7   Declarations                                             [dcl.dcl]


1 Declarations specify how names are to  be  interpreted.   Declarations
  have the form
                  declaration-seq declaration
                  decl-specifier-seqopt init-declarator-listopt ;
  [Note:  asm-definitions are described in _dcl.asm_, and linkage-speci­
  fications  are  described  in  _dcl.link_.   Function-definitions  are
  described  in _dcl.fct.def_ and template-declarations are described in
  _temp_.   Namespace-definitions  are  described  in   _namespace.def_,
  using-declarations are described in _namespace.udecl_ and using-direc­
  tives are described in _namespace.udir_.  ] The simple-declaration
          decl-specifier-seqopt init-declarator-listopt ;
  is divided into two parts: decl-specifiers, the components of a  decl-
  specifier-seq, are described in _dcl.spec_ and declarators, the compo­
  nents of an init-declarator-list, are described in _dcl.decl_.

2 A declaration occurs in a scope (_basic.scope_); the scope  rules  are
  summarized  in _basic.lookup_.  A declaration that declares a function
  or defines a class, namespace, template, or function also has  one  or
  more  scopes  nested within it. These nested scopes, in turn, can have
  declarations nested within them. Unless otherwise  stated,  utterances
  in  this clause about components in, of, or contained by a declaration
  or subcomponent thereof refer only to those components of the declara­
  tion  that are not nested within scopes nested within the declaration.

3 In a simple-declaration,  the  optional  init-declarator-list  can  be
  omitted   only   when  declaring  a  class  (_class_)  or  enumeration
  (_dcl.enum_), that is, when the decl-specifier-seq contains  either  a
  class-specifier,   an   elaborated-type-specifier   with  a  class-key

  (_class.name_), or an enum-specifier.  In these cases and  whenever  a
  class-specifier  or  enum-specifier  is present in the decl-specifier-
  seq, the identifiers in these specifiers are  among  the  names  being
  declared  by  the declaration (as class-names, enum-names, or enumera­
  tors, depending on the syntax).  In such cases,  and  except  for  the
  declaration of an unnamed bit-field (_class.bit_), the decl-specified-
  seq shall introduce one or more names into the program, or shall rede­
  clare a name introduced by a previous declaration.  [Example:
          enum { };          // ill-formed
          typedef class { }; // ill-formed
   --end example]

4 Each  init-declarator in the init-declarator-list contains exactly one
  declarator-id, which is the name declared by that init-declarator  and
  hence  one  of the names declared by the declaration.  The type-speci­
  fiers  (_dcl.type_)  in  the  decl-specifier-seq  and  the   recursive
  declarator   structure   of   the   init-declarator  describe  a  type
  (_dcl.meaning_), which is then associated with the name being declared
  by the init-declarator.

5 If the decl-specifier-seq contains the typedef specifier, the declara­
  tion is called a typedef  declaration  and  the  name  of  each  init-
  declarator is declared to be a typedef-name, synonymous with its asso­
  ciated type (_dcl.typedef_).  If the  decl-specifier-seq  contains  no
  typedef specifier, the declaration is called a function declaration if
  the type associated with the name is a function type  (_dcl.fct_)  and
  an object declaration otherwise.

6 Syntactic  components beyond those found in the general form of decla­
  ration are added to a function declaration to make a  function-defini­
  tion.   An object declaration, however, is also a definition unless it
  contains the extern specifier and has no initializer (_basic.def_).  A
  definition causes the appropriate amount of storage to be reserved and
  any appropriate initialization (_dcl.init_) to be done.

7 Only in function declarations for constructors, destructors, and  type
  conversions can the decl-specifier-seq be omitted.1)

  7.1  Specifiers                                             [dcl.spec]

1 The specifiers that can be used in a declaration are
                  decl-specifier-seqopt decl-specifier

  1) The "implicit int" rule of C is no longer supported.

2 The  longest sequence of decl-specifiers that could possibly be a type
  name is  taken  as  the  decl-specifier-seq  of  a  declaration.   The
  sequence shall be self-consistent as described below.  [Example:
          typedef char* Pc;
          static Pc;              // error: name missing
  Here,  the  declaration  static  Pc  is ill-formed because no name was
  specified for the static variable of  type  Pc.   To  get  a  variable
  called  Pc,  a type-specifier (other than const or volatile) has to be
  present to indicate  that  the  typedef-name  Pc  is  the  name  being
  (re)declared,  rather  than being part of the decl-specifier sequence.
  For another example,
          void f(const Pc);       // void f(char* const)  (not const char*)
          void g(const int Pc);   // void g(const int)
   --end example]

3 [Note: since signed, unsigned, long, and short by default imply int, a
  type-name  appearing  after  one of those specifiers is treated as the
  name being (re)declared.  [Example:
          void h(unsigned Pc);       // void h(unsigned int)
          void k(unsigned int Pc);   // void k(unsigned int)
   --end example]  --end note]

  7.1.1  Storage class specifiers                              [dcl.stc]

1 The storage class specifiers are
  At most one storage-class-specifier shall appear in a given decl-spec­
  ifier-seq.   If a storage-class-specifier appears in a decl-specifier-
  seq, there can be no typedef specifier in the same  decl-specifier-seq
  and  the  init-declarator-list  of  the declaration shall not be empty
  (except for global anonymous unions, which shall  be  declared  static
  (_class.union_).   The  storage-class-specifier  applies  to  the name
  declared by each init-declarator in the list  and  not  to  any  names
  declared  by other specifiers.  A storage-class-specifier shall not be
  specified in  an  explicit  specialization  (_temp.expl.spec_)  or  an
  explicit instantiation (_temp.explicit_) directive.

2 The  auto  or  register  specifiers  can  be  applied only to names of
  objects declared in a block (_stmt.block_) or to  function  parameters
  (_dcl.fct.def_).   They  specify  that  the named object has automatic
  storage duration (_basic.stc.auto_).  An  object  declared  without  a
  storage-class-specifier  at  block  scope  or  declared  as a function
  parameter has automatic storage duration by default.  Hence, the  auto
  specifier  is  almost  always redundant and not often used; one use of
  auto is to distinguish a  declaration-statement  from  an  expression-
  statement (_stmt.expr_) explicitly.

3 A  register  specifier  has  the  same  semantics as an auto specifier
  together with a hint to the implementation that the object so declared

  will be heavily used.  The hint can be ignored and in most implementa­
  tions it will be ignored if the address of the object is taken.

