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  9   Classes                                                    [class]

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1 A class is a type.   Its  name  becomes  a  class-name  (_class.name_)
  within its scope.
          class-name:
                  identifier
                  template-id
  Class-specifiers  and elaborated-type-specifiers (_dcl.type.elab_) are
  used to make class-names.  An object of a class consists of a  (possi-
  bly empty) sequence of members and base class objects.
          class-specifier:
                  class-head { member-specificationopt }
          class-head:
                  class-key identifieropt base-clauseopt
                  class-key nested-name-specifier identifier base-clauseopt
          class-key:
                  class
                  struct
                  union

2 A  class-name is inserted into the scope in which it is declared imme-
  diately after the class-name is seen.  The class-name is also inserted
  into  the scope of the class itself.  For purposes of access checking,
  the inserted 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-spec-
  ifier  has  been  seen even though its member functions are in general
  not yet defined.

3 A class with an empty sequence of members and base class objects is an
  empty class.  Complete objects and member subobjects of an empty class
  type  shall have nonzero size.1) [Note: class objects can be assigned,
  passed as arguments to functions, and returned  by  functions  (except
  objects  of  classes  for  which  copying  has  been  restricted;  see
  _class.copy_).  Other plausible operators, such as  equality  compari-
  son, can be defined by the user; see _over.oper_.  ]

4 A  structure is a class defined with the class-key struct; its members
  and   base   classes   (_class.derived_)   are   public   by   default
  (_class.access_).   A  union  is  a  class  defined with the class-key
  union; its members are public by default and it holds  only  one  data
  _________________________
  1) That is, a base class subobject of an empty class type may have ze-
  ro size.

  member at a time (_class.union_).  [Note: aggregates of class type are
  described in _dcl.init.aggr_.  ] A POD-struct2) is an aggregate  class
  that  has  no  non-static data members of type pointer to member, non-
  POD-struct, non-POD-union (or array of such types) or  reference,  and
  has  no  user-defined  copy  assignment  operator  and no user-defined
  destructor.  Similarly, a POD-union is an aggregate union that has  no
  non-static  data  members  of  type pointer to member, non-POD-struct,
  non-POD-union (or array of such types) or reference, and has no  user-
  defined  copy  assignment  operator and no user-defined destructor.  A
  POD class is a class that is either a POD-struct or a POD-union.

  9.1  Class names                                          [class.name]

1 A class definition introduces a new type.  [Example:
          struct X { int a; };
          struct Y { int a; };
          X a1;
          Y a2;
          int a3;
  declares three variables of three different types.  This implies that
          a1 = a2;        // error: Y assigned to X
          a1 = a3;        // error: int assigned to X
  are type mismatches, and that
          int f(X);
          int f(Y);
  declare an overloaded (_over_) function f() and not  simply  a  single
  function f() twice.  For the same reason,
          struct S { int a; };
          struct S { int a; };  // error, double definition
  is ill-formed because it defines S twice.  ]

2 A  class  definition introduces the class name into the scope where it
  is defined and hides any class, object, function, or other declaration
  of  that  name in an enclosing scope (_basic.scope_).  If a class name
  is declared in a scope where an object, function, or enumerator of the
  same  name is also declared, then when both declarations are in scope,
  the class can be referred to only using  an  elaborated-type-specifier
  (_basic.lookup.elab_).  [Example:
          struct stat {
              // ...
          };
          stat gstat;             // use plain `stat' to
                                  // define variable

          int stat(struct stat*); // redeclare `stat' as function

  _________________________
  2) The acronym POD stands for "plain ol' data."

          void f()
          {
              struct stat* ps;    // `struct' prefix needed
                                  // to name struct stat
              // ...
              stat(ps);           // call stat()
              // ...
          }
      --end    example]    A    declaration    consisting    solely   of
  class-key identifier ; is either a redeclaration of the  name  in  the
  current  scope  or  a forward declaration of the identifier as a class
  name.  It introduces the class name into the current scope.  [Example:
          struct s { int a; };

          void g()
          {
              struct s;               // hide global struct `s'
                                      //  with a local declaration
              s* p;                   // refer to local struct `s'
              struct s { char* p; };  // define local struct `s'
              struct s;               // redeclaration, has no effect
          }
    --end  example] [Note: Such declarations allow definition of classes
  that refer to each other.  [Example:
          class Vector;

          class Matrix {
              // ...
              friend Vector operator*(Matrix&, Vector&);
          };
          class Vector {
              // ...
              friend Vector operator*(Matrix&, Vector&);
          };
  Declaration of friends is described in _class.friend_, operator  func-
  tions in _over.oper_.  ] ]

