11 Classes [class]

11.1 Preamble [class.pre]

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A class is a type.
Its name becomes a class-name ([class.name]) within its scope.
An object of a class consists of a (possibly empty) sequence of members and base class objects.
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.spec.partial]).
[Note 1: 
An unnamed class thus can't be final.
— end note]
Otherwise, the class-name is an identifier; it is not looked up, and the class-specifier introduces it.
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The component name of the class-name is also bound in the scope of the class (template) 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.
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If a class-head-name contains a nested-name-specifier, the class-specifier shall not inhabit a class scope.
If its class-name is an identifier, the class-specifier shall correspond to one or more declarations nominable in the class, class template, or namespace to which the nested-name-specifier refers; they shall all have the same target scope, and the target scope of the class-specifier is that scope.
[Example 1: namespace N { template<class> struct A { struct B; }; } using N::A; template<class T> struct A<T>::B {}; // OK template<> struct A<void> {}; // OK — end example]
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[Note 2: 
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]
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If a class is marked with the class-virt-specifier final and it appears as a class-or-decltype in a base-clause ([class.derived]), 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 2: 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]
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[Note 3: 
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]
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[Note 4: 
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]