4 The static specifier can be applied only to names of objects and func­
  tions and to anonymous unions (_class.union_).  There can be no static
  function declarations within a block, nor any static function  parame­
  ters.   A  static  specifier  used  in  the  declaration  of an object
  declares   the   object    to    have    static    storage    duration
  (_basic.stc.static_).   A static specifier can be used in declarations
  of class members; _class.static_ describes its effect.  For the  link­
  age of a name declared with a static specifier, see _basic.link_.

5 The  extern  specifier can be applied only to the names of objects and
  functions.  The extern specifier cannot be used in the declaration  of
  class  members  or  function  parameters.   For  the linkage of a name
  declared with an extern specifier, see _basic.link_.

6 A name declared in a namespace scope without a storage-class-specifier
  has  external linkage unless it has internal linkage because of a pre­
  vious declaration and provided it  is  not  declared  const.   Objects
  declared  const and not explicitly declared extern have internal link­

7 The linkages implied by successive declarations  for  a  given  entity
  shall  agree.  That is, within a given scope, each declaration declar­
  ing the same object name or the same overloading of  a  function  name
  shall  imply  the same linkage.  Each function in a given set of over­
  loaded functions can have a different linkage, however.  [Example:
          static char* f(); // f() has internal linkage
          char* f()         // f() still has internal linkage
              { /* ... */ }
          char* g();        // g() has external linkage
          static char* g()  // error: inconsistent linkage
              { /* ... */ }
          void h();
          inline void h();  // external linkage
          inline void l();
          void l();         // external linkage
          inline void m();
          extern void m();  // external linkage
          static void n();
          inline void n();  // internal linkage
          static int a;     // `a' has internal linkage
          int a;            // error: two definitions
          static int b;     // `b' has internal linkage
          extern int b;     // `b' still has internal linkage
          int c;            // `c' has external linkage
          static int c;     // error: inconsistent linkage
          extern int d;     // `d' has external linkage
          static int d;     // error: inconsistent linkage
   --end example]

8 The name of a declared but undefined class can be used  in  an  extern
  declaration.   Such  a declaration, however, cannot be used before the

  class has been defined.  [Example:
          struct S;
          extern S a;
          extern S f();
          extern void g(S);

          void h()
              g(a);       // error: S undefined
              f();        // error: S undefined
   --end example] The mutable specifier can be applied only to names  of
  class  data  members  (_class.mem_)  and  can  not be applied to names
  declared const or static.  [Example:
          class X {
                  mutable const int* p;   // ok
                  mutable int* const q;   // ill-formed
   --end example]

9 The mutable specifier on a class data member nullifies a const  speci­
  fier  applied  to the containing class object and permits modification
  of the mutable class member even though the  rest  of  the  object  is
  const (_dcl.type.cv_).

  7.1.2  Function specifiers                              [dcl.fct.spec]

1 Function-specifiers can be used only in function declarations.

2 A  function declaration (_dcl.fct_, _class.mfct_, _class.friend_) with
  an inline specifier declares an inline function.  The inline specifier
  indicates  to the implementation that inline substitution of the func­
  tion body at the point of call is to be preferred to the  usual  func­
  tion  call  mechanism.   An  implementation is not required to perform
  this inline substitution at the point of call; however, even  if  this
  inline  substitution  is omitted, the other rules for inline functions
  defined by this subclause shall still be respected.

3 A function defined within a class definition is  an  inline  function.
  The  inline  specifier  shall  not  appear  on  a block scope function

4 An inline function shall be defined in every translation unit in which
  it  is used (_basic.def.odr_), and shall have exactly the same defini­
  tion in every case (see one definition rule,  _basic.def.odr_).  If  a
  function  with  external linkage is declared inline in one translation
  unit, it shall be declared inline in all translation units in which it
  2) The inline keyword has no effect on the linkage of a function.

  appears.   [Note: a static local variable in an extern inline function
  always refers to the same object.  ]

5 The virtual specifier shall only be used in declarations of  nonstatic
  class  member functions that appear within a member-specification of a
  class declaration; see _class.virtual_.

6 The explicit specifier shall be used only in declarations of construc­
  tors within a class declaration; see _class.conv.ctor_.

  7.1.3  The typedef specifier                             [dcl.typedef]

1 Declarations containing the decl-specifier typedef declare identifiers
  that can be used later for naming fundamental (_basic.fundamental_) or
  compound (_basic.compound_) types.  The typedef specifier shall not be
  used in a function-definition (_dcl.fct.def_), and  it  shall  not  be
  combined  in  a  decl-specifier-seq  with  any other kind of specifier
  except a type-specifier.
  A name declared with the typedef  specifier  becomes  a  typedef-name.
  Within  the  scope of its declaration, a typedef-name is syntactically
  equivalent to a keyword and names the type associated with the identi­
  fier  in  the  way  described in _dcl.decl_.  A typedef-name is thus a
  synonym for another type.  A typedef-name does  not  introduce  a  new
  type  the  way  a class declaration (_class.name_) or enum declaration
  does.  [Example: after
          typedef int MILES, *KLICKSP;
  the constructions
          MILES distance;
          extern KLICKSP metricp;
  are all correct declarations; the type of distance  is  int;  that  of
  metricp is "pointer to int."  ]

2 In a given scope, a typedef specifier can be used to redefine the name
  of any type declared in that scope to refer to the type  to  which  it
  already refers.  [Example:
          typedef struct s { /* ... */ } s;
          typedef int I;
          typedef int I;
          typedef I I;
   --end example]

3 In  a  given  scope, a typedef specifier shall not be used to redefine
  the name of any type declared in that scope to refer  to  a  different
  type.  [Example:
          class complex { /* ... */ };
          typedef int complex;    // error: redefinition
    --end  example]  Similarly, in a given scope, a class or enumeration
  shall not be declared with the same name as  a  typedef-name  that  is
  declared  in  that  scope and refers to a type other than the class or
  enumeration itself.  [Example:
          typedef int complex;
          class complex { /* ... */ };  // error: redefinition

   --end example]

4 A typedef-name that names a class is a class-name (_class.name_).  The
  typedef-name  shall not be used after a class, struct, or union prefix
  and not in the names for constructors and destructors within the class
  declaration itself.  [Example:
          struct S {

          typedef struct S T;

          S a = T();      // ok
          struct T * p;   // error
   --end example]