3 An elaborated-type-specifier (_dcl.type.elab_) can also be used in the
  declarations of objects and functions.  It differs from a class decla-
  ration in that if a class of the elaborated name is in scope the elab-
  orated name will refer to it.  [Example:
          struct s { int a; };

          void g(int s)
          {
              struct s* p = new struct s;    // global `s'
              p->a = s;                      // local `s'
          }
   --end example]

4 [Note: The declaration of a class name takes effect immediately  after
  the  identifier  is  seen  in the class definition or elaborated-type-
  specifier.  For example,
          class A * A;

  first specifies A to be the name of a class and then redefines  it  as
  the name of a pointer to an object of that class.  This means that the
  elaborated form class A must be used to  refer  to  the  class.   Such
  artistry with names can be confusing and is best avoided.  ]

5 A typedef-name (_dcl.typedef_) that names a class is a class-name, but
  shall not be used in an elaborated-type-specifier; see also _dcl.type-
  def_.

  9.2  Class members                                         [class.mem]
          member-specification:
                  member-declaration member-specificationopt
                  access-specifier : member-specificationopt
          member-declaration:
                  decl-specifier-seqopt member-declarator-listopt ;
                  function-definition ;opt
                  qualified-id ;
                  using-declaration
                  template-declaration
          member-declarator-list:
                  member-declarator
                  member-declarator-list , member-declarator
          member-declarator:
                  declarator pure-specifieropt
                  declarator constant-initializeropt
                  identifieropt : constant-expression
          pure-specifier:
                   = 0
          constant-initializer:
                   = constant-expression

1 The  member-specification  in a class definition declares the full set
  of members of the class; no member can be added elsewhere.  Members of
  a  class  are  data  members,  member functions (_class.mfct_), nested
  types, and enumerators.  Data members and member functions are  static
  or   nonstatic;   see   _class.static_.    Nested  types  are  classes
  (_class.name_, _class.nest_) and enumerations (_dcl.enum_) defined  in
  the class, and arbitrary types declared as members by use of a typedef
  declaration  (_dcl.typedef_).   The  enumerators  of  an   enumeration
  (_dcl.enum_)  defined  in  the class are members of the class.  Except
  when used to declare friends (_class.friend_) or to introduce the name
  of   a   member  of  a  base  class  into  a  derived  class  (_names-
  pace.udecl_,_class.access.dcl_), member-declarations  declare  members
  of  the class, and each such member-declaration shall declare at least
  one member name of the class.  A member shall not be declared twice in
  the  member-specification,  except that a nested class or member class
  template can be declared and then later defined.

2 A class is considered a completely-defined object type (_basic.types_)
  (or  complete  type)  at the closing } of the class-specifier.  Within
  the class member-specification, the  class  is  regarded  as  complete
  within  function  bodies,  default arguments and constructor ctor-ini-
  tializers (including such things in nested classes).  Otherwise it  is
  regarded as incomplete within its own class member-specification.

3 [Note:  a  single  name  can  denote several function members provided
  their types are sufficiently different (_over_).  ]

4 A member-declarator can contain  a  constant-initializer  only  if  it
  declares  a  static member (_class.static_) of integral or enumeration
  type, see _class.static.data_.

5 A member can be initialized using  a  constructor;  see  _class.ctor_.
  [Note:  see  clause  _special_  for  a description of constructors and
  other special member functions.  ]

6 A member shall not be auto, extern, or register.

7 The decl-specifier-seq is omitted in constructor, destructor, and con-
  version function declarations only.  The member-declarator-list can be
  omitted only after a class-specifier, an enum-specifier,  or  a  decl-
  specifier-seq  of  the form friend elaborated-type-specifier.  A pure-
  specifier shall be used only in the declaration of a virtual  function
  (_class.virtual_).

8 Non-static  (_class.static_)  members  that are class objects shall be
  objects of previously defined classes.   In  particular,  a  class  cl
  shall  not contain an object of class cl, but it can contain a pointer
  or reference to an object of class cl.  When an array is used  as  the
  type of a nonstatic member all dimensions shall be specified.

