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  3   Basic concepts                                             [basic]

  ______________________________________________________________________

1 [Note: this clause presents the basic concepts of  the  C++  language.
  It  explains  the difference between an object and a name and how they
  relate to the notion of an lvalue.  It introduces the  concepts  of  a
  declaration and a definition and presents C++'s notion of type, scope,
  linkage, and storage duration.  The mechanisms for starting and termi-
  nating  a  program  are  discussed.  Finally, this clause presents the
  fundamental types of the language and lists the ways  of  constructing
  compound types from these.

2 This  clause does not cover concepts that affect only a single part of
  the language.  Such concepts are discussed in the relevant clauses.  ]

3 An  entity  is a value, object, subobject, base class subobject, array
  element, variable, function, instance of a function, enumerator, type,
  class member, template, or namespace.

4 A  name  is a use of an identifier (_lex.name_) that denotes an entity
  or label (_stmt.goto_, _stmt.label_).  A variable is introduced by the
  declaration of an object.  The variable's name denotes the object.

5 Every  name  that  denotes  an  entity is introduced by a declaration.
  Every name that denotes a label is introduced either by a goto  state-
  ment (_stmt.goto_) or a labeled-statement (_stmt.label_).

6 Some names denote types, classes, enumerations, or templates.  In gen-
  eral, it is necessary to determine whether or not a name  denotes  one
  of  these  entities  before parsing the program that contains it.  The
  process that determines this is called name lookup (_basic.lookup_).

7 Two names are the same if

  --they are identifiers composed of the same character sequence; or

  --they are the names of overloaded operator functions formed with  the
    same operator; or

  --they  are the names of user-defined conversion functions formed with
    the same type.

8 An identifier used in more than one translation unit  can  potentially
  refer  to  the same entity in these translation units depending on the
  linkage (_basic.link_) of the identifier specified in each translation
  unit.

  3.1  Declarations and definitions                          [basic.def]

1 A  declaration (_dcl.dcl_) introduces names into a translation unit or
  redeclares names introduced by previous declarations.   A  declaration
  specifies the interpretation and attributes of these names.

2 A  declaration  is  a definition unless it declares a function without
  specifying the function's body (_dcl.fct.def_), it contains the extern
  specifier (_dcl.stc_) or a  linkage-specification1)  (_dcl.link_)  and
  neither  an initializer nor a function-body, it declares a static data
  member in a class declaration (_class.static_), it  is  a  class  name
  declaration (_class.name_), or it is a typedef declaration (_dcl.type-
  def_), a using-declaration (_namespace.udecl_), or  a  using-directive
  (_namespace.udir_).

3 [Example: all but one of the following are definitions:
          int a;                       // defines a
          extern const int c = 1;      // defines c
          int f(int x) { return x+a; } // defines f and defines x
          struct S { int a; int b; };  // defines S, S::a, and S::b
          struct X {                   // defines X
              int x;                   // defines nonstatic data member x
              static int y;            // declares static data member y
              X(): x(0) { }            // defines a constructor of X
          };
          int X::y = 1;                // defines X::y
          enum { up, down };           // defines up and down
          namespace N { int d; }       // defines N and N::d
          namespace N1 = N;            // defines N1
          X anX;                       // defines anX
  whereas these are just declarations:
          extern int a;                // declares a
          extern const int c;          // declares c
          int f(int);                  // declares f
          struct S;                    // declares S
          typedef int Int;             // declares Int
          extern X anotherX;           // declares anotherX
          using N::d;                  // declares N::d
   --end example]

4 [Note:  in  some  circumstances, C++ implementations implicitly define
  the   default    constructor    (_class.ctor_),    copy    constructor
  (_class.copy_),  assignment  operator  (_class.copy_),  or  destructor
  (_class.dtor_) member functions.  [Example: given

  _________________________
  1)  Appearing inside the braced-enclosed declaration-seq in a linkage-
  specification does not affect whether a declaration is a definition.

          struct C {
              string s;    // string is the standard library class (_lib.strings_)
          };

          int main()
          {
              C a;
              C b = a;
              b = a;
          }
  the implementation will implicitly define functions to make the  defi-
  nition of C equivalent to
          struct C {
              string s;
              C(): s() { }
              C(const C& x): s(x.s) { }
              C& operator=(const C& x) { s = x.s; return *this; }
              ~C() { }
          };
   --end example]  --end note]

5 [Note:  a class name can also be implicitly declared by an elaborated-
  type-specifier (_basic.scope.pdecl_).  ]

6 A program is ill-formed if the definition  of  any  object  gives  the
  object an incompletely-defined object type (_basic.types_).

  3.2  One definition rule                               [basic.def.odr]

1 No  translation  unit  shall  contain  more than one definition of any
  variable, function, class type, enumeration type or template.

2 An expression is potentially evaluated unless either it is the operand
  of  the  sizeof  operator (_expr.sizeof_), or it is the operand of the
  typeid operator and does not designate an lvalue of polymorphic  class
  type (_expr.typeid_).  An object or non-overloaded function is used if
  its name appears in a  potentially-evaluated  expression.   A  virtual
  member  function is used if it is not pure.  An overloaded function is
  used if it is selected by overload resolution when referred to from  a
  potentially-evaluated  expression.   [Note: this covers calls to named
  functions (_expr.call_), operator overloading  (_over_),  user-defined
  conversions  (_class.conv.fct_), allocation function for placement new
  (_expr.new_), as well as non-default initialization  (_dcl.init_).   A
  copy  constructor  is  used even if the call is actually elided by the
  implementation.  ] An allocation or deallocation function for a  class
  is  used  by  a  new  expression  appearing in a potentially-evaluated
  expression as specified in _expr.new_ and _class.free_.   A  dealloca-
  tion  function for a class is used by a delete expression appearing in
  a potentially-evaluated expression as specified in  _expr.delete_  and
  _class.free_.   A  copy-assignment  function for a class is used by an
  implicitly-defined copy-assignment function for another class as spec-
  ified  in  _class.copy_.  A default constructor for a class is used by
  default initialization as specified in _dcl.init_.  A constructor  for
  a  class is used as specified in _dcl.init_.  A destructor for a class

  is used as specified in _class.dtor_.

3 Every program shall contain exactly one definition of every non-inline
  function  or  object  that  is  used  in  that  program; no diagnostic
  required.  The definition can appear explicitly in the program, it can
  be found in the standard or a user-defined library, or (when appropri-
  ate) it is implicitly  defined  (see  _class.ctor_,  _class.dtor_  and
  _class.copy_).   An inline function shall be defined in every transla-
  tion unit in which it is used.

4 Exactly one definition of a class is required in a translation unit if
  the  class  is  used  in a way that requires the class type to be com-
  plete.  [Example: the following complete  translation  unit  is  well-
  formed, even though it never defines X:
          struct X;      // declare X as a struct type
          struct X* x1;  // use X in pointer formation
          X* x2;         // use X in pointer formation
    --end  example]  [Note:  the  rules for declarations and expressions
  describe in which contexts complete class types are required.  A class
  type T must be complete if:

  --an object of type T is defined (_basic.def_, _expr.new_), or

  --an  lvalue-to-rvalue conversion is applied to an lvalue referring to
    an object of type T (_conv.lval_), or

  --an expression is converted (either implicitly or explicitly) to type
    T        (_conv_,       _expr.type.conv_,       _expr.dynamic.cast_,
    _expr.static.cast_, _expr.cast_), or

  --an expression is converted to the type pointer to T or reference  to
    T   using   an   implicit   conversion   (_conv_),   a  dynamic_cast
    (_expr.dynamic.cast_) or a static_cast (_expr.static.cast_), or

  --a class member access operator is applied to an expression of type T
    (_expr.ref_), or

  --the   typeid   operator   (_expr.typeid_)  or  the  sizeof  operator
    (_expr.sizeof_) is applied to an operand of type T, or

  --a function with a return type or argument type of type T is  defined
    (_basic.def_) or called (_expr.call_), or

  --an lvalue of type T is assigned to (_expr.ass_).  ]

5 There  can be more than one definition of a class type (_class_), enu-
  meration type (_dcl.enum_),  inline  function  with  external  linkage
  (_dcl.fct.spec_),  class  template  (_temp_), non-static function tem-
  plate  (_temp.fct_),  static  data  member   of   a   class   template
  (_temp.static_),  member  function template (_temp.mem.func_), or tem-
  plate specialization for which some template parameters are not speci-
  fied  (_temp.spec_, _temp.class.spec_) in a program provided that each
  definition appears in a different translation unit, and  provided  the
  definitions  satisfy the following requirements.  Given such an entity

  named D defined in more than one translation unit, then

  --each definition of D shall consist of the same sequence  of  tokens;
    and

  --in each definition of D, corresponding names, looked up according to
    _basic.lookup_, shall refer to an entity defined within the  defini-
    tion of D, or shall refer to the same entity, after overload resolu-
    tion (_over.match_) and after matching of partial template  special-
    ization  (_temp.over_),  except  that  a  name  can refer to a const
    object with internal or no linkage if the object has the same  inte-
    gral  or enumeration type in all definitions of D, and the object is
    initialized with a constant expression (_expr.const_), and the value
    (but  not the address) of the object is used, and the object has the
    same value in all definitions of D; and

  --in each definition of D, the overloaded operators referred  to,  the
    implicit  calls  to conversion operators, constructors, operator new
    functions and operator delete functions, shall  refer  to  the  same
    function, or to a function defined within the definition of D; and

  --in  each definition of D, a default argument used by an (implicit or
    explicit) function call is treated as if  its  token  sequence  were
    present  in  the  definition  of D; that is, the default argument is
    subject to the three  requirements  described  above  (and,  if  the
    default  argument  has  sub-expressions with default arguments, this
    requirement applies recursively).2)

  --if   D   is   a   class   with  an  implicitly-declared  constructor
    (_class.ctor_), it is as if the constructor was  implicitly  defined
    in every translation unit where it is used, and the implicit defini-
    tion in every translation unit shall call the same constructor for a
    base class or a class member of D.  [Example:

  _________________________
  2) _dcl.fct.default_  describes how default argument names are  looked
  up.

              // translation unit 1:
              struct X {
                      X(int);
                      X(int, int);
              };
              X::X(int = 0) { }
              class D: public X { };
              D d2; // X(int) called by D()

              // translation unit 2:
              struct X {
                      X(int);
                      X(int, int);
              };
              X::X(int = 0, int = 0) { }
              class D: public X { };     // X(int, int) called by D();
                                         // D()'s implicit definition
                                         // violates the ODR
      --end example] If D is a template, and is defined in more than one
    translation unit, then the last  four  requirements  from  the  list
    above  shall apply to names from the template's enclosing scope used
    in the template definition (_temp.nondep_), and  also  to  dependent
    names  at  the  point of instantiation (_temp.dep_).  If the defini-
    tions of D satisfy all these requirements, then  the  program  shall
    behave  as  if  there were a single definition of D.  If the defini-
    tions of D do not satisfy these requirements, then the  behavior  is
    undefined.