5 If  the  typedef  declaration  defines an unnamed class (or enum), the
  first typedef-name declared by the declaration to be that  class  type
  (or  enum  type)  is  used to denote the class type (or enum type) for
  linkage purposes only (_basic.link_).  [Example:
          typedef struct { } *ps, S; // 'S' is the class name for linkage purposes
   --end example] If the typedef-name is used  where  a  class-name  (or
  enum-name) is required, the program is ill-formed.  [Example:
          typedef struct {
              S();    // error: requires a return type since S is
                      // an ordinary member function, not a constructor
          } S;
   --end example]

  7.1.4  The friend specifier                               [dcl.friend]

1 The  friend  specifier is used to specify access to class members; see

  7.1.5  Type specifiers                                      [dcl.type]

1 The type-specifiers are
  As a general rule, at most one type-specifier is allowed in  the  com­
  plete  decl-specifier-seq  of  a  declaration.  The only exceptions to
  this rule are the following:

  --const or volatile can be combined  with  any  other  type-specifier.
    However,  redundant  cv-qualifiers are prohibited except when intro­
    duced through the use of typedefs (_dcl.typedef_) or  template  type
    arguments  (_temp.arg_),  in  which case the redundant cv-qualifiers
    are ignored.

  --signed or unsigned can be combined with char, long, short, or int.

  --short or long can be combined with int.

  --long can be combined with double.

3 At least one type-specifier that is not a cv-qualifier is required  in
  a  declaration  unless  it  declares a constructor, destructor or type
  conversion operator.3)

4 [Note: class-specifiers and enum-specifiers are discussed  in  _class_
  and  _dcl.enum_, respectively.  The remaining type-specifiers are dis­
  cussed in the rest of this section.  ]  The cv-qualifiers                               [dcl.type.cv]

1 There are two cv-qualifiers, const and volatile.   If  a  cv-qualifier
  appears  in a decl-specifier-seq, the init-declarator-list of the dec­
  laration shall not be empty.  [Note: _basic.type.qualifier_  describes
  how cv-qualifiers affect object and function types.  ]

2 An  object  declared  with a const-qualified type has internal linkage
  unless it is explicitly declared extern or unless  it  was  previously
  declared  to  have  external  linkage.   A variable of const-qualified
  integral or enumeration  type  initialized  by  an  integral  constant
  expression    can   be   used   in   integral   constant   expressions
  (_expr.const_).  [Note: as described in _dcl.init_, the definition  of
  an  object  or  subobject of const-qualified type must specify an ini­
  tializer or be subject to default-initialization.  ]

3 A pointer or reference to a cv-qualified type need not actually  point
  or  refer to a cv-qualified object, but it is treated as if it does; a
  const-qualified access path cannot be used to modify an object even if
  the  object  referenced  is  a  non-const  object  and can be modified
  through some other access path.  [Note: cv-qualifiers are supported by
  the  type  system  so  that  they  cannot be subverted without casting
  (_expr.const.cast_).  ]

4 Except that any class member declared mutable (_dcl.stc_) can be modi­
  fied,  any  attempt  to  modify  a  const  object  during its lifetime
  (_basic.life_) results in undefined behavior.

5 [Example:
          const int ci = 3;  // cv-qualified (initialized as required)
          ci = 4;            // ill-formed: attempt to modify const
          int i = 2;         // not cv-qualified
          const int* cip;    // pointer to const int
          cip = &i;          // okay: cv-qualified access path to unqualified
          *cip = 4;          // ill-formed: attempt to modify through ptr to const
  3) There is no special provision for a decl-specifier-seq that lacks a
  type-specifier  or  that  has a type-specifier that only specifies cv-
  qualifiers.  The "implicit int" rule of C is no longer supported.

          int* ip;
          ip = const_cast<int*>(cip); // cast needed to convert const int* to int*
          *ip = 4;           // defined: *ip points to i, a non-const object
          const int* ciq = new const int (3); // initialized as required
          int* iq = const_cast<int*>(ciq);    // cast required
          *iq = 4;           // undefined: modifies a const object

6 For another example
          class X {
                  mutable int i;
                  int j;
          class Y {
                  X x;
          const Y y;
          y.x.i++;        // well-formed: mutable member can be modified
          y.x.j++;        // ill-formed: const-qualified member modified
          Y* p = const_cast<Y*>(&y);      // cast away const-ness of y
          p->x.i = 99;    // well-formed: mutable member can be modified
          p->x.j = 99;    // undefined: modifies a const member
   --end example]

7 [Note: volatile is a hint to the implementation  to  avoid  aggressive
  optimization  involving  the  object  because  the value of the object
  might be changed by means  undetectable  by  an  implementation.   See
  _intro.execution_  for  detailed semantics.  In general, the semantics
  of volatile are intended to be the same in C++ as they are in C.  ]  Simple type specifiers                      [dcl.type.simple]

1 The simple type specifiers are
                  ::opt nested-name-specifieropt type-name
  The simple-type-specifiers specify either a previously-declared  user-
  defined  type  or  one of the fundamental types (_basic.fundamental_).

  Table 1 summarizes the valid  combinations  of  simple-type-specifiers
  and the types they specify.

        Table 1--simple-type-specifiers and the types they specify

               |Specifier(s)       | Type                 |
               |type-name          | the type named       |
               |char               | "char"               |
               |unsigned char      | "unsigned char"      |
               |signed char        | "signed char"        |
               |bool               | "bool"               |
               |unsigned           | "unsigned int"       |
               |unsigned int       | "unsigned int"       |
               |signed             | "int"                |
               |signed int         | "int"                |
               |int                | "int"                |
               |unsigned short int | "unsigned short int" |
               |unsigned short     | "unsigned short int" |
               |unsigned long int  | "unsigned long int"  |
               |unsigned long      | "unsigned long int"  |
               |signed long int    | "long int"           |
               |signed long        | "long int"           |
               |long int           | "long int"           |
               |long               | "long int"           |
               |signed short int   | "short int"          |
               |signed short       | "short int"          |
               |short int          | "short int"          |
               |short              | "short int"          |
               |wchar_t            | "wchar_t"            |
               |float              | "float"              |
               |double             | "double"             |
               |long double        | "long double"        |
               |void               | "void"               |
  When  multiple  simple-type-specifiers are allowed, they can be freely
  intermixed with other decl-specifiers in any order.  It is implementa­
  tion-defined  whether  bit-fields  and objects of char type are repre­
  sented as signed or unsigned quantities.  The signed specifier  forces
  char  objects  and bit-fields to be signed; it is redundant with other
  integral types.  Elaborated type specifiers                    [dcl.type.elab]