9 Except  when  used to form a pointer to member (_expr.unary.op_), when
  used in the body of a nonstatic member function of its class or  of  a
  class derived from its class (_class.mfct.nonstatic_), or when used in
  a mem-initializer for a constructor for  its  class  or  for  a  class
  derived  from its class (_class.base.init_), a nonstatic data or func-
  tion member of a class shall only be referred to with the class member
  access syntax (_expr.ref_).

10[Note: the type of a nonstatic member function is an ordinary function
  type, and the type of a nonstatic data member is  an  ordinary  object
  type.   There  are  no  special  member  function types or data member
  types.  ]

11[Example: A simple example of a class definition is
          struct tnode {
              char tword[20];
              int count;
              tnode *left;
              tnode *right;
          };
  which contains an array of twenty  characters,  an  integer,  and  two
  pointers  to similar structures.  Once this definition has been given,
  the declaration
          tnode s, *sp;
  declares s to be a tnode and sp to be a  pointer  to  a  tnode.   With
  these declarations, sp->count refers to the count member of the struc-
  ture to which sp points; s.left refers to the left subtree pointer  of
  the structure s; and s.right->tword[0] refers to the initial character

  of the tword member of the right subtree of s.  ]

12Nonstatic data members of a  (non-union)  class  declared  without  an
  intervening  access-specifier are allocated so that later members have
  higher addresses within a class object.  The order  of  allocation  of
  nonstatic data members separated by an access-specifier is unspecified
  (_class.access.spec_).  Implementation  alignment  requirements  might
  cause  two adjacent members not to be allocated immediately after each
  other; so might requirements for space for managing virtual  functions
  (_class.virtual_) and virtual base classes (_class.mi_).

13If  T  is the name of a class, then each of the following shall have a
  name different from T:

  --every data member of class T;

  --every member of class T that is itself a type;

  --every enumerator of every member of class T that  is  an  enumerated
    type; and

  --every member of every anonymous union that is a member of class T.

14Two  POD-struct (_class_) types are layout-compatible if they have the
  same number of members, and corresponding members (in order) have lay-
  out-compatible types (_basic.types_).

15Two  POD-union  (_class_) types are layout-compatible if they have the
  same number of members, and corresponding members (in any order)  have
  layout-compatible types (_basic.types_).

16If  a  POD-union  contains two or more POD-structs that share a common
  initial sequence, and if the POD-union object currently  contains  one
  of  these  POD-structs,  it is permitted to inspect the common initial
  part of any of them.  Two POD-structs share a common initial  sequence
  if  corresponding  members have layout-compatible types (and, for bit-
  fields, the same widths) for a sequence of one or  more  initial  mem-
  bers.

17A  pointer  to  a POD-struct object, suitably converted, points to its
  initial member (or if that member is a bit-field, then to the unit  in
  which  it  resides)  and  vice versa.  [Note: There might therefore be
  unnamed padding within a POD-struct object, but not at its  beginning,
  as necessary to achieve appropriate alignment.  ]

  9.3  Member functions                                     [class.mfct]

1 Functions  declared  in  the  definition  of  a class, excluding those
  declared with a friend specifier (_class.friend_), are  called  member
  functions  of that class.  A member function may be declared static in
  which  case  it  is  a   static   member   function   of   its   class
  (_class.static_);  otherwise  it is a nonstatic member function of its
  class (_class.mfct.nonstatic_, _class.this_).

2 A member function may be defined (_dcl.fct.def_) in its class  defini-
  tion,  in which case it is an inline member function (_dcl.fct.spec_),
  or it may be defined outside of its class definition if it has already
  been declared but not defined in its class definition.  A member func-
  tion definition that appears outside of  the  class  definition  shall
  appear  in  a  namespace scope enclosing the class definition.  Except
  for member function definitions that appear outside of a class defini-
  tion, and except for explicit specializations of template member func-
  tions (_temp.spec_) appearing outside of the class definition, a  mem-
  ber function shall not be redeclared.