  3.3  Declarative regions and scopes                      [basic.scope]

1 Every  name  is  introduced  in  some portion of program text called a
  declarative region, which is the largest part of the program in  which
  that  name  is  valid,  that  is, in which that name may be used as an
  unqualified name to refer to the same entity.  In general,  each  par-
  ticular  name is valid only within some possibly discontiguous portion
  of program text called its scope.  To determine the scope of a  decla-
  ration,  it is sometimes convenient to refer to the potential scope of
  a declaration.  The scope of a declaration is the same as  its  poten-
  tial  scope unless the potential scope contains another declaration of
  the same name.  In that case, the potential scope of  the  declaration
  in the inner (contained) declarative region is excluded from the scope
  of the declaration in the outer (containing) declarative region.

2 [Example: in
          int j = 24;
          int main()
          {
                  int i = j, j;
                  j = 42;
          }
  the identifier j is declared twice as a name (and  used  twice).   The
  declarative  region  of  the first j includes the entire example.  The
  potential scope of the first j begins immediately  after  that  j  and
  extends to the end of the program, but its (actual) scope excludes the

  text between the , and the }.  The declarative region  of  the  second
  declaration of j (the j immediately before the semicolon) includes all
  the text between { and }, but its potential scope excludes the  decla-
  ration  of i.  The scope of the second declaration of j is the same as
  its potential scope.  ]

3 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-speci-
  fier  (_basic.scope.pdecl_),  and  using-directives (_namespace.udir_)
  alter this general behavior.

4 [Note: the name look up rules are summarized in _basic.lookup_.  ]

  3.3.1  Point of declaration                        [basic.scope.pdecl]

1 The point of declaration for a name is immediately after its  complete
  declarator (_dcl.decl_) and before its initializer (if any), except as
  noted below.  [Example:
      int x = 12;
      { int x = x; }
  Here the second x is initialized with its own  (indeterminate)  value.
  ]

2 [Note:  a nonlocal name remains visible up to the point of declaration
  of the local name that hides it.  [Example:
      const int  i = 2;
      { int  i[i]; }
  declares a local array of two integers.  ] ]

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 After the point of declaration of a class member, the member name  can
  be  looked  up in the scope of its class.  [Note: this is true even if
  the class is an incomplete class.  For example,
          struct X {
                  enum E { z = 16 };
                  int b[X::z]; //ok
          };
   --end note]

5 The point of declaration of a class first declared in  an  elaborated-
  type-specifier is as follows:

  --for an elaborated-type-specifier of the form
              class-key identifier ;
    the elaborated-type-specifier declares the identifier to be a class-
    name in the scope that contains the declaration, otherwise

  --for an elaborated-type-specifier of the form
              class-key identifier
    if the elaborated-type-specifier is used in  the  decl-specifier-seq
    or  parameter-declaration-clause  of a function defined in namespace
    scope, the identifier is declared as a class-name in  the  namespace
    that  contains the declaration; otherwise, except as a friend decla-
    ration, the identifier is declared in the smallest  non-class,  non-
    function-prototype  scope  that contains the declaration.  [Note: if
    the elaborated-type-specifier designates an enumeration, the identi-
    fier must refer to an already declared enum-name.  If the identifier
    in the elaborated-type-specifier is a qualified-id, it must refer to
    an     already    declared    class-name    or    enum-name.     See
    _basic.lookup.elab_.  ]

6 [Note: friend declarations refer to functions or classes that are mem-
  bers of the nearest enclosing namespace, but they do not introduce new
  names into that namespace (_namespace.memdef_).  Function declarations
  at  block  scope  and object declarations with the extern specifier at
  block scope refer to delarations that  are  members  of  an  enclosing
  namespace, but they do not introduce new names into that scope.  ]

7 [Note: For point of instantiation of a template, see _temp.inst_.  ]

  3.3.2  Local scope                                 [basic.scope.local]

1 A name declared in a block (_stmt.block_) is local to that block.  Its
  potential   scope    begins    at    its    point    of    declaration
  (_basic.scope.pdecl_) and ends at the end of its declarative region.

2 The potential scope of a function parameter name in a function defini-
  tion (_dcl.fct.def_) begins at its point of declaration.  If the func-
  tion  has a function try-block the potential scope of a parameter ends
  at the end of the last associated handler, else it ends at the end  of
  the  outermost  block  of  the  function definition.  A parameter name
  shall not be redeclared in the outermost block of the function defini-
  tion nor in the outermost block of any handler associated with a func-
  tion try-block .

3 The name in a catch exception-declaration is local to the handler  and
  shall not be redeclared in the outermost block of the handler.

4 Names  declared in the for-init-statement, and in the condition of if,
  while, for, and switch statements are local to the if, while, for,  or
  switch  statement  (including the controlled statement), and shall not
  be redeclared in a subsequent condition of that statement nor  in  the
  outermost  block  (or,  for  the  if  statement,  any of the outermost
  blocks) of the controlled statement; see _stmt.select_.

  3.3.3  Function prototype scope                    [basic.scope.proto]

1 In a function declaration, or in any function  declarator  except  the
  declarator  of a function definition (_dcl.fct.def_), names of parame-
  ters (if supplied) have function prototype scope, which terminates  at
  the end of the nearest enclosing function declarator.

  3.3.4  Function scope

1 Labels  (_stmt.label_) have function scope and may be used anywhere in
  the function in which they are declared.  Only  labels  have  function
  scope.

  3.3.5  Namespace scope                         [basic.scope.namespace]

1 The  declarative  region  of  a namespace-definition is its namespace-
  body.  The potential scope denoted by  an  original-namespace-name  is
  the  concatenation  of  the declarative regions established by each of
  the namespace-definitions in the same  declarative  region  with  that
  original-namespace-name.   Entities  declared  in a namespace-body are
  said to be members of the namespace, and  names  introduced  by  these
  declarations  into the declarative region of the namespace are said to
  be member names of the namespace.  A namespace member name has  names-
  pace  scope.   Its  potential  scope  includes  its namespace from the
  name's point of declaration  (_basic.scope.pdecl_)  onwards;  and  for
  each  using-directive  (_namespace.udir_)  that nominates the member's
  namespace, the member's potential scope includes that portion  of  the
  potential scope of the using-directive that follows the member's point
  of declaration.  [Example:
          namespace N {
                  int i;
                  int g(int a) { return a; }
                  int j();
                  void q();
          }
          namespace { int l=1; }
          // the potential scope of l is from its point of declaration
          // to the end of the translation unit
          namespace N {
                  int g(char a)         // overloads N::g(int)
                  {
                          return k+a;   // k is from unnamed namespace
                  }
                  int i;                // error: duplicate definition
                  int j();              // ok: duplicate function declaration
                  int j()               // ok: definition of N::j()
                  {
                          return g(i);  // calls N::g(int)
                  }
                  int q();              // error: different return type
          }
   --end example]

2 A namespace member can also be referred to after the :: scope  resolu-
  tion  operator  (_expr.prim_)  applied to the name of its namespace or
  the name of a namespace which nominates the member's  namespace  in  a
  using-directive; see _namespace.qual_.

3 A name declared outside all named or unnamed namespaces (_basic.names-
  pace_),  blocks  (_stmt.block_),  function  declarations  (_dcl.fct_),
  function  definitions (_dcl.fct.def_) and classes (_class_) has global

  namespace scope (also called global scope).  The  potential  scope  of
  such  a  name begins at its point of declaration (_basic.scope.pdecl_)
  and ends at the end of the translation unit that  is  its  declarative
  region.   Names  declared in the global namespace scope are said to be
  global.

  3.3.6  Class scope                                 [basic.scope.class]

1 The following rules describe the scope of names declared in classes.

  1)The potential scope of a name declared in a class consists not  only
    of  the declarative region following the name's declarator, but also
    of all function bodies, default arguments, and constructor ctor-ini-
    tializers in that class (including such things in nested classes).

  2)A  name  N  used in a class S shall refer to the same declaration in
    its context and when re-evaluated in the completed scope of  S.   No
    diagnostic is required for a violation of this rule.

  3)If  reordering  member  declarations  in a class yields an alternate
    valid program under (1) and (2),  the  program's  behavior  is  ill-
    formed, no diagnostic is required.

  4)A  name declared within a member function hides a declaration of the
    same name whose scope extends to or past the end of the member func-
    tion's class.

  5)The potential scope of a declaration that extends to or past the end
    of a class definition also extends to the  regions  defined  by  its
    member  definitions,  even if the members are defined lexically out-
    side the class (this includes static data member definitions, nested
    class definitions, member function definitions (including the member
    function body and, for  constructor  functions  (_class.ctor_),  the
    ctor-initializer   (_class.base.init_))   and  any  portion  of  the
    declarator part of such definitions which  follows  the  identifier,
    including  a  parameter-declaration-clause and any default arguments
    (_dcl.fct.default_).  [Example:
              typedef int  c;
              enum { i = 1 };
              class X {
                  char  v[i];  // error: 'i' refers to ::i
                               // but when reevaluated is X::i
                  int  f() { return sizeof(c); }  // okay: X::c
                  char  c;
                  enum { i = 2 };
              };
              typedef char*  T;
              struct Y {
                  T  a;    // error: 'T' refers to ::T
                           // but when reevaluated is Y::T
                  typedef long  T;
                  T  b;
              };
     --end example]

2 The name of a class member shall only be used as follows:

  --in the scope of its class (as described above) or  a  class  derived
    (_class.derived_) from its class,

  --after  the  .  operator  applied to an expression of the type of its
    class (_expr.ref_) or a class derived from its class,

  --after the -> operator applied to a pointer to an object of its class
    (_expr.ref_) or a class derived from its class,

  --after  the :: scope resolution operator (_expr.prim_) applied to the
    name of its class or a class derived from its class.

  3.3.7  Name hiding                                [basic.scope.hiding]

1 A name can be hidden by an explicit declaration of that same name in a
  nested declarative region or derived class (_class.member.lookup_).

2 A  class  name  (_class.name_) or enumeration name (_dcl.enum_) can be
  hidden by the name of an object, function, or enumerator  declared  in
  the  same  scope.  If a class or enumeration name and an object, func-
  tion, or enumerator are declared in the same scope (in any order) with
  the  same  name,  the class or enumeration name is hidden wherever the
  object, function, or enumerator name is visible.

3 In a member function definition, the declaration of a local name hides
  the  declaration  of  a  member  of  the class with the same name; see
  _basic.scope.class_.  The declaration of a member in a  derived  class
  (_class.derived_) hides the declaration of a member of a base class of
  the same name; see _class.member.lookup_.