1         elaborated-type-specifier:
                  class-key ::opt nested-name-specifieropt identifier
                  enum ::opt nested-name-specifieropt identifier

2 If an elaborated-type-specifier is the sole constituent of a  declara­
  tion,   the  declaration  is  ill-formed  unless  it  is  an  explicit

  specialization   (_temp.expl.spec_),   an    explicit    instantiation
  (_temp.explicit_) or it has one of the following forms:
  --      class-key identifier ;

3 --      friend class-key identifier ;

4 --      friend class-key ::identifier ;
          friend class-key nested-name-specifier identifier ;

5 _basic.lookup.elab_  describes how name look up proceeds for the iden­
  tifier in an elaborated-type-specifier.  If the identifier resolves to
  a class-name or enum-name, the elaborated-type-specifier introduces it
  into the declaration the same way a  simple-type-specifier  introduces
  its type-name.  If the identifier resolves to a typedef-name or a tem­
  plate type-parameter, the elaborated-type-specifier is ill-formed.  If
  name look up does not find a declaration for the name, the elaborated-
  type-specifier is ill-formed unless it is of the simple form class-key
  identifier  in  which  case the identifier is declared as described in

6 The class-key or enum keyword present in the elaborated-type-specifier
  shall  agree  in  kind  with  the declaration to which the name in the
  elaborated-type-specifier refers.  This rule also applies to the  form
  of  elaborated-type-specifier  that  declares  a  class-name or friend
  class since it can be construed as referring to the definition of  the
  class.  Thus, in any elaborated-type-specifier, the enum keyword shall
  be used to refer to an enumeration (_dcl.enum_), the  union  class-key
  shall  be  used to refer to a union (_class_), and either the class or
  struct class-key shall be used to refer to a class (_class_)  declared
  using the class or struct class-key.

  7.2  Enumeration declarations                               [dcl.enum]

1 An  enumeration  is  a  distinct type (_basic.fundamental_) with named
  constants.  Its name becomes an enum-name, within its scope.
                  enum identifieropt { enumerator-listopt }
                  enumerator-list , enumerator-definition
                  enumerator = constant-expression
  The identifiers in an enumerator-list are declared as  constants,  and
  can  appear wherever constants are required.  If no enumerator-defini­
  tions with = appear, then the values of  the  corresponding  constants
  begin  at zero and increase by one as the enumerator-list is read from

  left to right.  An enumerator-definition with = gives  the  associated
  enumerator  the value indicated by the constant-expression; subsequent
  enumerators without initializers continue  the  progression  from  the
  assigned  value.  The constant-expression shall be of integral or enu­
  meration type.

2 [Example:
          enum { a, b, c=0 };
          enum { d, e, f=e+2 };
  defines a, c, and d to be zero, b and e to be 1, and f to be 3.  ]

3 The point of declaration for an enumerator is  immediately  after  its
  enumerator-definition.  [Example:
          const int x = 12;
          { enum { x = x }; }
  Here,  the  enumerator x is initialized with the value of the constant
  x, namely 12.  ]

4 Each enumeration defines a type  that  is  different  from  all  other
  types.  The type of an enumerator is its enumeration.

5 The  underlying  type  of  an enumeration is an integral type that can
  represent all the enumerator values defined in the enumeration.  It is
  implementation-defined  which  integral type is used as the underlying
  type for an enumeration except that the underlying type shall  not  be
  larger than int unless the value of an enumerator cannot fit in an int
  or unsigned int.  If the enumerator-list is empty, the underlying type
  is  as  if  the enumeration had a single enumerator with value 0.  The
  value of sizeof() applied to an enumeration type, an object of enumer­
  ation  type, or an enumerator, is the value of sizeof() applied to the
  underlying type.

6 For an enumeration where emin is the smallest enumerator and  emax  is
  the  largest,  the  values  of  the  enumeration are the values of the
  underlying type in the range bmin to bmax, where bmin  and  bmax  are,
  respectively,  the  smallest  and  largest values of the smallest bit-
  field that can store emin and emax.4) It is possible to define an enu­
  meration that has values not defined by any of its enumerators.

7 Two  enumeration  types  are  layout-compatible  if they have the same
  underlying type.

8 The value of an enumerator or an object of an enumeration type is con­
  verted to an integer by integral promotion (_conv.prom_).  [Example:
      enum color { red, yellow, green=20, blue };
      color col = red;
      color* cp = &col;
      if (*cp == blue) // ...
  makes color a type describing various colors, and then declares col as
  4) On a two's-complement machine, bmax is the smallest  value  greater
  than  or equal to max(abs(emin)-1,abs(emax)) of the form 2M-1; bmin is
  zero if emin is non-negative and -(bmax+1) otherwise.

  an object of that type, and cp as a pointer to an object of that type.
  The possible values of an object of type color are red, yellow, green,
  blue; these values can be converted to the integral values 0,  1,  20,
  and  21.  Since enumerations are distinct types, objects of type color
  can be assigned only values of type color.
          color c = 1;     // error: type mismatch,
                           // no conversion from int to color
          int i = yellow;  // ok: yellow converted to integral value 1
                           // integral promotion
  See also _diff.anac_.  ]

9 An expression of arithmetic or enumeration type can be converted to an
  enumeration  type  explicitly.  The value is unchanged if it is in the
  range of enumeration values of the  enumeration  type;  otherwise  the
  resulting enumeration value is unspecified.

10The  enum-name  and  each  enumerator declared by an enum-specifier is
  declared in the scope that immediately  contains  the  enum-specifier.
  These   names   obey   the  scope  rules  defined  for  all  names  in
  (_basic.scope_) and (_basic.lookup_).  An enumerator declared in class
  scope can be referred to using the class member access operators ::, .
  (dot) and -> (arrow)), see _expr.ref_.  [Example:
          class X {
              enum direction { left='l', right='r' };
              int f(int i)
                  { return i==left ? 0 : i==right ? 1 : 2; }
          void g(X* p)
              direction d;        // error: `direction' not in scope
              int i;
              i = p->f(left);     // error: `left' not in scope
              i = p->f(X::right); // ok
              i = p->f(p->left);  // ok
              // ...
   --end example]

  7.3  Namespaces                                      [basic.namespace]

1 A namespace is an optionally-named declarative region.  The name of  a
  namespace  can  be used to access entities declared in that namespace;
  that is, the members  of  the  namespace.   Unlike  other  declarative
  regions, the definition of a namespace can be split over several parts
  of one or more translation units.

