std::strong_order

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) 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) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

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).

See also

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