4 During the lookup of a name qualified by a  namespace  name,  declara-
  tions that would otherwise be made visible by a using-directive can be
  hidden by declarations with the same name in the namespace  containing
  the using-directive; see (_namespace.qual_).

5 If a name is in scope and is not hidden it is said to be visible.

  3.4  Name look up                                       [basic.lookup]

1 The  name  look up rules apply uniformly to all names (including type-
  def-names  (_dcl.typedef_),  namespace-names  (_basic.namespace_)  and
  class-names  (_class.name_)) wherever the grammar allows such names in
  the context discussed by a particular rule.  Name look  up  associates
  the use of a name with a declaration (_basic.def_) of that name.  Name
  look up shall find  an  unambiguous  declaration  for  the  name  (see
  _class.member.lookup_).  Name look up may associate more than one dec-
  laration with a name if it finds the name to be a function  name;  the
  declarations   are   said  to  form  a  set  of  overloaded  functions
  (_over.load_).  Overload resolution (_over.match_) takes  place  after
  name  look  up  has  succeeded.  The access rules (_class.access_) are
  considered only once name look up and function overload resolution (if
  applicable)  have  succeeded.   Only  after  name  look  up,  function

  overload resolution (if applicable) and access checking have succeeded
  are  the  attributes introduced by the name's declaration used further
  in expression processing (_expr_).

2 A name "looked up in the context of an expression" is looked up as  an
  unqualified name in the scope where the expression is found.

3 [Note:  _basic.link_  discusses linkage issues.  The notions of scope,
  point of declaration and name hiding are discussed  in  _basic.scope_.
  ]

  3.4.1  Unqualified name look up                  [basic.lookup.unqual]

1 In all the cases listed in this subclause, the scopes are searched for
  a declaration in the order listed in  each  of  the  respective  cate-
  gories;  name  look  up ends as soon as a declaration is found for the
  name.  If no declaration is found, the program is ill-formed.

2 The declarations from the namespace  nominated  by  a  using-directive
  become  visible  in  a  namespace  enclosing  the using-directive; see
  _namespace.udir_.  For the purpose of the  unqualified  name  look  up
  rules described in this subclause, the declarations from the namespace
  nominated by  the  using-directive  are  considered  members  of  that
  enclosing namespace.

3 The lookup for an unqualified name used as the postfix-expression of a
  function call is described in _basic.lookup.koenig_.  [Note: for  pur-
  poses of determining (during parsing) whether an expression is a post-
  fix-expression for a function call, the usual name lookup rules apply.
  The  rules  in  _basic.lookup.koenig_  have no effect on the syntactic
  interpretation of an expression.  For example,
          typedef int f;
          struct A {
                  friend void f(A &);
                  operator int();
                  void g(A a) {
                          f(a);
                  }
          };
  The  expression  f(a)  is  a  cast-expression  equivalent  to  int(a).
  Because  the expression is not a function call, the argument-dependent
  name lookup (_basic.lookup.koenig_) does  not  apply  and  the  friend
  function f is not found.  ]

4 A  name  used in global scope, outside of any function, class or user-
  declared namespace, shall be declared before its use in global  scope.

5 A  name used in a user-declared namespace outside of the definition of
  any function or class shall be declared before its use in that  names-
  pace or before its use in a namespace enclosing its namespace.

6 A  name  used  in  the  definition of a function3) that is a member of
  namespace N (where, only  for  the  purpose  of  exposition,  N  could
  _________________________

  represent  the  global  scope) shall be declared before its use in the
  block in  which  it  is  used  or  in  one  of  its  enclosing  blocks
  (_stmt.block_) or, shall be declared before its use in namespace N or,
  if N is a nested namespace, shall be declared before its use in one of
  N's enclosing namespaces.  [Example:
          namespace A {
                  namespace N {
                          void f();
                  }
          }
          void A::N::f() {
                  i = 5;
                  // The following scopes are searched for a declaration of i:
                  // 1) outermost block scope of A::N::f, before the use of i
                  // 2) scope of namespace N
                  // 3) scope of namespace A
                  // 4) global scope, before the definition of A::N::f
          }
   --end example]

7 A  name  used in the definition of a class X outside of a member func-
  tion body or nested class definition4) shall be declared in one of the
  following ways:

  --before  its  use  in  class  X  or  be a member of a base class of X
    (_class.member.lookup_), or

  --if X is a nested class of class Y (_class.nest_), before the defini-
    tion of X in Y, or shall be a member of a base class of Y (this look
    up applies in turn to  Y's  enclosing  classes,  starting  with  the
    innermost enclosing class),5) or

  --if  X  is  a  local  class (_class.local_) or is a nested class of a
    local class, before the definition of class X in a  block  enclosing
    the definition of class X, or

  --if  X  is  a  member of namespace N, or is a nested class of a class
    that is a member of N, or is a local class or a nested class  within
    a  local class of a function that is a member of N, before the defi-
    nition of class X in namespace N or in one of N's  enclosing  names-
    paces.

  [Example:
  _________________________
  3) This refers to unqualified names following the function declarator;
  such a name may be used as a type or as a default argument name in the
  parameter-declaration-clause, or may be used in the function body.
  4) This refers to unqualified names following the class name;  such  a
  name  may be used in the base-clause or may be used in the class defi-
  nition.
  5)  This  look up applies whether the definition of X is nested within
  Y's definition or whether X's definition appears in a namespace  scope
  enclosing Y's definition (_class.nest_).

          namespace M {
                  class B { };
          }
          namespace N {
                  class Y : public M::B {
                          class X {
                                  int a[i];
                          };
                  };
          }
          // The following scopes are searched for a declaration of i:
          // 1) scope of class N::Y::X, before the use of i
          // 2) scope of class N::Y, before the definition of N::Y::X
          // 3) scope of N::Y's base class M::B
          // 4) scope of namespace N, before the definition of N::Y
          // 5) global scope, before the definition of N
   --end example] [Note: when looking for a prior declaration of a class
  or function introduced by a friend declaration, scopes outside of  the
  innermost  enclosing  namespace  scope are not considered; see _names-
  pace.memdef_.  ]  [Note:  _basic.scope.class_  further  describes  the
  restrictions  on the use of names in a class definition.  _class.nest_
  further describes the restrictions on the use of names in nested class
  definitions.   _class.local_ further describes the restrictions on the
  use of names in local class definitions.  ]

8 A name used in the definition of a function that is a member  function
  (_class.mfct_)6)  of class X shall be declared in one of the following
  ways:

  --before its use in the block in which it is used or in  an  enclosing
    block (_stmt.block_), or

  --shall  be  a  member  of class X or be a member of a base class of X
    (_class.member.lookup_), or

  --if X is a nested class of class Y (_class.nest_), shall be a  member
    of  Y,  or  shall  be  a  member  of a base class of Y (this look up
    applies in turn to Y's enclosing classes, starting with  the  inner-
    most enclosing class),7) or

  --if  X  is  a  local  class (_class.local_) or is a nested class of a
    local class, before the definition of class X in a  block  enclosing
    the definition of class X, or

  _________________________
  6)  That  is,  an  unqualified name following the function declarator;
  such a name may be used as a type or as a default argument name in the
  parameter-declaration-clause, or may be used in the function body, or,
  if the function is a constructor, may be used in the expression  of  a
  mem-initializer.
  7)  This look up applies whether the member function is defined within
  the definition of class X or whether the member function is defined in
  a namespace scope enclosing X's definition.

  --if X is a member of namespace N, or is a nested  class  of  a  class
    that  is a member of N, or is a local class or a nested class within
    a local class of a function that is a member of N, before the member
    function  definition,  in  namespace  N  or  in one of N's enclosing
    namespaces.

  [Example:
          class B { };
          namespace M {
                  namespace N {
                          class X : public B {
                                  void f();
                          };
                  }
          }
          void M::N::X::f() {
                  i = 16;
          }
          // The following scopes are searched for a declaration of i:
          // 1) outermost block scope of M::N::X::f, before the use of i
          // 2) scope of class M::N::X
          // 3) scope of M::N::X's base class B
          // 4) scope of namespace M::N
          // 5) scope of namespace M
          // 6) global scope, before the definition of M::N::X::f
    --end  example]  [Note:  _class.mfct_  and  _class.static_   further
  describe the restrictions on the use of names in member function defi-
  nitions.  _class.nest_ further describes the restrictions on  the  use
  of  names  in  the  scope  of  nested  classes.  _class.local_ further
  describes the restrictions on the use of names in local class  defini-
  tions.  ]

9 Name  look  up  for a name used in the definition of a friend function
  (_class.friend_) defined inline in the class granting friendship shall
  proceed  as  described for look up in member function definitions.  If
  the friend function is not defined in the class  granting  friendship,
  name  look  up  in  the  friend  function  definition shall proceed as
  described for look up in namespace member function definitions.

10During  the  lookup  for  a  name   used   as   a   default   argument
  (_dcl.fct.default_) in a function parameter-declaration-clause or used
  in  the  expression   of   a   mem-initializer   for   a   constructor
  (_class.base.init_), the function parameter names are visible and hide
  the names of entities declared in the block, class or namespace scopes
  containing the function declaration.  [Note: _dcl.fct.default_ further
  describes the restrictions on the use of names in  default  arguments.
  _class.base.init_  further  describes  the  restrictions on the use of
  names in a ctor-initializer.  ]

11A name used in the definition of a  static  data  member  of  class  X
  (_class.static.data_) (after the qualified-id of the static member) is
  looked up as if the name was used in a member function of  X.   [Note:
  _class.static.data_  further  describes the restrictions on the use of
  names in the definition of a static data member.  ]

12A name used in the handler  for  a  function-try-block  (_except_)  is
  looked  up as if the name was used in the outermost block of the func-
  tion definition.  In particular, the function  parameter  names  shall
  not  be  redeclared  in the exception-declaration nor in the outermost
  block of a handler for the function-try-block.  Names declared in  the
  outermost  block  of the function definition are not found when looked
  up in the scope of a handler for the function-try-block.   [Note:  but
  function parameter names are found.   --end note]

13[Note:  the  rules  for  name  look  up  in  template  definitions are
  described in _temp.res_.  ]

  3.4.2  Argument-dependent name lookup            [basic.lookup.koenig]

1 When an unqualified name is used as the postfix-expression in a  func-
  tion  call  (_expr.call_),  other namespaces not considered during the
  usual unqualified look up  (_basic.lookup.unqual_)  may  be  searched;
  this search depends on the types of the arguments.

2 For  each argument type T in the function call, there is a set of zero
  or more associated namespaces to be considered.  The set of namespaces
  is  determined entirely by the types of the function arguments.  Type-
  def names used to specify the types do not  contribute  to  this  set.
  The set of namespaces are determined in the following way:

  --If  T  is  a  fundamental  type, its associated set of namespaces is
    empty.