2 A name declared outside all named  namespaces,  blocks  (_stmt.block_)
  and  classes (_class_) has global namespace scope (_basic.scope.names­

  7.3.1  Namespace definition                            [namespace.def]

1 The grammar for a namespace-definition is



                  namespace identifier { namespace-body }

                  namespace original-namespace-name  { namespace-body }

                  namespace { namespace-body }


2 The identifier in an original-namespace-definition shall not have been
  previously defined in the declarative region in  which  the  original-
  namespace-definition  appears.   The  identifier in an original-names­
  pace-definition is the name of the namespace.   Subsequently  in  that
  declarative region, it is treated as an original-namespace-name.

3 The original-namespace-name in an extension-namespace-definition shall
  have previously been defined in  an  original-namespace-definition  in
  the same declarative region.

4 Every  namespace-definition  shall  appear in the global scope or in a
  namespace scope (_basic.scope.namespace_).

5 Because a namespace-definition contains declarations in its namespace-
  body  and  a  namespace-definition is itself a declaration, it follows
  that namespace-definitions can be nested.  [Example:
          namespace Outer {
                  int i;
                  namespace Inner {
                          void f() { i++; } // Outer::i
                          int i;
                          void g() { i++; } // Inner::i
   --end example]  Unnamed namespaces                        [namespace.unnamed]

1 An unnamed-namespace-definition behaves as if it were replaced by
          namespace unique { /* empty body */ }
          using namespace unique;
          namespace unique { namespace-body }
  where all occurrences of unique in a translation unit are replaced  by
  the same identifier and this identifier differs from all other identi­
  fiers in the entire program.5) [Example:
          namespace { int i; }       // unique::i
          void f() { i++; }          // unique::i++

          namespace A {
                  namespace {
                          int i;     // A::unique::i
                          int j;     // A::unique::j
                  void g() { i++; }  // A::unique::i++
          using namespace A;
          void h() {
                  i++;               // error: unique::i or A::unique::i
                  A::i++;            // A::unique::i
                  j++;               // A::unique::j
   --end example]

2 The  use of the static keyword is deprecated when declaring objects in
  a namespace scope (see Annex _depr_); the unnamed-namespace provides a
  superior alternative.  Namespace member definitions               [namespace.memdef]

1 Members  of  a namespace can be defined within that namespace.  [Exam­
          namespace X {
                  void f() { /* ... */ }
   --end example]

2 Members of a named namespace can also be defined outside  that  names­
  pace  by  explicit  qualification (_namespace.qual_) of the name being
  defined, provided that the entity being defined was  already  declared
  in  the namespace and the definition appears after the point of decla­
  ration in a  namespace  that  encloses  the  declaration's  namespace.

  5) Although entities in an unnamed namespace might have external link­
  age, they are effectively qualified by a name unique to their transla­
  tion unit and therefore can never be seen from any  other  translation

          namespace Q {
                  namespace V {
                          void f();
                  void V::f() { /* ... */ }  // fine
                  void V::g() { /* ... */ }  // error: g() is not yet a member of V
                  namespace V {
                          void g();
          namespace R {
                  void Q::V::g() { /* ... */ } // error: R doesn't enclose Q
   --end example]

3 Every  name  first  declared in a namespace is a member of that names­
  pace.  If a friend declaration first declares a class or function, and
  the  name of the class or function is unqualified, the friend class or
  function is a member of the innermost enclosing namespace.   The  name
  of  the  friend is not found by simple name lookup in the scope of the
  namespace until a matching declaration is provided in  that  namespace
  scope  (either  before or after the class declaration granting friend­
  ship).  If a friend function is called, its name may be found  by  the
  name  lookup  that considers functions from namespaces associated with
  the types of the  function  arguments  (_lookup.basic.koenig_).   When
  looking for a prior declaration of a class or a function declared as a
  friend, scopes outside the innermost enclosing namespace scope are not
  considered.  [Example:
          // Assume f and g have not yet been defined.
          friend void h(int);
          namespace A {
                  class X {
                          friend void f(X);  // A::f is a friend
                          class Y {
                                  friend void g();    // A::g is a friend
                                  friend void h(int); // A::h is a friend
                                                      // ::h not considered

                  // A::f, A::g and A::h are not visible here
                  void f(X) { /* ... */}     // definition of A::f
                  X x;
                  void g() { f(x); }         // definition of A::g
                  void h(int) { /* ... */ }  // definition of A::h
                  // A::f, A::g and A::h are visible here and known to be friends

          using A::x;

          void h()
                  A::X::f(x);    // error: f is not a member of A::X
                  A::X::Y::g();  // error: g is not a member of A::X::Y
   --end example]

4 When an entity declared with a block scope extern declaration  is  not
  found to refer to some other declaration, then that entity is a member
  of the innermost enclosing namespace.  However such a declaration does
  not introduce the member name in its namespace scope.  [Example:
          namespace X {
                  void p()
                          q();              // error: q not yet declared
                          extern void q();  // q is a member of namespace X
                  void middle()
                          q();              // error: q not yet declared
                  void q() { /* ... */ }    // definition of X::q

          void q() { /* ... */ }            // some other, unrelated q
   --end example]

  7.3.2  Namespace alias                               [namespace.alias]

1 A  namespace-alias-definition  declares an alternate name for a names­
  pace according to the following grammar:

                  namespace identifier = qualified-namespace-specifier ;

                  ::opt nested-name-specifieropt namespace-name

2 The identifier in a namespace-alias-definition is a  synonym  for  the
  name of the namespace denoted by the qualified-namespace-specifier and
  becomes a namespace-alias.  [Note: when looking up a namespace-name in
  a namespace-alias-definition, only namespace names are considered, see
  _basic.lookup.udir_.  ]

3 In a declarative region, a namespace-alias-definition can be  used  to
  redefine  a  namespace-alias  declared  in  that declarative region to
  refer to the namespace to which it already refers.  [Example: the fol­
  lowing declarations are well-formed:

          namespace Company_with_very_long_name { /* ... */ }
          namespace CWVLN = Company_with_very_long_name;
          namespace CWVLN = Company_with_very_long_name;  // ok: duplicate
          namespace CWVLN = CWVLN;
   --end example]

4 A  namespace-name or namespace-alias shall not be declared as the name
  of any other entity in the same declarative region.  A  namespace-name
  defined at global scope shall not be declared as the name of any other
  entity in any global scope of the program.  No diagnostic is  required
  for  a violation of this rule by declarations in different translation

  7.3.3  The using declaration                         [namespace.udecl]

1 A using-declaration introduces a name into the declarative  region  in
  which  the  using-declaration appears.  That name is a synonym for the
  name of some entity declared elsewhere.  A name specified in a  using-
  declaration  in a class or namespace scope shall not already be a mem­
  ber of that scope.
                  using typenameopt ::opt nested-name-specifier unqualified-id ;
                  using ::  unqualified-id ;

2 The member name specified in a using-declaration is  declared  in  the
  declarative region in which the using-declaration appears.