3 An  inline  member  function (whether static or nonstatic) may also be
  defined outside of its class definition provided either  its  declara-
  tion  in  the  class definition or its definition outside of the class
  definition declares the function as inline.  [Note:  member  functions
  of a class in namespace scope have external linkage.  Member functions
  of a local class (_class.local_) have no linkage.   See  _basic.link_.
  ]

4 There  shall be at most one definition of a non-inline member function
  in a program; no diagnostic is required.  There may be more  than  one
  inline  member  function definition in a program.  See _basic.def.odr_
  and _dcl.fct.spec_.

5 If the definition of a member function is lexically outside its  class
  definition,  the  member function name shall be qualified by its class
  name using the :: operator.  [Note: a name used in a  member  function
  definition (that is, in the parameter-declaration-clause including the
  default arguments (_dcl.fct.default_), or in the member function body,
  or,  for  a  constructor function (_class.ctor_), in a mem-initializer
  expression  (_class.base.init_))  is  looked  up   as   described   in
  _basic.lookup_.  ] [Example:
          struct X {
                  typedef int T;
                  static T count;
                  void f(T);
          };
          void X::f(T t = count) { }
  The member function f of class X is defined in global scope; the nota-
  tion X::f specifies that the function f is a member of class X and  in
  the  scope of class X.  In the function definition, the parameter type
  T refers to the typedef member T declared in class X and  the  default
  argument  count  refers  to  the  static data member count declared in
  class X.  ]

6 A static local variable in a member function always refers to the same
  object, whether or not the member function is inline.

7 Member  functions  may be mentioned in friend declarations after their
  class has been defined.

8 Member functions of a local class shall be  defined  inline  in  their
  class definition, if they are defined at all.

9 [Note:  a  member  function  can be declared (but not defined) using a
  typedef for a  function  type.   The  resulting  member  function  has
  exactly the same type as it would have if the function declarator were
  provided explicitly, see _dcl.fct_.  For example,
          typedef void fv(void);
          typedef void fvc(void) const;
          struct S {
                  fv memfunc1;  // equivalent to: void memfunc1(void);
                  void memfunc2();
                  fvc memfunc3; // equivalent to: void memfunc3(void) const;
          };
          fv  S::* pmfv1 = &S::memfunc1;
          fv  S::* pmfv2 = &S::memfunc2;
          fvc S::* pmfv3 = &S::memfunc3;
  Also see _temp.arg_.  ]

  9.3.1  Nonstatic member functions               [class.mfct.nonstatic]

1 A nonstatic member function may be called for an object of  its  class
  type,  or  for an object of a class derived (_class.derived_) from its
  class  type,  using  the  class  member  access  syntax   (_expr.ref_,
  _over.match.call_).   A  nonstatic  member function may also be called
  directly   using    the    function    call    syntax    (_expr.call_,
  _over.match.call_)

  --from within the body of a member function of its class or of a class
    derived from its class, or

  --from a mem-initializer (_class.base.init_) for a constructor for its
    class or for a class derived from its class.

  If  a  nonstatic  member function of a class X is called for an object
  that is not of type X, or of a type derived from X,  the  behavior  is
  undefined.

2 When an id-expression (_expr.prim_) that is not part of a class member
  access syntax (_expr.ref_) and not used to form a  pointer  to  member
  (_expr.unary.op_)  is  used in the body of a nonstatic member function
  of class X or used in the mem-initializer for a constructor  of  class
  X, if name lookup (_basic.lookup.unqual_) resolves the name in the id-
  expression to a nonstatic nontype member of class X or of a base class
  of  X,  the  id-expression  is  transformed into a class member access
  expression (_expr.ref_) using (*this) (_class.this_) as  the  postfix-
  expression  to  the  left  of  the  .  operator.  The member name then
  refers to the member of the object for which the function  is  called.
  Similarly  during  name  lookup,  when an unqualified-id (_expr.prim_)
  used in the definition of a member function for class X resolves to  a
  static  member, an enumerator or a nested type of class X or of a base
  class of X, the unqualified-id  is  transformed  into  a  qualified-id
  (_expr.prim_)  in  which  the nested-name-specifier names the class of
  the member function.  [Example:

          struct tnode {
                  char tword[20];
                  int count;
                  tnode *left;
                  tnode *right;
                  void set(char*, tnode* l, tnode* r);
          };
          void tnode::set(char* w, tnode* l, tnode* r)
          {
                  count = strlen(w)+1;
                  if (sizeof(tword)<=count)
                          perror("tnode string too long");
                  strcpy(tword,w);
                  left = l;
                  right = r;
          }
          void f(tnode n1, tnode n2)
          {
                  n1.set("abc",&n2,0);
                  n2.set("def",0,0);
          }
  In the body of the member function tnode::set, the member names tword,
  count,  left,  and  right refer to members of the object for which the
  function is called.  Thus,  in  the  call  n1.set("abc",&n2,0),  tword
  refers  to  n1.tword,  and in the call n2.set("def",0,0), it refers to
  n2.tword.  The functions strlen, perror, and strcpy are not members of
  the class tnode and should be declared elsewhere.3) ]

