std::strong_order

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Library feature-test macros
Date and time
Function objects
Formatting library (C++20)
initializer_list (C++11)
source_location (C++20)
bitset
hash (C++11)
integer_sequence (C++14)
Relational operators (deprecated in C++20)
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
three_way_comparablethree_way_comparable_with
strong_ordering weak_ordering partial_ordering common_comparison_category compare_three_way_result compare_three_way	strong_order weak_order partial_order compare_strong_order_fallback compare_weak_order_fallback compare_partial_order_fallback	is_eqis_neqis_ltis_lteqis_gtis_gteq
Common vocabulary types
pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)
launder (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>> == 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,
  • other the expression is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
• Otherwise, std::strong_order(t, u) is ill-formed.

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Customization point objects

The name std::strong_order denotes a customization point object, which is a function object of a literal semiregular class type (denoted, for exposition purposes, as strong_order_ftor). All instances of strong_order_ftor are equal. 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_ftor will satisfy std::invocable<const strong_order_ftor&, Args...>. Otherwise, no function call operator of strong_order_ftor 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,
    • 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)