3 Every  using-declaration is a declaration and a member-declaration and
  so can be used in a class definition.  [Example:
          struct B {
                  void f(char);
                  void g(char);
                  enum E { e };
                  union { int x; };
          struct D : B {
                  using B::f;
                  void f(int) { f('c'); } // calls B::f(char)
                  void g(int) { g('c'); } // recursively calls D::g(int)
   --end example]

4 A using-declaration used as a member-declaration shall refer to a mem­
  ber  of a base class of the class being defined, shall refer to a mem­
  ber of an anonymous union that is a member of  a  base  class  of  the
  class  being  defined, or shall refer to an enumerator for an enumera­
  tion type that is a member of a base class of the class being defined.
          class C {
                  int g();

          class D2 : public B {
                  using B::f;  // ok: B is a base of D2
                  using B::e;  // ok: e is an enumerator of base B
                  using B::x;  // ok: x is a union member of base B
                  using C::g;  // error: C isn't a base of D2
    --end example] [Note: since constructors and destructors do not have
  names, a using-declaration cannot refer to a constructor or a destruc­
  tor  for  a base class.  A using-declaration can refer to a base class
  copy-assignment operator; however, this  copy-assignment  operator  is
  never  used as the copy-assignment operator for the derived class that
  contains the using-declaration (_over.ass_).  ]

5 A using-declaration  for  a  member  shall  be  a  member-declaration.
          struct X {
                  int i;
                  static int s;
          void f()
                  using X::i;  // error: X::i is a class member
                               // and this is not a member declaration.
                  using X::s;  // error: X::s is a class member
                               // and this is not a member declaration.
   --end example]

6 Members declared by a using-declaration can be referred to by explicit
  qualification just like other member names (_namespace.qual_).   In  a
  using-declaration, a prefix :: refers to the global namespace.  [Exam­
          void f();

          namespace A {
                  void g();
          namespace X {
                  using ::f;   // global f
                  using A::g;  // A's g
          void h()
                  X::f();      // calls ::f
                  X::g();      // calls A::g
   --end example]

7 A using-declaration is a declaration and can therefore be used repeat­
  edly where (and only where) multiple declarations are allowed.  [Exam­

          namespace A {
                  int i;

          namespace A1 {
                  using A::i;
                  using A::i; // ok: double declaration

          void f()
                  using A::i;
                  using A::i; // error: double declaration
          class B {
                  int i;

          class X : public B {
                  using B::i;
                  using B::i;  // error: double member declaration
   --end example]

8 The  entity declared by a using-declaration shall be known in the con­
  text using it according to its definition at the point of  the  using-
  declaration.   Definitions added to the namespace after the using-dec­
  laration are not considered when a use of the name is made.  [Example:
          namespace A {
                  void f(int);
          using A::f;              // f is a synonym for A::f;
                                   // that is, for A::f(int).
          namespace A {
                  void f(char);
          void foo()
                  f('a');          // calls f(int),
          }                        // even though f(char) exists.
          void bar()
                  using A::f;      // f is a synonym for A::f;
                                   // that is, for A::f(int) and A::f(char).
                  f('a');          // calls f(char)
   --end example]

9 A  name  declared  by a using-declaration is an alias for its original
  declarations so that the using-declaration does not affect  the  type,
  linkage or other attributes of the members referred to.

10If  the  set  of declarations and using-declarations for a single name
  are given in a declarative region,

  --they shall all refer to the same entity, or all refer to  functions;

  --exactly  one  declaration  shall declare a class name or enumeration
    name and the other declarations shall all refer to the  same  entity
    or  all  refer to functions; in this case the class name or enumera­
    tion name is hidden (_basic.scope.hiding_).

          namespace A {
                  int x;
          namespace B {
                  int i;
                  struct g { };
                  struct x { };
                  void f(int);
                  void f(double);
                  void g(char);   // OK: hides struct g
          void func()
                  int i;
                  using B::i;     // error: i declared twice
                  void f(char);
                  using B::f;     // fine: each f is a function
                  f(3.5);         // calls B::f(double)
                  using B::g;
                  g('a');         // calls B::g(char)
                  struct g g1;    // g1 has class type B::g
                  using B::x;
                  using A::x;     // fine: hides struct B::x
                  x = 99;         // assigns to A::x
                  struct x x1;    // x1 has class type B::x
   --end example]

12If a function declaration in namespace scope or block  scope  has  the
  same  name  and the same parameter types as a function introduced by a
  using-declaration, the program is ill-formed.  [Note: two using-decla­
  rations may introduce functions with the same name and the same param­
  eter types.  A call to such a function is ill-formed unless name  look
  up  can  unambiguously  select  the function to be called (because the
  function name is qualified by its namespace  name,  for  example).   ]

          namespace B {
                  void f(int);
                  void f(double);
          namespace C {
                  void f(int);
                  void f(double);
                  void f(char);
          void h()
                  using B::f;   // B::f(int) and B::f(double)
                  using C::f;   // C::f(int), C::f(double), and C::f(char)
                  f('h');       // calls C::f(char)
                  f(1);         // error: ambiguous: B::f(int) or C::f(int) ?
                  void f(int);  // error:
                                // f(int) conflicts with C::f(int) and B::f(int)
   --end example]