3 A  nonstatic member function may be declared const, volatile, or const
  volatile.  These cv-qualifiers affect the type  of  the  this  pointer
  (_class.this_).  They also affect the function type (_dcl.fct_) of the
  member function; a member function declared const is  a  const  member
  function,  a  member  function  declared volatile is a volatile member
  function and a member function declared  const  volatile  is  a  const
  volatile member function.  [Example:
          struct X {
                  void g() const;
                  void h() const volatile;
          };
  X::g  is  a  const member function and X::h is a const volatile member
  function.  ]

4 A nonstatic member function may be declared virtual  (_class.virtual_)
  or pure virtual (_class.abstract_).

  9.3.2  The this pointer                                   [class.this]

1 In the body of a nonstatic (_class.mfct_) member function, the keyword
  this is a non-lvalue expression whose value  is  the  address  of  the
  object for which the function is called.  The type of this in a member
  function of a class X is X*.   If  the  member  function  is  declared
  _________________________
  3) See, for example, <cstring> (_lib.c.strings_).

  const,  the  type  of  this  is  const  X*,  if the member function is
  declared volatile, the type of this is volatile X*, and if the  member
  function  is  declared  const  volatile,  the  type  of  this is const
  volatile X*.

2 In a const member function, the  object  for  which  the  function  is
  called  is  accessed  through  a const access path; therefore, a const
  member function shall not modify the object and  its  non-static  data
  members.  [Example:
          struct s {
              int a;
              int f() const;
              int g() { return a++; }
              int h() const { return a++; } // error
          };

          int s::f() const { return a; }
  The  a++  in the body of s::h is ill-formed because it tries to modify
  (a part of) the object for  which  s::h()  is  called.   This  is  not
  allowed in a const member function because this is a pointer to const;
  that is, *this has const type.  ]

3 Similarly, volatile semantics (_dcl.type.cv_) apply in volatile member
  functions when accessing the object and its non-static data members.

4 A  cv-qualified  member function can be called on an object-expression
  (_expr.ref_) only if the object-expression is as cv-qualified or less-
  cv-qualified than the member function.  [Example:
          void k(s& x, const s& y)
          {
              x.f();
              x.g();
              y.f();
              y.g();      // error
          }
  The  call  y.g() is ill-formed because y is const and s::g() is a non-
  const member function, that is,  s::g()  is  less-qualified  than  the
  object-expression y.  ]

5 Constructors  (_class.ctor_)  and destructors (_class.dtor_) shall not
  be declared const, volatile or const volatile.  [Note: However,  these
  functions  can be invoked to create and destroy objects with cv-quali-
  fied types, see (_class.ctor_) and (_class.dtor_).  ]

  9.4  Static members                                     [class.static]

1 A data or function member of a class may be declared static in a class
  definition, in which case it is a static member of the class.

2 A static member s of class X may be referred to using the qualified-id
  expression X::s; it is not necessary to use the  class  member  access
  syntax  (_expr.ref_) to refer to a static member.  A static member may
  be referred to using the class member access syntax, in which case the
  object-expression is always evaluated.  [Example:

          class process {
          public:
                  static void reschedule();
          };
          process& g();
          void f()
          {
                  process::reschedule(); // ok: no object necessary
                  g().reschedule();      // g() is called
          }
    --end  example]  A  static member may be referred to directly in the
  scope  of  its  class  or  in   the   scope   of   a   class   derived
  (_class.derived_)  from  its class; in this case, the static member is
  referred to as if a qualified-id expression was used, with the nested-
  name-specifier  of  the qualified-id naming the class scope from which
  the static member is referenced.  [Example:
          int g();
          struct X {
                  static int g();
          };
          struct Y : X {
                  static int i;
          };
          int Y::i = g(); // equivalent to Y::g();
   --end example]

