std::compare_strong_order_fallback
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 */); |
Performs three-way comparison on t
and u
and produces a result of type std::strong_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_strong_order_fallback(t, u)
is expression-equivalent to:
- If
std::is_same_v<std::decay_t<T>, std::decay_t<U>>
istrue
:-
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
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_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.
Expression-equivalent
Expression e
is expression-equivalent to expression f
, if.
-
e
andf
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_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.
Example
See also
(C++20)
|
performs 3-way comparison and produces a result of type std::strong_ordering (customization point object) |
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