  --If T is a class type, its associated namespaces are  the  namespaces
    in  which  the  class  and  its direct and indirect base classes are
    defined.

  --If T is a union or enumeration type, its associated namespace is the
    namespace in which it is defined.

  --If  T  is  a  pointer  to U, a reference to U, or an array of U, its
    associated namespaces are the namespaces associated with U.

  --If T is a pointer to function type, its  associated  namespaces  are
    the  namespaces associated with the function parameter types and the
    namespaces associated with the return type.

  --If T is a pointer to a member function of a class X, its  associated
    namespaces are the namespaces associated with the function parameter
    types and return type, together with the namespaces associated  with
    X.

  --If T is a pointer to a data member of class X, its associated names-
    paces are the namespaces associated with the  member  type  together
    with the namespaces associated with X.

  --If  T is a template-id, its associated namespaces are the namespaces
    in which the template is defined, the namespaces associated with the
    types   of   the  template  arguments  provided  for  template  type

    parameters (excluding template template parameters), and the  names-
    paces  of any template template arguments.  [Note: non-type template
    arguments do not contribute to the set of associated namespaces.  ]

  If the ordinary unqualified lookup of the name finds  the  declaration
  of  a class member function, the associated namespaces are not consid-
  ered.  Otherwise the set of names found by the lookup of the  function
  name is the union of the set of names found using ordinary unqualified
  lookup and the set of names found in the  namespaces  associated  with
  the argument types.

3 When  considering  an  associated namespace, the lookup is the same as
  the lookup performed when the associated namespace is used as a quali-
  fier (_namespace.qual_) except that using-directives in the associated
  namespace are ignored.

  3.4.3  Qualified name look up                      [basic.lookup.qual]

1 The name of a class or namespace member can be referred to  after  the
  ::  scope  resolution operator (_expr.prim_) applied to a nested-name-
  specifier that nominates its class or namespace.  During the  look  up
  for  a  name preceding the :: scope resolution operator, object, func-
  tion, and enumerator names are ignored.  If the name found  is  not  a
  class-name  (_class_) or namespace-name (_namespace.def_), the program
  is ill-formed.  [Example:
          class A {
          public:
                  static int n;
          };
          int main()
          {
                  int A;
                  A::n = 42;          // OK
                  A b;                // ill-formed: A does not name a type
          }
   --end example]

2 [Note: Multiply qualified names, such as N1::N2::N3::n, can be used to
  refer to members of nested classes (_class.nest_) or members of nested
  namespaces.  ]

3 In a declaration in which the declarator-id is a  qualified-id,  names
  used  before  the  qualified-id  being  declared  are looked up in the
  defining namespace scope; names following the qualified-id are  looked
  up in the scope of the member's class or namespace.  [Example:

          class X { };
          class C {
                  class X { };
                  static const int number = 50;
                  static X arr[number];
          };
          X C::arr[number];  // ill-formed:
                             // equivalent to: ::X  C::arr[C::number];
                             // not to:  C::X  C::arr[C::number];
   --end example]

4 A name prefixed by the unary scope operator :: (_expr.prim_) is looked
  up in global scope, in the translation unit where  it  is  used.   The
  name  shall  be  declared in global namespace scope or shall be a name
  whose declaration is visible in global scope because of a using-direc-
  tive  (_namespace.qual_).   The  use  of :: allows a global name to be
  referred to even if its identifier has been hidden  (_basic.scope.hid-
  ing_).

5 A nested-name-specifier that names a scalar type, followed by ::, fol-
  lowed by ~type-name is a  pseudo-destructor-name  for  a  scalar  type
  (_expr.pseudo_).  The type-name is looked up as a type in the scope of
  the nested-name-specifier.  [Example:
          struct A {
                  typedef int I;
          };
          typedef int I1, I2;
          extern int* p;
          extern int* q;
          p->A::I::~I(); // I is looked up in the scope of A
          q->I1::~I2();  // I2 is looked up in the scope of
                         // the postfix-expression
   --end example] [Note: _basic.lookup.classref_ describes how name look
  up proceeds after the .  and -> operators.  ]

  3.4.3.1  Class members                                    [class.qual]

1 If  the nested-name-specifier of a qualified-id nominates a class, the
  name specified after the nested-name-specifier is  looked  up  in  the
  scope  of the class (_class.member.lookup_).  The name shall represent
  one or more members of that class  or  of  one  of  its  base  classes
  (_class.derived_).   [Note:  a class member can be referred to using a
  qualified-id    at    any    point    in    its    potential     scope
  (_basic.scope.class_).  ]

2 A class member name hidden by a name in a nested declarative region or
  by the name of a derived class member can still be found if  qualified
  by the name of its class followed by the :: operator.

  3.4.3.2  Namespace members                            [namespace.qual]

1 If  the nested-name-specifier of a qualified-id nominates a namespace,
  the name specified after the nested-name-specifier is looked up in the
  scope of the namespace.

2 Given X::m (where X is a user-declared namespace), or given ::m (where
  X is the global namespace), let S be the set of all declarations of  m
  in  X  and  in  the  transitive closure of all namespaces nominated by
  using-directives in X and its  used  namespaces,  except  that  using-
  directives  are  ignored  in any namespace, including X, directly con-
  taining one or more declarations of m.  No namespace is searched  more
  than once in the lookup of a name.  If S is the empty set, the program
  is ill-formed.  Otherwise, if S has exactly one member, or if the con-
  text of the reference is a using-declaration (_namespace.udecl_), S is
  the required set of declarations of m.  Otherwise if the use of  m  is
  not one that allows a unique declaration to be chosen from S, the pro-
  gram is ill-formed.  [Example:
          int x;
          namespace Y {
                  void f(float);
                  void h(int);
          }
          namespace Z {
                  void h(double);
          }
          namespace A {
                  using namespace Y;
                  void f(int);
                  void g(int);
                  int i;
          }
          namespace B {
                  using namespace Z;
                  void f(char);
                  int i;
          }
          namespace AB {
                  using namespace A;
                  using namespace B;
                  void g();
          }

          void h()
          {
                  AB::g();     // g is declared directly in AB,
                               // therefore S is { AB::g() } and AB::g() is chosen
                  AB::f(1);    // f is not declared directly in AB so the rules are
                               // applied recursively to A and B;
                               // namespace Y is not searched and Y::f(float)
                               // is not considered;
                               // S is { A::f(int), B::f(char) } and overload
                               // resolution chooses A::f(int)
                  AB::f('c');  // as above but resolution chooses B::f(char)

                  AB::x++;     // x is not declared directly in AB, and
                               // is not declared in A or B, so the rules are
                               // applied recursively to Y and Z,
                               // S is { } so the program is ill-formed
                  AB::i++;     // i is not declared directly in AB so the rules are
                               // applied recursively to A and B,
                               // S is { A::i, B::i } so the use is ambiguous
                               // and the program is ill-formed
                  AB::h(16.8); // h is not declared directly in AB and
                               // not declared directly in A or B so the rules are
                               // applied recursively to Y and Z,
                               // S is { Y::h(int), Z::h(double) } and overload
                               // resolution chooses Z::h(double)
          }

3 The same declaration found more than once is not an ambiguity (because
  it is still a unique declaration). For example:
          namespace A {
                  int a;
          }
          namespace B {
                  using namespace A;
          }
          namespace C {
                  using namespace A;
          }
          namespace BC {
                  using namespace B;
                  using namespace C;
          }
          void f()
          {
                  BC::a++;  // ok: S is { A::a, A::a }
          }
          namespace D {
                  using A::a;
          }
          namespace BD {
                  using namespace B;
                  using namespace D;
          }

          void g()
          {
                  BD::a++;  // ok: S is { A::a, A::a }
          }

4 Because  each  referenced namespace is searched at most once, the fol-
  lowing is well-defined:
          namespace B {
                  int b;
          }
          namespace A {
                  using namespace B;
                  int a;
          }
          namespace B {
                  using namespace A;
          }
          void f()
          {
                  A::a++;  // ok: a declared directly in A, S is { A::a }
                  B::a++;  // ok: both A and B searched (once), S is { A::a }
                  A::b++;  // ok: both A and B searched (once), S is { B::b }
                  B::b++;  // ok: b declared directly in B, S is { B::b }
          }
   --end example]

5 During the look up of a qualified namespace member name, if  the  look
  up  finds more than one declaration of the member, and if one declara-
  tion introduces a class name or enumeration name and the other  decla-
  rations either introduce the same object, the same enumerator or a set
  of functions, the non-type name hides the class or enumeration name if
  and  only  if  the declarations are from the same namespace; otherwise
  (the declarations are from different namespaces), the program is  ill-
  formed.  [Example:
          namespace A {
                  struct x { };
                  int x;
                  int y;
          }
          namespace B {
                  struct y {};
          }
          namespace C {
                  using namespace A;
                  using namespace B;
                  int i = C::x; // ok, A::x (of type 'int')
                  int j = C::y; // ambiguous, A::y or B::y
          }
   --end example]

6 In  a declaration for a namespace member in which the declarator-id is
  a qualified-id, given that the qualified-id for the  namespace  member
  has the form
          nested-name-specifier unqualified-id

  the  unqualified-id shall name a member of the namespace designated by
  the nested-name-specifier.  [Example:
          namespace A {
                  namespace B {
                          void f1(int);
                  }
                  using namespace B;
          }
          void A::f1(int){}  // ill-formed, f1 is not a member of A
   --end example] However, in such namespace  member  declarations,  the
  nested-name-specifier  may rely on using-directives to implicitly pro-
  vide the initial part of the nested-name-specifier.  [Example:
          namespace A {
                  namespace B {
                          void f1(int);
                  }
          }
          namespace C {
                  namespace D {
                          void f1(int);
                  }
          }
          using namespace A;
          using namespace C::D;
          void B::f1(int){}  // okay, defines A::B::f1(int)
   --end example]

  3.4.4  Elaborated type specifiers                  [basic.lookup.elab]

1 An elaborated-type-specifier may be used  to  refer  to  a  previously
  declared  class-name or enum-name even though the name has been hidden
  by a non-type declaration (_basic.scope.hiding_).  The  class-name  or
  enum-name  in  the  elaborated-type-specifier  may  either be a simple
  identifer or be a qualified-id.

2 If the name in the elaborated-type-specifier is  a  simple  identifer,
  and unless the elaborated-type-specifier has the following form:
          class-key identifier ;
  the  identifier  is  looked  up according to _basic.lookup.unqual_ but
  ignoring any non-type names that have been  declared.   If  this  name
  look  up  finds  a typedef-name, the elaborated-type-specifier is ill-
  formed.  If the elaborated-type-specifier refers to an  enum-name  and
  this look up does not find a previously declared enum-name, the elabo-
  rated-type-specifier is ill-formed.  If the  elaborated-type-specifier
  refers  to  an  class-name and this look up does not find a previously
  declared class-name, or if the elaborated-type-specifier has the form:
          class-key identifier ;
  the  elaborated-type-specifier  is  a  declaration that introduces the
  class-name as described in _basic.scope.pdecl_.

