11   Member access control                    [class.access]


1 A member of a class can be

  --private; that is, its name can be used  only  by  member  functions,
    static  data  members,  and  friends  of  the  class  in which it is

  --protected; that is, its name can be used only by  member  functions,
    static  data  members,  and  friends  of  the  class  in which it is
    declared and by member functions, static data members,  and  friends
    of classes derived from this class (see _class.protected_).

  --public;  that  is,  its  name  can  be  used anywhere without access

2 Members of a class defined with  the  keyword  class  are  private  by
  default.  Members of a class defined with the keywords struct or union
  are public by default.  [Example:
          class X {
              int a;  // X::a is private by default
          struct S {
              int a;  // S::a is public by default
   --end example]

3 Access control is applied uniformly to all  names.   In  the  case  of
  overloaded  function  names, access control is applied to the function
  selected  by  overload  resolution.   [Note:  because  access  control
  applies to names, if access control is applied to a typedef name, only
  the accessibility of the  typedef  name  itself  is  considered.   The
  accessibility  of the entity referred to by the typedef is not consid­
  ered.  For example,
          class A
                  class B { };
                  typedef B BB;

          void f()
                  A::BB x; // ok, typedef name A::BB is public
                  A::B y;  // access error, A::B is private
   --end note]

4 It should be noted that it is access to members and base classes  that
  is controlled, not their visibility.  Names of members are still visi­
  ble, and implicit conversions to base classes  are  still  considered,
  when those members and base classes are inaccessible.  The interpreta­
  tion of a given construct is established without regard to access con­
  trol.   If  the  interpretation  established makes use of inaccessible
  member names or base classes, the construct is ill-formed.

5 All access controls in this clause affect  the  ability  to  access  a
  class member name from a particular scope.  In particular, access con­
  trols apply as usual to member names accessed as part  of  a  function
  return  type,  even  though it is not possible to determine the access
  privileges of that use without first parsing the rest of the function.
          class A {
              typedef int I;      // private member
              I f();
              friend I g(I);
              static I x;

          A::I A::f() { return 0; }
          A::I g(A::I);
          A::I g(A::I p) { return 0; }
          A::I A::x = 0;
  Here,  all  the uses of A::I are well-formed because A::f and A::x are
  members of class A and g is a friend of class A.   This  implies,  for
  example,  that  access  checking  on  the  first  use  of A::I must be
  deferred until it is determined that this use of A::I is as the return
  type of a member of class A.   --end example]

6 It  is  necessary  to  name a class member to define it outside of the
  definition of its class.  For this reason, no access checking is  per­
  formed  on  the components of the qualified-id used to name the member
  in the declarator of such a definition.  [Example:
          class D {
              class E {
                  static int m;
          int D::E::m = 1;   // Okay, no access error on private `E'
   --end example]

  11.1  Access specifiers                            [class.access.spec]

1 Member  declarations   can   be   labeled   by   an   access-specifier
          access-specifier : member-specificationopt
  An  access-specifier  specifies the access rules for members following
  it until the end of the class or  until  another  access-specifier  is
  encountered.  [Example:
          class X {
              int a;  // X::a is private by default: `class' used
              int b;  // X::b is public
              int c;  // X::c is public
   --end example] Any number of access specifiers is allowed and no par­
  ticular order is required.  [Example:
          struct S {
              int a;  // S::a is public by default: `struct' used
              int b;  // S::b is protected
              int c;  // S::c is private
              int d;  // S::d is public
   --end example]

2 The order of allocation of data members with separate access-specifier
  labels is unspecified (_class.mem_).

3 When  a  member  is redeclared within its class definition, the access
  specified at its redeclaration shall be the same  as  at  its  initial
  declaration.  [Example:
  struct S {
          class A;
          class A { };    // error: cannot change access
   --end example]

  11.2  Access specifiers for base classes           [class.access.base]

1 If  a  class  is  declared  to  be  a base class (_class.derived_) for
  another class using the public access specifier, the public members of
  the  base  class are accessible as public members of the derived class
  and protected members of the base class are  accessible  as  protected
  members  of  the  derived  class.  If a class is declared to be a base
  class for another class using the protected access specifier, the pub­
  lic  and  protected  members  of the base class are accessible as pro­
  tected members of the derived class.  If a class is declared to  be  a
  base  class  for another class using the private access specifier, the
  public and protected members of the base class are accessible as  pri­
  vate members of the derived class1).
  1)  As  specified  previously  in _class.access_, private members of a

2 In the absence of an access-specifier for  a  base  class,  public  is
  assumed  when  the  derived  class  is  declared struct and private is
  assumed when the class is declared class.  [Example:
          class B { /* ... */ };
          class D1 : private B { /* ... */ };
          class D2 : public B { /* ... */ };
          class D3 : B { /* ... */ };    // `B' private by default
          struct D4 : public B { /* ... */ };
          struct D5 : private B { /* ... */ };
          struct D6 : B { /* ... */ };   // `B' public by default
          class D7 : protected B { /* ... */ };
          struct D8 : protected B { /* ... */ };
  Here B is a public base of D2, D4, and D6, a private base of  D1,  D3,
  and D5, and a protected base of D7 and D8.   --end example]

