10 Classes [class]

10.10 Comparisons [class.compare]

10.10.2 Three-way comparison [class.spaceship]

The direct base class subobjects of C, in the order of their declaration in the base-specifier-list of C, followed by the non-static data members of C, in the order of their declaration in the member-specification of C, form a list of subobjects.
In that list, any subobject of array type is recursively expanded to the sequence of its elements, in the order of increasing subscript.
Let be an lvalue denoting the element in the expanded list of subobjects for an object x (of length n), where is formed by a sequence of derived-to-base conversions ([over.best.ics]), class member access expressions ([expr.ref]), and array subscript expressions ([expr.sub]) applied to x.
The type of the expression <=> is denoted by .
It is unspecified whether virtual base class subobjects are compared more than once.
If the declared return type of a defaulted three-way comparison operator function is auto, then the return type is deduced as the common comparison type (see below) of , , , .
[Note
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Otherwise, the program will be ill-formed if the expression <=> is not implicitly convertible to the declared return type for any i.
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]
If the return type is deduced as void, the operator function is defined as deleted.
The return value V 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 elements and in the expanded lists of subobjects for x and y until the first index i where <=> yields a result value where , contextually converted to bool, yields true; V is converted to R.
If no such index exists, V is std::strong_­ordering::equal converted to R.
The common comparison type U of a possibly-empty list of n types , , , is defined as follows:
  • If any is not a comparison category type ([cmp.categories]), U is void.
  • Otherwise, if at least one is std::weak_­equality, or at least one is std::strong_­equality and at least one is std::partial_­ordering or std::weak_­ordering, U is std::weak_­equality ([cmp.weakeq]).
  • Otherwise, if at least one is std::strong_­equality, U is std::strong_­equality ([cmp.strongeq]).
  • Otherwise, if at least one is std::partial_­ordering, U is std::partial_­ordering ([cmp.partialord]).
  • Otherwise, if at least one is std::weak_­ordering, U is std::weak_­ordering ([cmp.weakord]).
  • Otherwise, U is std::strong_­ordering ([cmp.strongord]).
    [Note
    :
    In particular, this is the result when n is 0.
    end note
    ]