3 If the name is a qualified-id, the name is  looked  up  according  its
  qualifications,  as described in _basic.lookup.qual_, but ignoring any
  non-type names that have been declared.  If this name lookup  finds  a
  typedef-name,  the  elaborated-type-specifier  is ill-formed.  If this

  name look up does not find a previously declared class-name  or  enum-
  name, the elaborated-type-specifier is ill-formed.  [Example:
          struct Node {
                  struct Node* Next;      // ok: Refers to Node at global scope
                  struct Data* Data;      // ok: Declares type Data
                                          // at global scope and member Data
          };
          struct Data {
                  struct Node* Node;      // ok: Refers to Node at global scope
                  friend struct ::Glob;   // error: Glob is not declared
                                          // cannot introduce a qualified type (_decl.type.elab_)
                  friend struct Glob;     // ok: Refers to (as yet) undeclared Glob
                                          // at global scope.
                  /* ... */
          };
          struct Base {
                  struct Data;                    // ok: Declares nested Data
                  struct ::Data*     thatData;    // ok: Refers to ::Data
                  struct Base::Data* thisData;    // ok: Refers to nested Data
                  friend class ::Data;            // ok: global Data is a friend
                  friend class Data;              // ok: nested Data is a friend
                  struct Data { /* ... */ };      // Defines nested Data
                  struct Data;                    // ok: Redeclares nested Data
          };
          struct Data;            // ok: Redeclares Data at global scope
          struct ::Data;          // error: cannot introduce a qualified type (_decl.type.elab_)
          struct Base::Data;      // error: cannot introduce a qualified type (_decl.type.elab_)
          struct Base::Datum;     // error: Datum undefined
          struct Base::Data* pBase;       // ok: refers to nested Data
   --end example]

  3.4.5  Class member access                     [basic.lookup.classref]

1 If  the  id-expression  in  a  class  member access (_expr.ref_) is an
  unqualified-id, and the type of the object expression is  of  a  class
  type C (or of pointer to a class type C), the unqualified-id is looked
  up in the scope of class C.  If the type of the object  expression  is
  of pointer to scalar type, the unqualified-id is looked up in the con-
  text of the complete postfix-expression.

2 If the unqualified-id is  ~type-name,  and  the  type  of  the  object
  expression is of a class type C (or of pointer to a class type C), the
  type-name is looked up in the context of the entire postfix-expression
  and  in  the  scope of class C.  The type-name shall refer to a class-
  name.  If type-name is found in both contexts, the name shall refer to
  the  same  class  type.   If  the  type of the object expression is of
  scalar type, the type-name is looked up in the scope of  the  complete
  postfix-expression.

3 If the id-expression in a class member access is a qualified-id of the
  form
          class-name-or-namespace-name::...
  the class-name-or-namespace-name following the .  or  ->  operator  is
  looked  up both in the context of the entire postfix-expression and in

  the scope of the class of the object expression.  If the name is found
  only  in  the  scope  of  the class of the object expression, the name
  shall refer to a class-name.  If the name is found only in the context
  of the entire postfix-expression, the name shall refer to a class-name
  or namespace-name.  If the name is found in both contexts, the  class-
  name-or-namespace-name shall refer to the same entity.  [Note: because
  the name of a class is inserted in its class scope (_class_), the name
  of a class is also considered a nested member of that class.  ] [Note:
  the result of  looking  up  the  class-name-or-namespace-name  is  not
  required  to  be  a  unique base class of the class type of the object
  expression, as long as the entity or entities named by the  qualified-
  id  are members of the class type of the object expression and are not
  ambiguous according to _class.member.lookup_.
          struct A {
                  int a;
          };
          struct B: virtual A { };
          struct C: B { };
          struct D: B { };
          struct E: public C, public D { };
          struct F: public A { };
          void f() {
                  E e;
                  e.B::a = 0;     // ok, only one A::a in E

                  F f;
                  f.B::a = 1;     // ok, A::a is a member of F
          }
   --end note]

4 If the qualified-id has the form
          ::class-name-or-namespace-name::...
  the class-name-or-namespace-name is looked up in  global  scope  as  a
  class-name or namespace-name.

5 If    the   nested-name-specifier   contains   a   class   template-id
  (_temp.names_), its template-arguments are evaluated in the context in
  which the entire postfix-expression occurs.

6 If the id-expression is a conversion-function-id, its conversion-type-
  id shall denote the same type in both the context in which the  entire
  postfix-expression  occurs  and  in  the  context  of the class of the
  object expression (or the class pointed to by the pointer expression).

  3.4.6  Using-directives and namespace aliases      [basic.lookup.udir]

1 When looking up a namespace-name in a  using-directive  or  namespace-
  alias-definition, only namespace names are considered.

  3.5  Program and linkage                                  [basic.link]

1 A  program  consists  of  one or more translation units (_lex_) linked
  together.  A translation unit consists of a sequence of  declarations.

          translation-unit:
                  declaration-seqopt

2 A  name  is said to have linkage when it might denote the same object,
  reference, function, type, template, namespace  or  value  as  a  name
  introduced by a declaration in another scope:

  --When  a  name  has  external  linkage,  the entity it denotes can be
    referred to by names from scopes of other translation units or  from
    other scopes of the same translation unit.

  --When  a  name  has  internal  linkage,  the entity it denotes can be
    referred to by names from other scopes in the same translation unit.

  --When a name has no linkage, the entity it denotes cannot be referred
    to by names from other scopes.

3 A name having namespace scope (_basic.scope.namespace_)  has  internal
  linkage if it is the name of

  --an  object, reference, function or function template that is explic-
    itly declared static or,

  --an object or reference that is explicitly declared const and neither
    explicitly  declared extern nor previously declared to have external
    linkage; or

  --the name of a data member of an anonymous union.

4 A name having namespace scope has external linkage if it is  the  name
  of

  --an object or reference, unless it has internal linkage; or

  --a function, unless it has internal linkage; or

  --a  named  class  (_class_), or an unnamed class defined in a typedef
    declaration in which the class has the typedef name for linkage pur-
    poses (_dcl.typedef_); or

  --a  named enumeration (_dcl.enum_), or an unnamed enumeration defined
    in a typedef declaration in which the enumeration  has  the  typedef
    name for linkage purposes (_dcl.typedef_); or

  --an enumerator belonging to an enumeration with external linkage; or

  --a template, unless it is a function template that has internal link-
    age (_temp_); or

  --a namespace (_basic.namespace_), unless it  is  declared  within  an
    unnamed namespace.

5 In  addition, a member function, static data member, class or enumera-
  tion of class scope has external linkage if the name of the class  has

  external linkage.

6 The name of a function declared in block scope, and  the  name  of  an
  object declared by a block scope extern declaration, have linkage.  If
  there is a visible declaration of an entity with  linkage  having  the
  same  name  and type, ignoring entities declared outside the innermost
  enclosing namespace scope, the block scope declaration  declares  that
  same  entity and receives the linkage of the previous declaration.  If
  there is more than one such  matching  entity,  the  program  is  ill-
  formed.   Otherwise,  if  no matching entity is found, the block scope
  entity receives external linkage.  [Example:
          static void f();
          static int i = 0; //1
          void g() {
                  extern void f(); // internal linkage
                  int i; //2: 'i' has no linkage
                  {
                          extern void f(); // internal linkage
                          extern int i; //3: external linkage
                  }
          }
  There are three objects named i in  this  program.   The  object  with
  internal  linkage  introduced by the declaration in global scope (line
  //1), the object with automatic storage duration and no linkage intro-
  duced by the declaration on line //2, and the object with static stor-
  age duration and external linkage introduced  by  the  declaration  on
  line //3.  ]

7 When  a block scope declaration of an entity with linkage 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]

8 Names not covered by these rules have no linkage.  Moreover, except as
  noted,  a  name declared in a local scope (_basic.scope.local_) has no
  linkage.  A name with no linkage (notably, the name of a class or enu-
  meration declared in a local scope (_basic.scope.local_)) shall not be
  used to declare an entity with linkage.  If a declaration uses a type-
  def  name,  it  is  the  linkage of the type name to which the typedef

  refers that is considered.  [Example:
          void f()
          {
              struct A { int x; };       // no linkage
              extern A a;                // ill-formed
              typedef A B;
              extern B b;                // ill-formed
          }
   --end example] This implies that names with no linkage cannot be used
  as template arguments (_temp.arg_).

9 Two names that are the same (_basic_) and that are declared in differ-
  ent scopes shall denote the same object,  reference,  function,  type,
  enumerator, template or namespace if

  --both  names  have  external linkage or else both names have internal
    linkage and are declared in the same translation unit; and

  --both names refer to members of the same namespace or to members, not
    by inheritance, of the same class; and

  --when  both  names denote functions, the function types are identical
    for purposes of overloading; and

  --when  both  names  denote   function   templates,   the   signatures
    (_temp.over.link_) are the same.

10After  all adjustments of types (during which typedefs (_dcl.typedef_)
  are replaced by their definitions), the types specified by all  decla-
  rations of a name in a given namespace shall be identical, except that
  declarations for an array object can specify array types  that  differ
  by  the presence or absence of a major array bound (_dcl.array_), dec-
  larations for functions with the same name can specify different  num-
  bers  and  types of parameters (_dcl.fct_), and a class or enumeration
  may  have  the  same  name  as  an  object,  function  or   enumerator
  (_basic.scope.hiding_).   A  violation  of  this rule on type identity
  does not require a diagnostic.