3 [Note:  A  member  of a private base class might be inaccessible as an
  inherited member name, but accessible directly.  Because of the  rules
  on  pointer conversions (_conv.ptr_) and explicit casts (_expr.cast_),
  a conversion from a pointer to a derived class  to  a  pointer  to  an
  inaccessible  base class might be ill-formed if an implicit conversion
  is used, but well-formed if an explicit cast is used.  For example,
          class B {
                  int mi;         // nonstatic member
                  static int si;  // static member
          class D : private B {
          class DD : public D {
                  void f();
          void DD::f() {
                  mi = 3;        // error: mi is private in D
                  si = 3;        // error: si is private in D
                  B  b;
                  b.mi = 3;      // okay (b.mi is different from this->mi)
                  b.si = 3;      // okay (b.si is different from this->si)
                  B::si = 3;     // okay
                  B* bp1 = this; // error: B is a private base class
                  B* bp2 = (B*)this;  // okay with cast
                  bp2->mi = 3;   // okay: access through a pointer to B.
   --end note]

4 A base class is said to be accessible if an invented public member  of
  the  base class is accessible.  If a base class is accessible, one can
  implicitly convert a pointer to a derived class to a pointer  to  that
  base  class  (_conv.ptr_, _conv.mem_).  [Note: It follows that members
  base class remain inaccessible even to derived classes  unless  friend
  declarations  within  the base class declaration are used to grant ac­
  cess explicitly.

  and friends of a class X can implicitly convert an X* to a pointer  to
  a private or protected immediate base class of X.  ]

  11.3  Access declarations                           [class.access.dcl]

1 The  access  of a member of a base class can be changed in the derived
  class by mentioning its qualified-id in the derived class declaration.
  Such  mention  is called an access declaration.  The base class member
  is given, in the derived class, the access in effect  in  the  derived
  class  declaration at the point of the access declaration.  The effect
  of an access declaration qualified-id ; is defined to be equivalent to
  the declaration using qualified-id ;.2)

2 [Example:
          class A {
              int z;
              int z1;
          class B : public A {
              int a;
              int b, c;
              int bf();
              int x;
              int y;
          class D : private B {
              int d;
              B::c;  // adjust access to `B::c'
              B::z;  // adjust access to `A::z'
              A::z1; // adjust access to `A::z1'
              int e;
              int df();
              B::x;  // adjust access to `B::x'
              int g;

  2)  Access  declarations  are  deprecated;  member  using-declarations
  (_namespace.udecl_) provide a better means of doing the  same  things.
  In earlier versions of the C++ language, access declarations were more
  limited;  they  were  generalized  and  made  equivalent   to   using-
  declarations  in  the interest of simplicity.  Programmers are encour­
  aged to use using, rather than the new capabilities of access declara­
  tions, in new code.

          class X : public D {
              int xf();

          int ef(D&);
          int ff(X&);
  The  external  function ef can use only the names c, z, z1, e, and df.
  Being a member of D, the function df can use the names b,  c,  z,  z1,
  bf,  x, y, d, e, df, and g, but not a.  Being a member of B, the func­
  tion bf can use the members a, b, c, z, z1, bf, x, and y.   The  func­
  tion  xf can use the public and protected names from D, that is, c, z,
  z1, e, and df (public), and x, and g (protected).  Thus  the  external
  function  ff has access only to c, z, z1, e, and df.  If D were a pro­
  tected or private base class of X, xf would have the  same  privileges
  as before, but ff would have no access at all.  ]

  11.4  Friends                                           [class.friend]

1 A friend of a class is a function or class that is not a member of the
  class but is permitted to use the private and protected  member  names
  from  the  class.   The  name  of  a friend is not in the scope of the
  class, and the friend is not called with the member  access  operators
  (_expr.ref_)  unless  it  is a member of another class.  [Example: the
  following example illustrates  the  differences  between  members  and
          class X {
              int a;
              friend void friend_set(X*, int);
              void member_set(int);
          void friend_set(X* p, int i) { p->a = i; }
          void X::member_set(int i) { a = i; }
          void f()
              X obj;
   --end example]

2 When  a  friend  declaration refers to an overloaded name or operator,
  only the function specified by the parameter types becomes  a  friend.
  A  member  function of a class X can be a friend of a class Y.  [Exam­
          class Y {
              friend char* X::foo(int);
              // ...
   --end example] Declaring a class to be a friend implies that  private
  and protected names from the class granting friendship can be accessed
  in declarations of members of the befriended  class.   An  elaborated-
  type-specifier  shall be used in a friend declaration for a class; see

