11 Classes [class]

11.1 Preamble [class.pre]

A class is a type.

Its name becomes a class-name ([class.name]) within its scope.

class-name:
	identifier
	simple-template-id

A class-specifier or an elaborated-type-specifier is used to make a class-name .

An object of a class consists of a (possibly empty) sequence of members and base class objects.

class-specifier:
	class-head { member-specification $_{o p t}$  }

class-head:
	class-key attribute-specifier-seq $_{o p t}$  class-head-name class-virt-specifier $_{o p t}$  base-clause $_{o p t}$ 
	class-key attribute-specifier-seq $_{o p t}$  base-clause $_{o p t}$

class-head-name:
	nested-name-specifier $_{o p t}$  class-name

class-virt-specifier:
	final

class-key:
	class
	struct
	union

A class declaration where the class-name in the class-head-name is a simple-template-id shall be an explicit specialization ([temp.expl.spec]) or a partial specialization ([temp.class.spec]).

A class-specifier whose class-head omits the class-head-name defines an unnamed class.

[ Note

An unnamed class thus can't be final.

— end note

]

A class-name is inserted into the scope in which it is declared immediately after the class-name is seen.

The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name.

For purposes of access checking, the injected-class-name is treated as if it were a public member name.

A class-specifier is commonly referred to as a class definition.

A class is considered defined after the closing brace of its class-specifier has been seen even though its member functions are in general not yet defined.

The optional attribute-specifier-seq appertains to the class; the attributes in the attribute-specifier-seq are thereafter considered attributes of the class whenever it is named.

If a class-head-name contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set ([namespace.def]) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration.

In such cases, the nested-name-specifier of the class-head-name of the definition shall not begin with a decltype-specifier .

[ Note

The class-key determines whether the class is a union ([class.union]) and whether access is public or private by default ([class.access]).

A union holds the value of at most one data member at a time.

— end note

]

If a class is marked with the class-virt-specifier final and it appears as a class-or-decltype in a base-clause, the program is ill-formed.

Whenever a class-key is followed by a class-head-name, the identifier final, and a colon or left brace, final is interpreted as a class-virt-specifier .

[ Example

struct A;
struct A final {};      // OK: definition of struct A,
                        // not value-initialization of variable final

struct X {
 struct C { constexpr operator int() { return 5; } };
 struct B final : C{};  // OK: definition of nested class B,
                        // not declaration of a bit-field member final
};

— end example

]

[ Note

Complete objects of class type have nonzero size.

Base class subobjects and members declared with the no_unique_address attribute ([dcl.attr.nouniqueaddr]) are not so constrained.

— end note

]

[ Note

Class objects can be assigned ([over.ass], [class.copy.assign]), passed as arguments to functions ([dcl.init], [class.copy.ctor]), and returned by functions (except objects of classes for which copying or moving has been restricted; see [dcl.fct.def.delete] and [class.access]).

Other plausible operators, such as equality comparison, can be defined by the user; see [over.oper].

— end note

]