11[Note: linkage to non-C++ declarations can be achieved using  a  link-
  age-specification (_dcl.link_).  ]

  3.6  Start and termination                               [basic.start]

  3.6.1  Main function                                [basic.start.main]

1 A  program  shall  contain a global function called main, which is the
  designated start of the program.  It is implementation-defined whether
  a  program  in a freestanding environment is required to define a main
  function.  [Note: in a freestanding environment, start-up and termina-
  tion  is  implementation-defined;  start-up  contains the execution of
  constructors for objects of namespace scope with static storage  dura-
  tion;  termination  contains  the execution of destructors for objects
  with static storage duration.  ]

2 An implementation shall not predefine the main function.   This  func-
  tion  shall  not  be  overloaded.  It shall have a return type of type
  int, but otherwise its type is implementation-defined.  All  implemen-
  tations shall allow both of the following definitions of main:
          int main() { /* ... */ }
  and
          int main(int argc, char* argv[]) { /* ... */ }
  In the latter form argc shall be the number of arguments passed to the
  program from the environment in which the program is run.  If argc  is
  nonzero   these   arguments  shall  be  supplied  in  argv[0]  through
  argv[argc-1] as pointers to the initial characters of  null-terminated
  multibyte strings (NTMBSs) (_lib.multibyte.strings_) and argv[0] shall
  be the pointer to the initial character of a NTMBS that represents the
  name  used  to  invoke  the program or "".  The value of argc shall be
  nonnegative.  The value of argv[argc] shall be 0.  [Note: it is recom-
  mended that any further (optional) parameters be added after argv.  ]

3 The  function  main  shall  not  be called from within a program.  The
  linkage (_basic.link_) of main is implementation-defined.   A  program
  that  takes  the  address  of main, or declares it inline or static is
  ill-formed.  The name main is not otherwise reserved.  [Example:  mem-
  ber  functions,  classes,  and enumerations can be called main, as can
  entities in other namespaces.  ]

4 Calling the function
          void exit(int);
  declared in <cstdlib> (_lib.support.start.term_) terminates  the  pro-
  gram  without  leaving  the current block and hence without destroying
  any objects with automatic storage duration (_class.dtor_).   If  exit
  is  called  to  end a program during the destruction of an object with
  static storage duration, the program has undefined behavior.

5 A return statement in main has the effect of leaving the main function
  (destroying  any  objects with automatic storage duration) and calling
  exit with the return value as the argument.  If  control  reaches  the
  end  of  main  without  encountering a return statement, the effect is
  that of executing
          return 0;

  3.6.2  Initialization of non-local objects          [basic.start.init]

1 The   storage   for   objects    with    static    storage    duration
  (_basic.stc.static_) shall be zero-initialized (_dcl.init_) before any
  other   initialization   takes   place.    Objects   of   POD    types
  (_basic.types_) with static storage duration initialized with constant
  expressions (_expr.const_) shall be  initialized  before  any  dynamic
  initialization  takes  place.   Objects of namespace scope with static
  storage duration defined in the same translation unit and  dynamically
  initialized  shall  be initialized in the order in which their defini-
  tion  appears  in  the  translation  unit.    [Note:   _dcl.init.aggr_
  describes  the  order in which aggregate members are initialized.  The
  initialization of local static objects is described in _stmt.dcl_.  ]

2 An implementation is permitted to perform  the  initialization  of  an
  object  of  namespace  scope  with static storage duration as a static
  initialization even if such initialization is not required to be  done
  statically, provided that

  --the  dynamic version of the initialization does not change the value
    of any other object of namespace scope with static storage  duration
    prior to its initialization, and

  --the  static version of the initialization produces the same value in
    the initialized object as would be produced by the dynamic  initial-
    ization  if  all  objects  not required to be initialized statically
    were initialized dynamically.

  [Note: as a consequence, if  the  initialization  of  an  object  obj1
  refers  to an object obj2 of namespace scope with static storage dura-
  tion potentially requiring dynamic initialization and defined later in
  the same translation unit, it is unspecified whether the value of obj2
  used will be the value of the fully initialized obj2 (because obj2 was
  statically  initialized) or will be the value of obj2 merely zero-ini-
  tialized.  For example,
          inline double fd() { return 1.0; }
          extern double d1;
          double d2 = d1; // unspecified:
                          // may be statically initialized to 0.0 or
                          // dynamically initialized to 1.0
          double d1 = fd(); // may be initialized statically to 1.0
   --end note]

3 It  is  implementation-defined  whether  the  dynamic   initialization
  (_dcl.init_,  _class.static_,  _class.ctor_,  _class.expl.init_) of an
  object of namespace scope with static storage duration is done  before
  the first statement of main or deferred to any point in time after the
  first statement of main but before the first  use  of  a  function  or
  object defined in the same translation unit.  [Example:
          // -- File 1 --
          #include "a.h"
          #include "b.h"
          B b;
          A::A(){
                  b.Use();
          }
          // -- File 2 --
          #include "a.h"
          A a;
          // -- File 3 --
          #include "a.h"
          #include "b.h"
          extern A a;
          extern B b;
          main() {
                  a.Use();
                  b.Use();
          }

  It  is  implementation-defined  whether  a  is  defined before main is
  entered or whether its definition is delayed until a is first used  in
  main.   It  is implementation-defined whether b is defined before main
  is entered or whether its definition is delayed until b is first  used
  in main.  In particular, if a is defined before main is entered, it is
  not guaranteed that b will be initialized before it  is  used  by  the
  initialization of a, that is, before A::A is called.  ]

4 If  construction  or  destruction of a non-local static object ends in
  throwing an uncaught  exception,  the  result  is  to  call  terminate
  (_lib.terminate_).

  3.6.3  Termination                                  [basic.start.term]

1 Destructors  (_class.dtor_)  for initialized objects of static storage
  duration (declared at block scope or at namespace scope) are called as
  a  result  of  returning  from  main  and  as a result of calling exit
  (_lib.support.start.term_).   These  objects  are  destroyed  in   the
  reverse order of the completion of their constructor or of the comple-
  tion of their dynamic initialization.  If  an  object  is  initialized
  statically, the object is destroyed in the same order as if the object
  was dynamically initialized.  For an object of array  or  class  type,
  all  subobjects  of  that object are destroyed before any local object
  with static storage duration initialized during  the  construction  of
  the subobjects is destroyed.

2 If  a function contains a local object of static storage duration that
  has been destroyed and the function is called during  the  destruction
  of  an  object with static storage duration, the program has undefined
  behavior if the flow of control passes through the definition  of  the
  previously destroyed local object.

3 If  a  function  is  registered  with atexit (see <cstdlib>, _lib.sup-
  port.start.term_) then following the call to exit,  any  objects  with
  static  storage duration initialized prior to the registration of that
  function will not be destroyed until the registered function is called
  from  the  termination  process and has completed.  For an object with
  static storage duration constructed after  a  function  is  registered
  with  atexit, then following the call to exit, the registered function
  is not called until the execution of the object's destructor has  com-
  pleted.

4 Calling the function
          void abort();
  declared   in  <cstdlib>  terminates  the  program  without  executing
  destructors for objects of automatic or static  storage  duration  and
  without calling the functions passed to atexit().

  3.7  Storage duration                                      [basic.stc]

1 Storage duration is the property of an object that defines the minimum
  potential lifetime of the storage containing the object.  The  storage
  duration  is determined by the construct used to create the object and
  is one of the following:

  --static storage duration

  --automatic storage duration

  --dynamic storage duration

2 Static and automatic storage durations  are  associated  with  objects
  introduced by declarations (_basic.def_) and implicitly created by the
  implementation (_class.temporary_).  The dynamic storage  duration  is
  associated with objects created with operator new (_expr.new_).

3 The  storage  class  specifiers static and auto are related to storage
  duration as described below.

4 References (_dcl.ref_) might or might not  require  storage;  however,
  the storage duration categories apply to references as well.

  3.7.1  Static storage duration                      [basic.stc.static]

1 All  objects which neither have dynamic storage duration nor are local
  have static storage duration.  The storage  for  these  objects  shall
  last   for   the   duration   of   the   program  (_basic.start.init_,
  _basic.start.term_).

2 If an object of  static  storage  duration  has  initialization  or  a
  destructor  with  side  effects, it shall not be eliminated even if it
  appears to be unused, except that a class object or its  copy  may  be
  eliminated as specified in _class.copy_.

3 The keyword static can be used to declare a local variable with static
  storage duration.  [Note: _stmt.dcl_ describes the  initialization  of
  local  static  variables; _basic.start.term_ describes the destruction
  of local static variables.  ]

4 The keyword static applied to a class data member in a  class  defini-
  tion gives the data member static storage duration.

  3.7.2  Automatic storage duration                     [basic.stc.auto]

1 Local  objects  explicitly declared auto or register or not explicitly
  declared static or extern have automatic storage duration.  The  stor-
  age  for these objects lasts until the block in which they are created
  exits.

2 [Note: these objects are initialized and  destroyed  as  described  in
  _stmt.dcl_.  ]

3 If  a  named  automatic object has initialization or a destructor with
  side effects, it shall not be destroyed before the end of  its  block,
  nor shall it be eliminated as an optimization even if it appears to be
  unused, except that a class object or its copy may  be  eliminated  as
  specified in _class.copy_.

  3.7.3  Dynamic storage duration                    [basic.stc.dynamic]

1 Objects   can   be   created   dynamically  during  program  execution
  (_intro.execution_), using new-expressions (_expr.new_), and destroyed
  using  delete-expressions  (_expr.delete_).  A C++ implementation pro-
  vides access to, and management of, dynamic  storage  via  the  global
  allocation  functions  operator  new and operator new[] and the global
  deallocation functions operator delete and operator delete[].

2 The library provides default definitions for the global allocation and
  deallocation functions.  Some global allocation and deallocation func-
  tions are replaceable (_lib.new.delete_).  A C++ program shall provide
  at  most  one  definition  of a replaceable allocation or deallocation
  function.  Any such function definition replaces the  default  version
  provided  in the library (_lib.replacement.functions_).  The following
  allocation  and  deallocation  functions  (_lib.support.dynamic_)  are
  implicitly declared in global scope in each translation unit of a pro-
  gram
          void* operator new(std::size_t) throw(std::bad_alloc);
          void* operator new[](std::size_t) throw(std::bad_alloc);
          void operator delete(void*) throw();
          void operator delete[](void*) throw();
  These implicit declarations introduce only the function names operator
  new,  operator  new[], operator delete, operator delete[].  [Note: the
  implicit declarations do not introduce the names std,  std::bad_alloc,
  and  std::size_t,  or any other names that the library uses to declare
  these names.  Thus, a new-expression,  delete-expression  or  function
  call  that  refers  to  one  of  these functions without including the
  header   <new>   is   well-formed.    However,   referring   to   std,
  std::bad_alloc, and std::size_t is ill-formed unless the name has been
  declared by including the appropriate  header.   ]  Allocation  and/or
  deallocation  functions can also be declared and defined for any class
  (_class.free_).

3 Any allocation and/or deallocation functions defined in a C++  program
  shall conform to the semantics specified in this subclause.

  3.7.3.1  Allocation functions           [basic.stc.dynamic.allocation]

1 An  allocation  function  shall be a class member function or a global
  function; a  program  is  ill-formed  if  an  allocation  function  is
  declared  in  a  namespace  scope  other than global scope or declared
  static in global scope.  The return type shall be  void*.   The  first
  parameter  shall  have  type  size_t (_lib.support.types_).  The first
  parameter   shall   not   have   an   associated   default    argument
  (_dcl.fct.default_).  The value of the first parameter shall be inter-
  preted as the requested size of the allocation.  An  allocation  func-
  tion  can  be  a function template.  Such a template shall declare its
  return type and first parameter as specified above (that is,  template
  parameter types shall not be used in the return type and first parame-
  ter type).  Template allocation  functions  shall  have  two  or  more
  parameters.

