7   Declarations                                   [dcl.dcl]


1 A declaration introduces one or more names into a program  and  speci­
  fies  how  those  names  are to be interpreted.  Declarations have the
                  declaration-seq declaration
                  decl-specifier-seqopt init-declarator-listopt ;
  [Note:  asm-definitions  are  described  in  _dcl.asm_,  and  linkage-
  specifications  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-
  directives   are   described   in  _namespace.udir_.   ]  The  simple-
          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).

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-
  specifiers  (_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-
  definition.  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

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

8 The  names  declared by a declaration are introduced into the scope in
  which the declaration occurs, except that the  presence  of  a  friend
  specifier  (_class.friend_),  certain  uses  of  the  elaborated-type-
  specifier   (_basic.scope.pdecl_),   and   using-directives   (_names­
  pace.udir_) alter this general behavior.

  7.1  Specifiers                                             [dcl.spec]

1 The specifiers that can be used in a declaration are

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

                  decl-specifier-seqopt decl-specifier

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  of
  type  int called Pc, the type-specifier int has to be present to indi­
  cate 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-
  specifier-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.

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_.  For
  a  nonmember  function,  an inline specifier is equivalent to a static
  specifier for linkage purposes (_basic.link_) unless the inline decla­
  ration  explicitly  includes  extern  as part of its decl-specifier or
  matches a previous declaration of the  function,  in  which  case  the
  function name retains the linkage of the previous declaration.

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();         // internal linkage
          inline void m();
          extern void m();  // internal 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 decla­
  ration.  For the linkage of inline  functions,  see  _basic.link_  and

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

5 The virtual specifier shall be used only in declarations of  nonstatic
  class    member    functions   within   a   class   declaration;   see

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 is required in a typedef declaration.   At
  least  one type-specifier is required in a function declaration unless
  it declares a constructor, destructor or type conversion operator.1)

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

1 There   are   two   cv-qualifiers,   const   and   volatile.    [Note:
  _basic.type.qualifier_  describes  how cv-qualifiers affect object and
  function types.  ]

2 Unless explicitly declared extern, an object declared  with  a  const-
  qualified   type   has  internal  linkage  and  shall  be  initialized
  (_dcl.init_, _class.ctor_).  For an object of const-qualified type  T,
  if  no explicit initializer is specified to initialize the object, and
  T is a class with a user-declared default constructor, the constructor
  for T is called; otherwise, the program is ill-formed.  An integral or
  enumeration object of const-qualified type initialized by an  integral
  or enumeration constant expression can be used in integral or enumera­
  tion constant expressions (_expr.const_).

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.

  1) There is no special provision for a decl-specifier-seq that lacks a
  type-specifier.   The "implicit int" rule of C is no longer supported.

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
          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 has one of the following
  --      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,  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 _basic.scope.pdecl_.

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-
  definitions 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 enumeration 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.  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.   It is possible to define an
  enumeration 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

  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

  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-
  namespace-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,  for  each  translation unit, all occurrences of unique in that
  translation unit are replaced by an identifier that differs  from  all
  other identifiers in the entire program.2) [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.
  2) 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.   A friend function first declared within a class is a member of
  the innermost enclosing namespace.  When looking for a prior  declara­
  tion 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);  // declaration of f
                          class Y {
                                  friend void g();    // A::g is a friend
                                  friend void h(int); // A::h is a friend
                                                      // ::h not considered

                  void f(X) { /* ... */}     // definition of f declared above
                  X x;
                  void g() { f(x); }         // f and g are members of A
                  void h(int) { /* ... */ }  // h is a member of A
          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] The scope of class names first  introduced  in  elabo­
  rated-type-specifiers is described in (_basic.scope.pdecl_).

