cppdraft_translate/cppdraft/class/spaceship.md

[class.spaceship]

11 Classes [class]

11.10 Comparisons [class.compare]

11.10.3 Three-way comparison [class.spaceship]

1

#

The synthesized three-way comparison of type R ([cmp.categories]) of glvalues a and b of the same type is defined as follows:

• (1.1)

  If a <=> b is usable ([class.compare.default]) and can be explicitly converted to R using static_cast,static_cast(a <=> b).

• (1.2)

  Otherwise, if a <=> b is usable or overload resolution for a <=> b is performed and finds at least one viable candidate, the synthesized three-way comparison is not defined.

• (1.3)

  Otherwise, if R is not a comparison category type, or either the expression a == b or the expression a < b is not usable, the synthesized three-way comparison is not defined.

• (1.4)

  Otherwise, if R is strong_ordering, thena == b ? strong_ordering::equal : a < b ? strong_ordering::less : strong_ordering::greater

• (1.5)

  Otherwise, if R is weak_ordering, thena == b ? weak_ordering::equivalent : a < b ? weak_ordering::less : weak_ordering::greater

• (1.6)

  Otherwise (when R is partial_ordering),a == b ? partial_ordering::equivalent : a < b ? partial_ordering::less : b < a ? partial_ordering::greater : partial_ordering::unordered

[Note 1:

A synthesized three-way comparison is ill-formed if overload resolution finds usable candidates that do not otherwise meet the requirements implied by the defined expression.

— end note]

2

#

Let R be the declared return type of a defaulted three-way comparison operator function, and let xi be the elements of the expanded list of subobjects for an object x of type C.

• (2.1)

  If R is auto, then let cvi Ri be the type of the expression xi <=> xi. The operator function is defined as deleted if that expression is not usable or if Ri is not a comparison category type ([cmp.categories.pre]) for any i. The return type is deduced as the common comparison type (see below) ofR0, R1, …, Rn−1.

• (2.2)

  Otherwise, R shall not contain a placeholder type. If the synthesized three-way comparison of type R between any objects xi and xi is not defined, the operator function is defined as deleted.

3

#

The return value of type R of the defaulted three-way comparison operator function with parameters x and y of the same type is determined by comparing corresponding elementsxi and yi in the expanded lists of subobjects for x and y (in increasing index order) until the first index i where the synthesized three-way comparison of type R between xi and yi yields a result value vi where vi!=0, contextually converted to bool, yields true.

The return value is a copy of vi if such an index exists andstatic_cast(std​::​strong_ordering​::​equal) otherwise.

4

#

The common comparison type U of a possibly-empty list of n comparison category typesT0, T1, …, Tn−1 is defined as follows:

• (4.1)

  If at least one Ti is std​::​partial_ordering,U is std​::​partial_ordering ([cmp.partialord]).

• (4.2)

  Otherwise, if at least one Ti is std​::​weak_ordering,U is std​::​weak_ordering ([cmp.weakord]).

• (4.3)

  Otherwise, U is std​::​strong_ordering ([cmp.strongord]). [Note 2: In particular, this is the result when n is 0. — end note]