2 The function shall return the address of the start of a block of stor-
  age whose length in bytes shall be at least as large as the  requested
  size.  There are no constraints on the contents of the allocated stor-
  age on return from the allocation function.   The  order,  contiguity,
  and initial value of storage allocated by successive calls to an allo-
  cation function is unspecified.  The pointer returned shall  be  suit-
  ably  aligned so that it can be converted to a pointer of any complete
  object type and then used to access the object or array in the storage
  allocated  (until the storage is explicitly deallocated by a call to a
  corresponding deallocation  function).   If  the  size  of  the  space
  requested  is  zero,  the  value  returned shall not be a null pointer
  value (_conv.ptr_).  The results of dereferencing a  pointer  returned
  as a request for zero size are undefined.8)

3 An allocation function that fails to allocate storage can  invoke  the
  currently  installed  new_handler  (_lib.new.handler_).  [Note: A pro-
  gram-supplied allocation function can obtain the address of  the  cur-
  rently   installed  new_handler  using  the  set_new_handler  function
  (_lib.set.new.handler_).   ]  If   a   nothrow   allocation   function
  (_lib.support.dynamic_)  fails  to allocate storage, it shall return a
  null pointer.  Any other allocation function that  fails  to  allocate
  storage  shall only indicate failure by throwing an exception of class
  std::bad_alloc   (_lib.bad.alloc_)   or   a   class    derived    from
  std::bad_alloc.

4 A  global  allocation  function  is only called as the result of a new
  expression (_expr.new_), or called directly using  the  function  call
  syntax  (_expr.call_), or called indirectly through calls to the func-
  tions in the C++ standard library.  [Note:  in  particular,  a  global
  allocation function is not called to allocate storage for objects with
  static storage duration  (_basic.stc.static_),  for  objects  of  type
  type_info (_expr.typeid_), for the copy of an object thrown by a throw
  expression (_except.throw_).  ]

  3.7.3.2  Deallocation functions       [basic.stc.dynamic.deallocation]

1 Deallocation functions shall be class member functions or global func-
  tions;  a program is ill-formed if deallocation functions are declared
  in a namespace scope other than global scope  or  declared  static  in
  global scope.

2 Each  deallocation  function shall return void and its first parameter
  shall be void*.  A deallocation function can have more than one param-
  eter.   If a class T has a member deallocation function named operator
  delete with exactly one parameter, then that function is a usual (non-
  placement) deallocation function.  If class T does not declare such an
  operator delete but does declare a member deallocation function  named
  operator  delete  with exactly two parameters, the second of which has
  type std::size_t (_lib.support.types_), then this function is a  usual
  _________________________
  8)  The intent is to have operator new() implementable by calling mal-
  loc() or calloc(), so the rules are substantially the same.  C++  dif-
  fers  from C in requiring a zero request to return a non-null pointer.

  deallocation function.  Similarly, if a class T has a member dealloca-
  tion function named operator delete[] with exactly one parameter, then
  that  function  is  a usual (non-placement) deallocation function.  If
  class T does not declare such an operator delete[] but does declare  a
  member  deallocation function named operator delete[] with exactly two
  parameters, the second of which has type std::size_t, then this  func-
  tion is a usual deallocation function.  A deallocation function can be
  an instance of a function template.  Neither the first  parameter  nor
  the return type shall depend on a template parameter.  [Note: that is,
  a deallocation function template shall have a first parameter of  type
  void*  and  a return type of void (as specified above).  ] A dealloca-
  tion function template shall have two or more function parameters.   A
  template  instance  is never a usual deallocation function, regardless
  of its signature.

3 The value of the first parameter supplied to a  deallocation  function
  shall  be  a  null pointer value, or refer to storage allocated by the
  corresponding allocation function (even if  that  allocation  function
  was  called with a zero argument).  If the value of the first argument
  is a null pointer value, the call to the deallocation function has  no
  effect.   If  the  value  of  the  first  argument refers to a pointer
  already deallocated, the effect is undefined.

4 If the argument given to a deallocation function is a pointer that  is
  not  the  null  pointer  value (_conv.ptr_), the deallocation function
  will deallocate the storage referenced by the pointer  thus  rendering
  the  pointer  invalid.  At the time storage is deallocated, the values
  of any pointers that refer to that deallocated storage become indeter-
  minate.  The effect of using the value of a pointer with an indetermi-
  nate value is undefined.9)

  3.7.4  Duration of sub-objects                     [basic.stc.inherit]

1 The  storage  duration of member subobjects, base class subobjects and
  array elements is that of their complete object (_intro.object_).

  3.8  Object Lifetime                                      [basic.life]

1 The lifetime of an object is a runtime property of  the  object.   The
  lifetime of an object of type T begins when:

  --storage  with  the proper alignment and size for type T is obtained,
    and

  --if T is a class type with a non-trivial constructor  (_class.ctor_),
    the constructor call has completed.

  The lifetime of an object of type T ends when:

  --if  T  is a class type with a non-trivial destructor (_class.dtor_),
  _________________________
  9) On some  implementations,  it  causes  a  system-generated  runtime
  fault.

    the destructor call starts, or

  --the storage which the object occupies is reused or released.

2 [Note: the lifetime of an  array  object  or  of  an  object  of  type
  (_basic.types_)  starts as soon as storage with proper size and align-
  ment is obtained, and its lifetime ends when  the  storage  which  the
  array  or  object  occupies  is reused or released.  _class.base.init_
  describes the lifetime of base and member subobjects.  ]

3 The properties ascribed to objects throughout this International Stan-
  dard  apply  for  a  given object only during its lifetime.  [Note: in
  particular, before the lifetime of an  object  starts  and  after  its
  lifetime  ends  there  are  significant restrictions on the use of the
  object, as described below, in _class.base.init_ and in _class.cdtor_.
  Also,  the  behavior  of  an object under construction and destruction
  might not be the same as the behavior of an object whose lifetime  has
  started  and  not ended.  _class.base.init_ and _class.cdtor_ describe
  the behavior  of  objects  during  the  construction  and  destruction
  phases.  ]

4 A  program  may  end the lifetime of any object by reusing the storage
  which the object occupies or by explicitly calling the destructor  for
  an  object  of  a  class  type  with a non-trivial destructor.  For an
  object of a class type with a non-trivial destructor, the  program  is
  not  required  to  call  the  destructor explicitly before the storage
  which the object occupies is reused or released; however, if there  is
  no   explicit  call  to  the  destructor  or  if  a  delete-expression
  (_expr.delete_) is not used to release  the  storage,  the  destructor
  shall  not  be  implicitly  called and any program that depends on the
  side effects produced by the destructor has undefined behavior.

5 Before the lifetime of an object has started  but  after  the  storage
  which the object will occupy has been allocated10) or, after the life-
  time  of  an  object has ended and before the storage which the object
  occupied is reused or released, any pointer that refers to the storage
  location  where the object will be or was located may be used but only
  in  limited  ways.   Such  a  pointer  refers  to  allocated   storage
  (_basic.stc.dynamic.deallocation_),  and  using  the pointer as if the
  pointer were of type void*, is well-defined.  Such a  pointer  may  be
  dereferenced  but  the  resulting  lvalue  may only be used in limited
  ways, as described below.  If the object will be or  was  of  a  class
  type  with  a  non-trivial  destructor, and the pointer is used as the
  operand of a delete-expression, the program  has  undefined  behavior.
  If  the object will be or was of a non-POD class type, the program has
  undefined behavior if:

  --the pointer is used to access a non-static data  member  or  call  a
    non-static member function of the object, or
  _________________________
  10) For example, before the construction of a global object of non-POD
  class type (_class.cdtor_).

  --the  pointer  is implicitly converted (_conv.ptr_) to a pointer to a
    base class type, or

  --the  pointer   is   used   as   the   operand   of   a   static_cast
    (_expr.static.cast_) (except when the conversion is to void*, char*,
    or unsigned char*).

  --the  pointer  is   used   as   the   operand   of   a   dynamic_cast
    (_expr.dynamic.cast_).  [Example:
              struct B {
                      virtual void f();
                      void mutate();
                      virtual ~B();
              };

              struct D1 : B { void f(); };
              struct D2 : B { void f(); };
              void B::mutate() {
                      new (this) D2;  // reuses storage - ends the lifetime of '*this'
                      f();            // undefined behavior
                      ... = this;     // ok, 'this' points to valid memory
              }
              void g() {
                      void* p = malloc(sizeof(D1) + sizeof(D2));
                      B* pb = new (p) D1;
                      pb->mutate();
                      &pb;            // ok: pb points to valid memory
                      void* q = pb;   // ok: pb points to valid memory
                      pb->f();        // undefined behavior, lifetime of *pb has ended
              }
     --end example]

6 Similarly,  before the lifetime of an object has started but after the
  storage which the object will occupy has been allocated or, after  the
  lifetime  of  an  object  has  ended  and before the storage which the
  object occupied is reused or released, any lvalue which refers to  the
  original  object may be used but only in limited ways.  Such an lvalue
  refers to allocated  storage  (_basic.stc.dynamic.deallocation_),  and
  using the properties of the lvalue which do not depend on its value is
  well-defined.  If  an  lvalue-to-rvalue  conversion  (_conv.lval_)  is
  applied  to such an lvalue, the program has undefined behavior; if the
  original object will be or was of a non-POD class  type,  the  program
  has undefined behavior if:

  --the lvalue is used to access a non-static data member or call a non-
    static member function of the object, or

  --the lvalue is implicitly converted (_conv.ptr_) to a reference to  a
    base class type, or

  --the   lvalue   is   used   as   the   operand   of   a   static_cast
    (_expr.static.cast_) (except when the  conversion  is  to  char&  or
    unsigned char&), or

  --the   lvalue   is   used   as   the   operand   of   a  dynamic_cast
    (_expr.dynamic.cast_) or as the operand of typeid.