3 If an unqualified-id (_expr.prim_) is used  in  the  definition  of  a
  static  member  following  the member's declarator-id, and name lookup
  (_basic.lookup.unqual_) finds that  the  unqualified-id  refers  to  a
  static member, enumerator, or nested type of the member's class (or of
  a base class of the member's class), the unqualified-id is transformed
  into  a  qualified-id  expression  in  which the nested-name-specifier
  names the class scope from which the member is referenced.  The  defi-
  nition of a static member shall not use directly the names of the non-
  static members of its class or of a base class of its class (including
  as  operands of the sizeof operator).  The definition of a static mem-
  ber may only refer  to  these  members  to  form  pointer  to  members
  (_expr.unary.op_) or with the class member access syntax (_expr.ref_).

4 Static  members   obey   the   usual   class   member   access   rules
  (_class.access_).  When used in the declaration of a class member, the
  static specifier shall only be used in the  member  declarations  that
  appear  within  the  member-specification  of  the  class declaration.
  [Note: it cannot be specified in member declarations  that  appear  in
  namespace scope.  ]

  9.4.1  Static member functions                     [class.static.mfct]

1 [Note:  the  rules  described  in  _class.mfct_ apply to static member
  functions.  ]

2 [Note:  a  static  member  function  does  not  have  a  this  pointer
  (_class.this_).   ]  A  static  member  function shall not be virtual.
  There shall not be a static and a nonstatic member function  with  the

  same name and the same parameter types (_over.load_).  A static member
  function shall not be declared const, volatile, or const volatile.

  9.4.2  Static data members                         [class.static.data]

1 A static data member is not part of the subobjects of a class.   There
  is  only one copy of a static data member shared by all the objects of
  the class.

2 The declaration of a static data member in its class definition is not
  a  definition and may be of an incomplete type other than cv-qualified
  void.  A definition shall be provided for the static data member if it
  is used (_basic.def.odr_) in the program.  The definition shall appear
  in a namespace scope enclosing the member's class definition.  In  the
  definition  at  namespace  scope,  the  name of the static data member
  shall be qualified by its class name using the :: operator.  The  ini-
  tializer  expression  in  the definition of a static data member is in
  the scope of its class (_basic.scope.class_).  [Example:
          class process {
                  static process* run_chain;
                  static process* running;
          };
          process* process::running = get_main();
          process* process::run_chain = running;
  The static data member run_chain of class process is defined in global
  scope;  the  notation  process::run_chain  specifies  that  the member
  run_chain is a member of class process and in the scope of class  pro-
  cess.   In  the static data member definition, the initializer expres-
  sion refers to the static data member running of class process.  ]

3 [Note: once the static data member has been defined, it exists even if
  no  objects  of its class have been created.  [Example: in the example
  above, run_chain and running exist even if no objects of class process
  are created by the program.  ] ]

4 If  a  static  data  member  is of const integral or const enumeration
  type, its declaration in the class definition can specify a  constant-
  initializer   which   shall   be   an   integral  constant  expression
  (_expr.const_).  In that case, the member can appear in integral  con-
  stant expressions within its scope.  The member shall still be defined
  in a namespace scope if it is used in the program  and  the  namespace
  scope definition shall not contain an initializer.

5 There  shall be exactly one definition of a static data member that is
  used in a program; no diagnostic  is  required;  see  _basic.def.odr_.
  Unnamed  classes  and  classes contained directly or indirectly within
  unnamed classes shall not contain static data members.  [Note: this is
  because  there  is  no  mechanism  to provide the definitions for such
  static data members.  ]

6 Static data members of a class in namespace scope have external  link-
  age (_basic.link_).  A local class shall not have static data members.

7 Static data members are initialized and destroyed  exactly  like  non-
  local objects (_basic.start.init_, _basic.start.term_).

8 A static data member shall not be mutable (_dcl.stc_).

  9.5  Unions                                              [class.union]

1 In a union, at most one of the data members can be active at any time,
  that is, the value of at most one of the data members can be stored in
  a union at any time.  The size of a union is sufficient to contain the
  largest of its data members.  Each data member is allocated as  if  it
  were  the  sole member of a struct.  A union can have member functions
  (including constructors and destructors), but not virtual (_class.vir-
  tual_) functions.  A union shall not have base classes.  A union shall
  not be used as a base class.  An object of a class with a  non-trivial
  default  constructor  (_class.ctor_),  a  non-trivial copy constructor
  (_class.copy_), a non-trivial destructor  (_class.dtor_),  or  a  non-
  trivial  copy assignment operator (_over.ass_, _class.copy_) cannot be
  a member of a union, nor can an array of such  objects.   If  a  union
  contains a static data member, or a member of reference type, the pro-
  gram is ill-formed.