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 is
  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 ::opt nested-name-specifier unqualified-id ;
                  using ::  unqualified-id ;

  +-------                      BEGIN BOX 1                     -------+
  There  is  still an open issue regarding the "opt" on the nested-name-
  +-------                       END BOX 1                      -------+

2 The member names specified in a using-declaration are 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);
          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.  [Example:
          class C {
                  int g();
          class D2 : public B {
                  using B::f;  // ok: B is a base of D2
                  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.  ]

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-
  declaration are not considered when a use of the name is made.  [Exam­
          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  defined  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 local 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 local function declaration has the same name and type as a  func­
  tion  introduced  by  a  using-declaration, the program is ill-formed.
          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)
   --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 virtual member functions with the same name and argument 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]

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-
  declaration  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.

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 ;
  [Note:  when  looking  up  a namespace-name is 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 name look up (_basic.lookup_), the  names
  appear  as  if  they  were declared in the nearest enclosing namespace
  which contains both the using-directive and the  nominated  namespace.
  [Note:  in  this context, "contains" means "contains directly or indi­
  rectly".  ] A using-directive does not add any members to the declara­
  tive region in which it appears.  [Example:
          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 namespace contains a using-
  directive 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  is  given,  the  additional  members  of the extended
  namespace 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.3) [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]

  3)  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-

  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.  ]

  7.5  Linkage specifications                                 [dcl.link]

1 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
                  declaration-seq declaration
  The string-literal indicates the required linkage.  The meaning of the
  string-literal  is implementation-defined.  Every implementation shall
  provide for linkage to functions written in  the  C  programming  lan­
  guage,  "C",  and linkage to C++ functions, "C++".  Default linkage is
  "C++".  [Example:
          complex sqrt(complex);    // C++ linkage by default
          extern "C" {
              double sqrt(double);  // C linkage
   --end example]

  +-------                      BEGIN BOX 2                     -------+
  This example might need to be revisited depending on  what  the  rules
  ultimately  are  concerning  C++ linkage to standard library functions
  from the C library.
  +-------                       END BOX 2                      -------+

2 Linkage specifications  nest.   When  linkage  spefications  nest  the
  innermost  one  determines  the linkage.  A linkage specification does
  not establish a scope.  A  linkage-specification  can  occur  only  in
  namespace  scope (_basic.scope_).  A linkage-specification for a class
  applies to nonmember functions and  objects  declared  within  it.   A
  linkage-specification  for  a  function  also applies to functions and
  objects declared within it.  A linkage declaration with a string  that
  is unknown to the implementation is ill-formed.

3 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.

4 At most one of a set of overloaded functions (_over_) with a  particu­
  lar name can have C linkage.

5 Linkage can be specified for objects.  [Example:
          extern "C" {
              // ...
              _iobuf _iob[_NFILE];
              // ...
              int _flsbuf(unsigned,_iobuf*);
              // ...
   --end example] Functions and objects can be declared static or inline
  within the {} of a linkage specification.  The  linkage  directive  is
  ignored for a function or object with internal linkage (_basic.link_).
  A function first declared in a  linkage  specification  behaves  as  a
  function with external linkage.  [Example:
          extern "C" double f();
          static double f();     // error
  is ill-formed (_dcl.stc_).  ] An object defined within an
          extern "C" { /* ... */ }
  construct is still defined (and not just declared).

6 The  linkage  of a pointer to function affects only the pointer.  When
  the pointer is dereferenced, the function to which it refers  is  con­
  sidered  to  be  a  C++ function.  There is no way to specify that the
  function to which a function pointer refers is written in another lan­

  +-------                      BEGIN BOX 3                     -------+
  This is not a wholly satisfactory state of affairs
  +-------                       END BOX 3                      -------+

7 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.
  Taking  the address of a function whose linkage is other than C++ or C
  produces undefined behavior.

8 When the name of a programming language is used to  name  a  style  of
  linkage in the string-literal in a linkage-specification, it is recom­
  mended that the spelling be taken from the document defining that lan­
  guage,  [Example:  Ada  (not ADA) and Fortran or FORTRAN (depending on
  the vintage).  ]