13When a using-declaration brings names from a base class into a derived
  class scope, member functions in the  derived  class  override  and/or
  hide member functions with the same name and parameter types in a base
  class (rather than conflicting).  [Example:
          struct B {
                  virtual void f(int);
                  virtual void f(char);
                  void g(int);
                  void h(int);
          struct D : B {
                  using B::f;
                  void f(int);   // ok: D::f(int) overrides B::f(int);

                  using B::g;
                  void g(char);  // ok

                  using B::h;
                  void h(int);   // ok: D::h(int) hides B::h(int)
          void k(D* p)
                  p->f(1);    // calls D::f(int)
                  p->f('a');  // calls B::f(char)
                  p->g(1);    // calls B::g(int)
                  p->g('a');  // calls D::g(char)
   --end example] [Note: two using-declarations may introduce  functions
  with  the  same  name  and the same parameter types.  A call to such a
  function is ill-formed unless name look up  can  unambiguously  select
  the  function  to be called (because the function name is qualified by
  its class name, for example).  ]

14For the purpose of overload resolution, the functions which are intro­
  duced by a using-declaration into a derived class will be  treated  as
  though  they  were  members  of the derived class.  In particular, the
  implicit this parameter shall be treated as if it were  a  pointer  to
  the  derived  class rather than to the base class.  This has no effect
  on the type of the function, and in all other  respects  the  function
  remains a member of the base class.

15All  instances  of  the name mentioned in a using-declaration shall be
  accessible.  In particular, if a derived class uses  a  using-declara­
  tion  to  access  a  member  of a base class, the member name shall be
  accessible.  If the name is that of  an  overloaded  member  function,
  then  all functions named shall be accessible.  The base class members
  mentioned by a using-declaration shall be visible in the scope  of  at
  least one of the direct base classes of the class where the using-dec­
  laration is specified.  [Note: because a using-declaration  designates
  a  base  class member (and not a member subobject or a member function
  of a base class subobject), a  using-declaration  cannot  be  used  to
  resolve inherited member ambiguities.  For example,
          struct A { int x(); };
          struct B : A { };
          struct C : A {
              using A::x;
              int x(int);
          struct D : B, C {
              using C::x;
              int x(double);
          int f(D* d) {
              return d->x(); // ambiguous: B::x or C::x

16The alias created by the using-declaration has the usual accessibility
  for a member-declaration.  [Example:
          class A {
                  void f(char);
                  void f(int);
                  void g();
          class B : public A {
                  using A::f; // error: A::f(char) is inaccessible
                  using A::g; // B::g is a public synonym for A::g
   --end example]

17[Note: use of access-declarations (_class.access.dcl_) is  deprecated;
  member using-declarations provide a better alternative.  ]

  7.3.4  Using directive                                [namespace.udir]

1         using-directive:
                  using  namespace ::opt nested-name-specifieropt namespace-name ;
  A  using-directive  shall not appear in class scope, but may appear in
  namespace scope or in block scope.  [Note: when looking  up  a  names­
  pace-name  in  a using-directive, only namespace names are considered,
  see _basic.lookup.udir_.  ]

2 A using-directive specifies that the names in the nominated  namespace
  can  be  used  in the scope in which the using-directive appears after
  the   using-directive.     During    unqualified    name    look    up
  (_basic.lookup.unqual_),  the names appear as if they were declared in
  the nearest enclosing namespace which contains both  the  using-direc­
  tive  and the nominated namespace.  [Note: in this context, "contains"
  means "contains directly or indirectly".  ] A using-directive does not
  add any members to the declarative region in which it appears.  [Exam­
          namespace A {
                  int i;
                  namespace B {
                          namespace C {
                                  int i;
                          using namespace A::B::C;
                          void f1() {
                                  i = 5; // ok, C::i visible in B and hides A::i
                  namespace D {
                          using namespace B;
                          using namespace C;
                          void f2() {
                                  i = 5; // ambiguous , B::C::i or A::i ?
                  void f3() {
                          i = 5; // uses A::i
          void f4() {
                  i = 5;  // ill-formed; neither "i" is visible

3 The using-directive is transitive: if a scope contains a  using-direc­
  tive  that  nominates  a  second namespace that itself contains using-
  directives, the effect is as if the using-directives from  the  second
  namespace also appeared in the first.  [Example:
          namespace M {
                  int i;

          namespace N {
                  int i;
                  using namespace M;
          void f()
                  using namespace N;
                  i = 7;    // error: both M::i and N::i are visible
  For another example,
          namespace A {
                  int i;
          namespace B {
                  int i;
                  int j;
                  namespace C {
                          namespace D {
                                  using namespace A;
                                  int j;
                                  int k;
                                  int a = i;  // B::i hides A::i
                          using namespace D;
                          int k = 89; // no problem yet
                          int l = k;  // ambiguous: C::k or D::k;
                          int m = i;  // B::i hides A::i
                          int n = j;  // D::j hides B::j
   --end example]

4 If a namespace is extended by an extended-namespace-definition after a
  using-directive for that namespace is given, the additional members of
  the  extended  namespace  and  the  members of namespaces nominated by
  using-directives in  the  extended-namespace-definition  can  be  used
  after the extended-namespace-definition.

5 If name look up finds a declaration for a name in two different names­
  paces, and the declarations do not declare the same entity and do  not
  declare  functions, the use of the name is ill-formed.  [Note: in par­
  ticular, the name of an object, function or enumerator does  not  hide
  the  name of a class or enumeration declared in a different namespace.
  For example,
          namespace A { class X { }; }
          namespace B { void X(int); }
          using namespace A;
          using namespace B;
          void f() {
                  X(1);        // error: name X found in two namespaces
   --end note]

6 During  overload  resolution, all functions from the transitive search
  are considered for argument matching.  The set of  declarations  found
  by  the  transitive  search  is  unordered.  [Note: in particular, the
  order in which namespaces were considered and the relationships  among
  the namespaces implied by the using-directives do not cause preference
  to be given to any of the declarations found  by  the  search.   ]  An
  ambiguity  exists  if the best match finds two functions with the same
  signature, even if one is in  a  namespace  reachable  through  using-
  directives in the namespace of the other.6) [Example:
          namespace D {
                  int d1;
                  void f(char);
          using namespace D;

          int d1;            // ok: no conflict with D::d1
          namespace E {
                  int e;
                  void f(int);
          namespace D {       // namespace extension
                  int d2;
                  using namespace E;
                  void f(int);
          void f()
                  d1++;      // error: ambiguous ::d1 or D::d1?
                  ::d1++;    // ok
                  D::d1++;   // ok
                  d2++;      // ok: D::d2
                  e++;       // ok: E::e
                  f(1);      // error: ambiguous: D::f(int) or E::f(int)?
                  f('a');    // ok: D::f(char)
   --end example]