2 A union of the form
          union { member-specification } ;
  is called an anonymous union; it defines an unnamed object of  unnamed
  type.   The  member-specification  of  an  anonymous  union shall only
  define non-static data members.  [Note:  nested  types  and  functions
  cannot be declared within an anonymous union.  ] The names of the mem-
  bers of an anonymous union shall be distinct from  the  names  of  any
  other  entity  in  the scope in which the anonymous union is declared.
  For the purpose of name look up, after the anonymous union definition,
  the members of the anonymous union are considered to have been defined
  in the scope in which the anonymous union is declared.  [Example:
          void f()
          {
              union { int a; char* p; };
              a = 1;
              // ...
              p = "Jennifer";
              // ...
          }
  Here a and p are used like ordinary (nonmember) variables,  but  since
  they are union members they have the same address.  ]

3 Anonymous unions declared at namespace scope shall be declared static.
  Anonymous unions declared at block scope shall be  declared  with  any
  storage  class  allowed for a block-scope variable, or with no storage
  class.  A storage class is not allowed in a declaration of  an  anony-
  mous  union  in a class scope.  An anonymous union shall not have pri-
  vate or protected members (_class.access_).  An anonymous union  shall
  not have function members.

4 A union for which objects or pointers are declared is not an anonymous
  union.  [Example:

          union { int aa; char* p; } obj, *ptr = &obj;
          aa = 1;       // error
          ptr->aa = 1;  // ok
  The assignment to plain aa is ill formed since the member name is  not
  visible  outside  the  union,  and  even if it were visible, it is not
  associated with any particular object.   ]  [Note:  Initialization  of
  unions   with   no   user-declared   constructors   is   described  in
  (_dcl.init.aggr_).  ]

  9.6  Bit-fields                                            [class.bit]

1 A member-declarator of the form
          identifieropt : constant-expression
  specifies a bit-field; its length is set off from the  bit-field  name
  by  a  colon.   The bit-field attribute is not part of the type of the
  class member.  The constant-expression shall be an integral  constant-
  expression  with a value greater than or equal to zero.  The constant-
  expression may be larger than the number of bits in the object  repre-
  sentation  (_basic.types_)  of the bit-field's type; in such cases the
  extra bits are used as padding bits and  do  not  participate  in  the
  value  representation (_basic.types_) of the bit-field.  Allocation of
  bit-fields within a class object is implementation-defined.  Alignment
  of  bit-fields  is implementation-defined.  Bit-fields are packed into
  some addressable allocation unit.  [Note: bit-fields straddle  alloca-
  tion  units  on  some  machines  and  not  on  others.  Bit-fields are
  assigned right-to-left on some machines, left-to-right on others.  ]

2 A declaration for a bit-field that omits the  identifier  declares  an
  unnamed  bit-field.   Unnamed bit-fields are not members and cannot be
  initialized.  [Note: an unnamed bit-field is  useful  for  padding  to
  conform  to  externally-imposed  layouts.   ]  As  a  special case, an
  unnamed bit-field with a width of zero specifies alignment of the next
  bit-field  at  an  allocation  unit  boundary.  Only when declaring an
  unnamed bit-field may the constant-expression  be  a  value  equal  to
  zero.

3 A  bit-field  shall  not  be  a static member.  A bit-field shall have
  integral or enumeration type (_basic.fundamental_).  It is implementa-
  tion-defined  whether a plain (neither explicitly signed nor unsigned)
  char, short, int or long bit-field is  signed  or  unsigned.   A  bool
  value  can  successfully be stored in a bit-field of any nonzero size.
  The address-of operator & shall not be  applied  to  a  bit-field,  so
  there  are no pointers to bit-fields.  A non-const reference shall not
  be bound to a bit-field (_dcl.init.ref_).  [Note: if  the  initializer
  for  a  reference  of type const T& is an lvalue that refers to a bit-
  field, the reference is bound to a temporary initialized to  hold  the
  value  of  the  bit-field; the reference is not bound to the bit-field
  directly.  See _dcl.init.ref_.  ]

