > 0) (only meaningful for decimal
floating-point number);

* if either x or y is NaN:

* if x is negative NaN and y is not negative NaN, then total_order_less(x, y)
== true,
* if x is not positive NaN and y is positive NaN, then total_order_less(x, y)
== true,
* if both x and y are NaNs with the same sign and x's mantissa field is less
than y's, then total_order_less(x, y) == !std::signbit(x).

Example

This section is incomplete
Reason: no example

See also

strong_ordering the result type of 3-way comparison that supports all
(C++20) 6 operators and is substitutable
(class)
weak_order performs 3-way comparison and produces a result of
(C++20) type std::weak_ordering
(customization point object)
partial_order performs 3-way comparison and produces a result of
(C++20) type std::partial_ordering
(customization point object)
performs 3-way comparison and produces a result of
compare_strong_order_fallback type std::strong_ordering, even if operator<=> is
(C++20) unavailable
(customization point object)