  7.4  The asm declaration                                     [dcl.asm]

1 An asm declaration has the form
                  asm ( string-literal ) ;
  The  meaning  of an asm declaration is implementation-defined.  [Note:
  Typically it is used to pass information through the implementation to
  an assembler.  ]

  6)  During  name  lookup in a class hierarchy, some ambiguities may be
  resolved by considering whether one member hides the other along  some
  paths  (_class.member.lookup_).   There is no such disambiguation when
  considering the set of names found as a result of following  using-di­

  7.5  Linkage specifications                                 [dcl.link]

1 All function types, function names, and variable names have a language
  linkage, the specific semantics of which  are  implementation-defined.
  [Note: a particular language linkage may be associated with a particu­
  lar form of representing names of objects and functions with  external
  linkage,  with  a  particular  calling convention, etc.  ] The default
  language linkage of all function types, function names,  and  variable
  names is C++ language linkage.  Two function types with different lan­
  guage linkages are distinct types even if they are  otherwise  identi­

2 Linkage  (_basic.link_) between C++ and  non-C++ code fragments can be
  achieved using a linkage-specification:
                  extern string-literal { declaration-seqopt }
                  extern string-literal declaration
  The string-literal indicates the required language linkage.  The mean­
  ing of the string-literal is implementation-defined.  A linkage-speci­
  fication with a string that is unknown to the implementation  is  ill-
  formed.   When  the  string-literal in a linkage-specification names a
  programming language, the spelling of the programming language's  name
  is implementation-defined.  [Note: it is recommended that the spelling
  be taken from the document defining that  language,  for  example  Ada
  (not ADA) and Fortran or FORTRAN (depending on the vintage).  ]

3 Every implementation shall provide for linkage to functions written in
  the C programming language, "C", and linkage to C++ functions,  "C++".
          complex sqrt(complex);    // C++ linkage by default
          extern "C" {
              double sqrt(double);  // C linkage
   --end example]

4 Linkage  specifications  nest.   When linkage specifications nest, the
  innermost one determines the language linkage.  A  linkage  specifica­
  tion  does  not  establish a scope.  A linkage-specification can occur
  only in namespace scope (_basic.scope_).  In a  linkage-specification,
  the  specified  language  linkage applies to the function types of all
  function declarators, function names, and variable names introduced by
  the declaration(s).  [Example:

          extern "C" void f1(void(*pf)(int));
                          // the name f1 and its function type have C language
                          // linkage; pf is a pointer to a C function
          extern "C" typedef void FUNC();
          FUNC f2;        // the name f2 has C++ language linkage and the
                          // function's type has C language linkage
          extern "C" FUNC f3;
                          // the name of function f3 and the function's type
                          // have C language linkage
          void (*pf2)(FUNC*);
                          // the name of the variable pf2 has C++ linkage and
                          // the type of pf2 is pointer to C++ function that
                          // takes one parameter of type pointer to C function
   --end example] A non-C++ language linkage is ignored for the names of
  class members and for the  function  type  of  class  member  function
  declarators.  [Example:
          extern "C" typedef void FUNC_c();
          class C {
               void mf1(FUNC_c*);
                          // the name of the function mf1 and the member
                          // function's type have C++ language linkage; the
                          // parameter has type pointer to C function
               FUNC_c mf2;
                          // the name of the function mf2 and the member
                          // function's type have C++ language linkage
               static FUNC_c* q;
                          // the name of the data member q has C++ language
                          // linkage and the data member's type is pointer to
                          // C function
          extern "C" {
              class X {
                  void mf();
                          // the name of the function mf and the member
                          // function's type have C++ language linkage
   --end example]

5 If  two  declarations of the same function or object specify different
  linkage-specifications (that is, the linkage-specifications  of  these
  declarations  specify  different string-literals), the program is ill-
  formed if the declarations appear in the same  translation  unit,  and
  the  one definition rule (_basic.def.odr_) applies if the declarations
  appear in different translation units.  Except for functions with  C++
  linkage,  a function declaration without a linkage specification shall
  not precede the first linkage  specification  for  that  function.   A
  function  can  be  declared  without  a linkage specification after an
  explicit linkage specification has been seen; the  linkage  explicitly
  specified  in  the earlier declaration is not affected by such a func­
  tion declaration.

6 At most one of a set of overloaded functions (_over_) with a  particu­
  lar  name  can have C linkage.  Two declarations for a function with C

  language  linkage  with the same function name (ignoring the namespace
  names that qualify it) and the same parameter-clause  that  appear  in
  different  namespace  scopes refer to the same function.  Two declara­
  tions for an object with C language linkage with the same name (ignor­
  ing  the  namespace  names  that  qualify it) that appear in different
  namespace scopes refer to the same object.  [Note: because of the  one
  definition  rule (_basic.def.odr_), only one definition for a function
  or object with C linkage may appear in the program; that  is,  such  a
  function  or  object  must  not  be defined in more than one namespace
  scope.  For example,
          namespace A {
              extern "C" int f();
              extern "C" int g() { return 1; }
              extern "C" int h();
          namespace B {
              extern "C" int f();              // A::f and B::f refer
                                               // to the same function
              extern "C" int g() { return 1; } // ill-formed, the function g
                                               // with C language linkage
                                               // has two definitions
          int A::f() { return 98; } // definition for the function f
                                    // with C language linkage
          extern "C" int h() { return 97; }
                                    // definition for the function h
                                    // with C language linkage
                                    // A::h and ::h refer to the same function
   --end note]

7 Except for functions with internal linkage, a function first  declared
  in  a  linkage-specification behaves as a function with external link­
  age.  [Example:
          extern "C" double f();
          static double f();     // error
  is ill-formed (_dcl.stc_).  ] An object defined within an
          extern "C" { /* ... */ }
  linkage-specification is still defined (and not just declared).

8 [Note: because the language linkage is part of a function type, when a
  pointer  to  C function (for example) is dereferenced, the function to
  which it refers is considered a C function.  ]

9 Linkage from C++ to objects defined in other languages and to  objects
  defined in C++ from other languages is implementation-defined and lan­
  guage-dependent.  Only where the object layout strategies of two  lan­
  guage implementations are similar enough can such linkage be achieved.