4 If the value true or false is stored into a bit-field of type bool  of
  any  size (including a one bit bit-field), the original bool value and
  the value of the bit-field shall compare equal.  If the  value  of  an
  enumerator is stored into a bit-field of the same enumeration type and
  the number of bits in the bit-field is large enough to  hold  all  the

  values of that enumeration type, the original enumerator value and the
  value of the bit-field shall compare equal.  [Example:
          enum BOOL { f=0, t=1 };
          struct A {
                  BOOL b:1;
          };
          A a;
          void f() {
                  a.b = t;
                  if (a.b == t) // shall yield true
                  { /* ... */ }
          }
   --end example]

  9.7  Nested class declarations                            [class.nest]

1 A class can be defined within another class.  A class  defined  within
  another is called a nested class.  The name of a nested class is local
  to its enclosing class.  The nested class  is  in  the  scope  of  its
  enclosing  class.   Except by using explicit pointers, references, and
  object names, declarations in a nested class can use only type  names,
  static members, and enumerators from the enclosing class.  [Example:
          int x;
          int y;

          class enclose {
          public:
              int x;
              static int s;
              class inner {
                  void f(int i)
                  {
                      int a = sizeof(x); // error: refers to enclose::x
                      x = i;   // error: assign to enclose::x
                      s = i;   // ok: assign to enclose::s
                      ::x = i; // ok: assign to global x
                      y = i;       // ok: assign to global y
                  }
                  void g(enclose* p, int i)
                  {
                      p->x = i;   // ok: assign to enclose::x
                  }
              };
          };

          inner* p = 0;   // error `inner' not in scope
   --end example]

2 Member  functions  and  static  data  members of a nested class can be
  defined in a namespace scope enclosing the definition of their  class.
  [Example:

          class enclose {
          public:
              class inner {
                  static int x;
                  void f(int i);
              };
          };
          int enclose::inner::x = 1;

          void enclose::inner::f(int i) { /* ... */ }
   --end example]

3 If  class  X  is defined in a namespace scope, a nested class Y may be
  declared in class X and later defined in the definition of class X  or
  be  later  defined  in  a  namespace scope enclosing the definition of
  class X.  [Example:
          class E {
              class I1;      // forward declaration of nested class
              class I2;
              class I1 {};  // definition of nested class
          };
          class E::I2 {};   // definition of nested class
   --end example]

4 Like a member function, a  friend  function  (_class.friend_)  defined
  within  a nested class is in the lexical scope of that class; it obeys
  the same rules for name binding as a static member  function  of  that
  class  (_class.static_) and has no special access rights to members of
  an enclosing class.

  9.8  Local class declarations                            [class.local]

1 A class can be defined within a function definition; such a  class  is
  called  a  local  class.   The  name  of a local class is local to its
  enclosing scope.  The local class is in the  scope  of  the  enclosing
  scope.   Declarations in a local class can use only type names, static
  variables, extern variables and functions, and  enumerators  from  the
  enclosing scope.  [Example:
          int x;
          void f()
          {
              static int s ;
              int x;
              extern int g();
              struct local {
                  int g() { return x; }    // error: `x' is auto
                  int h() { return s; }    // ok
                  int k() { return ::x; }  // ok
                  int l() { return g(); }  // ok
              };
              // ...
          }

          local* p = 0;   // error: `local' not in scope

   --end example]

2 An  enclosing  function  has no special access to members of the local
  class; it obeys the usual access rules (_class.access_).  Member func-
  tions of a local class shall be defined within their class definition,
  if they are defined at all.

3 If class X is a local class a nested class Y may be declared in  class
  X  and  later defined in the definition of class X or be later defined
  in the same scope as the definition of class X.  A class nested within
  a local class is a local class.

4 A local class shall not have static data members.

  9.9  Nested type names                             [class.nested.type]

1 Type  names obey exactly the same scope rules as other names.  In par-
  ticular, type names defined within a class definition cannot  be  used
  outside their class without qualification.  [Example:
          class X {
          public:
              typedef int I;
              class Y { /* ... */ };
              I a;
          };

          I b;     // error
          Y c;     // error
          X::Y d;  // ok
          X::I e;  // ok
   --end example]