  _dcl.type.elab_ .3) [Note: _dcl.type.elab_ further describes the  syn­
  tax of friend class declarations.  ] [Example:
          class X {
              enum { a=100 };
              friend class Y;
          class Y {
              int v[X::a];  // ok, Y is a friend of X
          class Z {
              int v[X::a];  // error: X::a is private
    --end example] Access to private and protected names is also granted
  to member functions of the friend class (as if the functions were each
  friends)  and  to  the  static  data  member definitions of the friend

3 [Note: _basic.scope.pdecl_ describes the point  of  declaration  of  a
  class  name  or  a function name first introduced by a friend declara­
  tion.  ]

4 A function first declared in a friend declaration has external linkage
  (_basic.link_).   Otherwise,   it   retains   its   previous   linkage
  (_dcl.stc_).  No storage-class-specifier shall  appear  in  the  decl-
  specifier-seq of a friend declaration.

5 A  function  can  be defined in a friend declaration of a class if and
  only if the class is a non-local class (_class.local_),  the  function
  name is unqualified, and the function has namespace scope.  [Example:
          class M {
                  friend void f() { } // definition of global f, a friend of M,
                                      // not the definition of a member function
    --end example] Such a function is implicitly inline.  A friend func­
  tion defined in a class is in the (lexical)  scope  of  the  class  in
  which  it  is defined.  A friend function defined outside the class is
  not (_basic.lookup.unqual_).

6 Friend   declarations   are   not   affected   by    access-specifiers

7 Friendship is neither inherited nor transitive.  [Example:
          class A {
              friend class B;
              int a;
          class B {
              friend class C;

  3) The class-key of the elaborated-type-specifier is required.

          class C  {
              void f(A* p)
                  p->a++;  // error: C is not a friend of A
                           // despite being a friend of a friend
          class D : public B  {
              void f(A* p)
                  p->a++;  // error: D is not a friend of A
                           // despite being derived from a friend
   --end example]

  11.5  Protected member access                        [class.protected]

1 When  a  friend  or  a member function of a derived class references a
  protected nonstatic member of a base class, an access check applies in
  addition to those described earlier  in  this  clause.4)  Except  when
  forming  a  pointer  to  member  (_expr.unary.op_), the access must be
  through a pointer to, reference to, or object  of  the  derived  class
  itself  (or  any  class derived from that class) (_expr.ref_).  If the
  access is to form a pointer to member, the nested-name-specifier shall
  name the derived class (or any class derived from that class).  [Exam­
          class B {
              int i;
              static int j;

          class D1 : public B {

          class D2 : public B {
              friend void fr(B*,D1*,D2*);
              void mem(B*,D1*);

  4) This additional check does not apply to other members, e.g.  static
  data members.

          void fr(B* pb, D1* p1, D2* p2)
              pb->i = 1;  // ill-formed
              p1->i = 2;  // ill-formed
              p2->i = 3;  // ok (access through a D2)
              p2->B::i = 4;  // ok (access through a D2, qualification ignored)
              int B::* pmi_B = &B::i;    // ill-formed
              int B::* pmi_B2 = &D2::i;  // ok (type of &D2::i is "int B::*")
              B::j = 5;   // ok (because refers to static member)
              D2::j =6;   // ok (because refers to static member)
          void D2::mem(B* pb, D1* p1)
              pb->i = 1;  // ill-formed
              p1->i = 2;  // ill-formed
              i = 3;      // ok (access through `this')
              B::i = 4;   // ok (access through `this', qualification ignored)
              j = 5;      // ok (because refers to static member)
              B::j = 6;   // ok (because refers to static member)
          void g(B* pb, D1* p1, D2* p2)
              pb->i = 1;  // ill-formed
              p1->i = 2;  // ill-formed
              p2->i = 3;  // ill-formed
   --end example]

  11.6  Access to virtual functions                  [class.access.virt]

1 The access rules (_class.access_) for a virtual  function  are  deter­
  mined by its declaration and are not affected by the rules for a func­
  tion that later overrides it.  [Example:
          class B {
              virtual int f();

          class D : public B {
              int f();
          void f()
              D d;
              B* pb = &d;
              D* pd = &d;

              pb->f();  // ok: B::f() is public,
                        // D::f() is invoked
              pd->f();  // error: D::f() is private
   --end example] Access is checked at the call point using the type  of

  the expression used to denote the object for which the member function
  is called (B* in the example above).  The access of the  member  func­
  tion  in the class in which it was defined (D in the example above) is
  in general not known.

  11.7  Multiple access                                    [class.paths]

1 If a name can be reached by several paths through a  multiple  inheri­
  tance  graph,  the  access is that of the path that gives most access.
          class W { public: void f(); };
          class A : private virtual W { };
          class B : public virtual W { };
          class C : public A, public B {
              void f() { W::f(); }  // ok
  Since W::f() is available to C::f() along the public path  through  B,
  access is allowed.  ]