10 Classes [class]

10.10 Comparisons [class.compare]

10.10.1 Defaulted comparison operator functions [class.compare.default]

A defaulted comparison operator function ([expr.spaceship], [expr.rel], [expr.eq]) for some class C shall be a non-template function declared in the member-specification of C that is

(1.1)
a non-static member of C having one parameter of type const C&, or
(1.2)
a friend of C having two parameters of type const C&.

A three-way comparison operator for a class type C is a structural comparison operator if it is defined as defaulted in the definition of C, and all three-way comparison operators it invokes are structural comparison operators.

A type T has strong structural equality if, for a glvalue x of type const T, x <=> x is a valid expression of type std::strong_ordering or std::strong_equality and either does not invoke a three-way comparison operator or invokes a structural comparison operator.

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

x_{i}

be an lvalue denoting the

i^{th}

element in the expanded list of subobjects for an object x (of length n), where

x_{i}

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

x_{i}

<=>

x_{i}

is denoted by

R_{i}

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

R_{0}

R_{1}

, …,

R_{n - 1}

[ Note

Otherwise, the program will be ill-formed if the expression

x_{i}

<=>

x_{i}

is not implicitly convertible to the declared return type for any i.

— end note

]

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

x_{i}

and

y_{i}

in the expanded lists of subobjects for x and y until the first index i where

x_{i}

<=>

y_{i}

yields a result value

v_{i}

where

v_{i}!= 0

, contextually converted to bool, yields true; V is

v_{i}

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

T_{0}

T_{1}

, …,

T_{n - 1}

is defined as follows:

(4.1)
If any $T_{i}$ is not a comparison category type ([cmp.categories]), U is void.
(4.2)
Otherwise, if at least one $T_{i}$ is std::weak_equality, or at least one $T_{i}$ is std::strong_equality and at least one $T_{j}$ is std::partial_ordering or std::weak_ordering, U is std::weak_equality ([cmp.weakeq]).
(4.3)
Otherwise, if at least one $T_{i}$ is std::strong_equality, U is std::strong_equality ([cmp.strongeq]).
(4.4)
Otherwise, if at least one $T_{i}$ is std::partial_ordering, U is std::partial_ordering ([cmp.partialord]).
(4.5)
Otherwise, if at least one $T_{i}$ is std::weak_ordering, U is std::weak_ordering ([cmp.weakord]).
(4.6)
Otherwise, U is std::strong_ordering ([cmp.strongord]).

[ Note
:
In particular, this is the result when n is 0.
— end note
]

10.10.3 Other comparison operators [class.rel.eq]

A defaulted relational ([expr.rel]) or equality ([expr.eq]) operator function for some operator @ shall have a declared return type bool.

The operator function with parameters x and y is defined as deleted if

(2.1)
overload resolution ([over.match]), as applied to x <=> y (also considering synthesized candidates with reversed order of parameters ([over.match.oper])), results in an ambiguity or a function that is deleted or inaccessible from the operator function, or
(2.2)
the operator @ cannot be applied to the return type of x <=> y or y <=> x.

Otherwise, the operator function yields x <=> y @ 0 if an operator<=> with the original order of parameters was selected, or 0 @ y <=> x otherwise.

[ Example

struct C {
  friend std::strong_equality operator<=>(const C&, const C&);
  friend bool operator==(const C& x, const C& y) = default; // OK, returns x <=> y == 0
  bool operator<(const C&) = default;                       // OK, function is deleted
};

— end example

]