std::strong_order
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 */ 

Compares two values using 3way 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 expressionequivalent 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 wellformed expression with overload resolution performed in a context that does not include a declaration of
std::strong_order
,  otherwise, if
T
is a floatingpoint type: if std::numeric_limits<T>::is_iec559 is true, performs the ISO/IEC/IEEE 60559 totalOrder comparison of floatingpoint 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 wellformed.
 std::strong_ordering(strong_order(t, u)), if it is a wellformed expression with overload resolution performed in a context that does not include a declaration of
 In all other cases, the expression is illformed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
Expressionequivalent
Expression e is expressionequivalent 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 potentiallythrowing or else neither is potentiallythrowing (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 cvunqualified 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 constqualified or not (however, a volatilequalified 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 floatingpoint types
Let x
and y
be values of same IEEE floatingpoint 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
nory
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 andy
is positive zero, total_order_less(x, y) == true,  if
x
is not zero andx
's exponent field is less thany
's, then total_order_less(x, y) == (x > 0) (only meaningful for decimal floatingpoint number);
 if
 if
 if either
x
ory
is NaN: if
x
is negative NaN andy
is not negative NaN, then total_order_less(x, y) == true,  if
x
is not positive NaN andy
is positive NaN, then total_order_less(x, y) == true,  if both
x
andy
are NaNs with the same sign andx
's mantissa field is less thany
's, then total_order_less(x, y) == !std::signbit(x).
 if
Example
This section is incomplete Reason: no example 
See also
(C++20) 
the result type of 3way comparison that supports all 6 operators and is substitutable (class) 
(C++20) 
performs 3way comparison and produces a result of type std::weak_ordering (customization point object) 
(C++20) 
performs 3way comparison and produces a result of type std::partial_ordering (customization point object) 
performs 3way comparison and produces a result of type std::strong_ordering , even if operator<=> is unavailable (customization point object) 