7 If, after the lifetime of an object has ended and before  the  storage
  which  the object occupied is reused or released, a new object is cre-
  ated at the storage location which the  original  object  occupied,  a
  pointer that pointed to the original object, a reference that referred
  to the original object, or the name of the original object will  auto-
  matically  refer  to  the new object and, once the lifetime of the new
  object has started, can be used to manipulate the new object, if:

  --the storage for the new object exactly overlays the storage location
    which the original object occupied, and

  --the  new object is of the same type as the original object (ignoring
    the top-level cv-qualifiers), and

  --the original object was a most derived  object  (_intro.object_)  of
    type  T  and the new object is a most derived object of type T (that
    is, they are not base class subobjects).  [Example:
              struct C {
                      int i;
                      void f();
                      const C& operator=( const C& );
              };
              const C& C::operator=( const C& other)
              {
                      if ( this != &other )
                      {
                              this->~C();          // lifetime of '*this' ends
                              new (this) C(other); // new object of type C created
                              f();                 // well-defined
                      }
                      return *this;
              }
              C c1;
              C c2;
              c1 = c2; // well-defined
              c1.f();  // well-defined; c1 refers to a new object of type C
     --end example]

8 If a program ends the lifetime of an object  of  type  T  with  static
  (_basic.stc.static_)  or automatic (_basic.stc.auto_) storage duration
  and if T has a non-trivial destructor,11) the program must ensure that
  an object of the original type occupies  that  same  storage  location
  when  the implicit destructor call takes place; otherwise the behavior
  of the program is undefined.  This is true even if the block is exited
  with an exception.  [Example:
  _________________________
  11) that is, an object for which a destructor will be called implicit-
  ly  --  either  upon  exit from the block for an object with automatic
  storage duration or upon exit from the  program  for  an  object  with
  static storage duration.

          class T { };
          struct B {
                  ~B();
          };
          void h() {
                  B b;
                  new (&b) T;
          } // undefined behavior at block exit
   --end example]

9 Creating a new object at the storage location that a const object with
  static or automatic storage duration occupies or, at the storage loca-
  tion that such a const object used to occupy before its lifetime ended
  results in undefined behavior.  [Example:
          struct B {
                  B();
                  ~B();
          };
          const B b;
          void h() {
                  b.~B();
                  new (&b) const B; // undefined behavior
          }
   --end example]

  3.9  Types                                               [basic.types]

1 [Note: these clauses impose requirements on implementations  regarding
  the  representation of types.  There are two kinds of types: fundamen-
  tal   types   and   compound   types.     Types    describe    objects
  (_intro.object_), references (_dcl.ref_), or functions (_dcl.fct_).  ]

2 For any complete POD object type T, whether or not the object holds  a
  valid value of type T, the underlying bytes (_intro.memory_) making up
  the object can be copied into an array of char or unsigned char.12) If
  the content of the array of char or unsigned char is copied back  into
  the  object,  the  object  shall subsequently hold its original value.
  [Example:
          #define N sizeof(T)
          char buf[N];
          T obj;  // obj initialized to its original value
          memcpy(buf, &obj, N);
                  // between these two calls to memcpy,
                  // obj might be modified
          memcpy(&obj, buf, N);
                  // at this point, each subobject of obj of scalar type
                  // holds its original value
   --end example]

  _________________________
  12) By using, for example, the library functions (_lib.headers_)  mem-
  cpy or memmove.

3 For any POD type T, if two pointers to T point to distinct  T  objects
  obj1  and  obj2,  if  the value of obj1 is copied into obj2, using the
  memcpy library function, obj2 shall subsequently hold the  same  value
  as obj1.  [Example:
          T* t1p;
          T* t2p;
                  // provided that t2p points to an initialized object ...
          memcpy(t1p, t2p, sizeof(T));
                  // at this point, every subobject of POD type in *t1p
                  // contains the same value as the corresponding subobject in
                  // *t2p
   --end example]

4 The  object representation of an object of type T is the sequence of N
  unsigned char objects taken up by the object of type T, where N equals
  sizeof(T).   The  value representation of an object is the set of bits
  that hold the value of type T.  For POD types, the  value  representa-
  tion  is  a set of bits in the object representation that determines a
  value, which is one discrete element of an implementation-defined  set
  of values.13)

5 Object  types  have   alignment   requirements   (_basic.fundamental_,
  _basic.compound_).   The  alignment  of  a  complete object type is an
  implementation-defined integer value representing a number  of  bytes;
  an object is allocated at an address that meets the alignment require-
  ments of its object type.

6 A class that has been declared but not defined, or an array of unknown
  size or of incomplete element type, is an  incomplete  type.14)  Also,
  the  void  type  is an incomplete type (_basic.fundamental_).  Objects
  shall not be defined to have an incomplete type.

7 A class type (such as "class X") might be incomplete at one point in a
  translation unit and complete later on; the type "class X" is the same
  type at both points.  The declared type of an array object might be an
  array  of incomplete class type and therefore incomplete; if the class
  type is completed later on in the translation  unit,  the  array  type
  becomes complete; the array type at those two points is the same type.
  The declared type of an array object might be an array of unknown size
  and  therefore  be  incomplete  at one point in a translation unit and
  complete later on; the array types at  those  two  points  ("array  of
  unknown bound of T" and "array of N T") are different types.  The type
  of a pointer to array of unknown size, or of a type defined by a type-
  def  declaration  to be an array of unknown size, cannot be completed.
  [Example:

  _________________________
  13) The intent is that the memory model of C++ is compatible with that
  of ISO/IEC 9899 Programming Language C.
  14)  The  size  and layout of an instance of an incomplete type is un-
  known.

          class X;             // X is an incomplete type
          extern X* xp;        // xp is a pointer to an incomplete type
          extern int arr[];    // the type of arr is incomplete
          typedef int UNKA[];  // UNKA is an incomplete type
          UNKA* arrp;          // arrp is a pointer to an incomplete type
          UNKA** arrpp;
          void foo()
          {
              xp++;             // ill-formed:  X is incomplete
              arrp++;           // ill-formed:  incomplete type
              arrpp++;          // okay: sizeof UNKA* is known
          }
          struct X { int i; };  // now X is a complete type
          int  arr[10];         // now the type of arr is complete
          X x;
          void bar()
          {
              xp = &x;          // okay; type is ``pointer to X''
              arrp = &arr;      // ill-formed: different types
              xp++;             // okay:  X is complete
              arrp++;           // ill-formed:  UNKA can't be completed
          }
   --end example]

8 [Note: the rules for declarations and expressions  describe  in  which
  contexts incomplete types are prohibited.  ]

9 An  object  type is a (possibly cv-qualified) type that is not a func-
  tion type, not a reference type, and not  incomplete  (except  for  an
  incompletely-defined object type).

10Arithmetic  types  (_basic.fundamental_),  enumeration  types, pointer
  types, and pointer to member types (_basic.compound_),  and  cv-quali-
  fied versions of these types (_basic.type.qualifier_) are collectively
  called scalar types.  Scalar types, POD-struct types, POD-union  types
  (_class_),  arrays  of  such  types and cv-qualified versions of these
  types (_basic.type.qualifier_) are collectively called POD types.

11If two types T1 and T2 are the same type, then T1 and T2  are  layout-
  compatible types.  [Note: Layout-compatible enumerations are described
  in  _dcl.enum_.   Layout-compatible  POD-structs  and  POD-unions  are
  described in _class.mem_.  ]

  3.9.1  Fundamental types                           [basic.fundamental]

1 Objects  declared  as  characters char) shall be large enough to store
  any member of the implementation's basic character set.  If a  charac-
  ter  from this set is stored in a character object, the integral value
  of that character object is equal to the value of the single character
  literal  form of that character.  It is implementation-defined whether
  a char object can hold negative values.  Characters can be  explicitly
  declared   unsigned   or   signed.    Plain   char,  signed char,  and
  unsigned char are three distinct types.  A char, a signed char, and an
  unsigned char  occupy  the  same  amount  of storage and have the same

  alignment requirements (_basic.types_); that is, they  have  the  same
  object  representation.   For  character types, all bits of the object
  representation participate in the value representation.  For  unsigned
  character types, all possible bit patterns of the value representation
  represent numbers. These requirements do not hold for other types.  In
  any  particular implementation, a plain char object can take on either
  the same values as a signed char or an  unsigned char;  which  one  is
  implementation-defined.

2 There  are  four  signed  integer  types:  "signed char", "short int",
  "int", and "long int."  In this list, each type provides at  least  as
  much  storage  as those preceding it in the list.  Plain ints have the
  natural  size  suggested  by  the  architecture   of   the   execution
  environment15) ; the other signed integer types are provided  to  meet
  special needs.

3 For  each  of  the  signed integer types, there exists a corresponding
  (but different) unsigned  integer  type:  "unsigned  char",  "unsigned
  short  int",  "unsigned  int",  and "unsigned long int," each of which
  occupies the same  amount  of  storage  and  has  the  same  alignment
  requirements  (_basic.types_)  as  the  corresponding  signed  integer
  type16) ; that is, each signed integer type has the same object repre-
  sentation as its corresponding unsigned integer type.   The  range  of
  nonnegative  values of a signed integer type is a subrange of the cor-
  responding unsigned integer type, and the value representation of each
  corresponding signed/unsigned type shall be the same.

4 Unsigned  integers,  declared  unsigned, shall obey the laws of arith-
  metic modulo 2n where n is the number of bits in the value representa-
  tion of that particular size of integer.17)

5 Type  wchar_t  is  a distinct type whose values can represent distinct
  codes for all members of the largest extended character set  specified
  among  the  supported locales (_lib.locale_).  Type wchar_t shall have
  the same size, signedness, and alignment requirements (_intro.memory_)
  as one of the other integral types, called its underlying type.

6 Values of type bool are either true or false.18) [Note: there  are  no
  signed, unsigned, short, or long bool types or values.  ] As described
  below, bool values behave as integral  types.   Values  of  type  bool
  _________________________
  15) that is, large enough to contain any value in the range of INT_MIN
  and INT_MAX, as defined in the header <climits>.
  16) See _dcl.type.simple_ regarding the correspondence  between  types
  and the sequences of type-specifiers that designate them.
  17)  This implies that unsigned arithmetic does not overflow because a
  result that cannot be represented by the  resulting  unsigned  integer
  type is reduced modulo the number that is one greater than the largest
  value that can be represented by the resulting unsigned integer  type.
  18)  Using  a bool value in ways described by this International Stan-
  dard as ``undefined,'' such as by examining the value of an uninitial-
  ized  automatic  variable,  might  cause it to behave as if is neither
  true nor false.

  participate in integral promotions (_conv.prom_).

7 Types  bool,  char, wchar_t, and the signed and unsigned integer types
  are collectively called integral types.19) A synonym for integral type
  is  integer  type.  The representations of integral types shall define
  values  by  use of  a pure binary numeration system.20) [Example: this
  International Standard permits  2's  complement,  1's  complement  and
  signed magnitude representations for integral types.  ]

8 There  are three floating point types: float, double, and long double.
  The type double provides at least as much precision as float, and  the
  type  long  double provides at least as much precision as double.  The
  set of values of the type float is a subset of the set  of  values  of
  the  type  double; the set of values of the type double is a subset of
  the set of values of the type long double.  The  value  representation
  of  floating-point  types  is  implementation-defined.   Integral  and
  floating types are collectively called arithmetic types.   Specializa-
  tions  of  the standard template numeric_limits (_lib.support.limits_)
  shall specify the maximum and minimum values of each  arithmetic  type
  for an implementation.

9 The  void type has an empty set of values.  The void type is an incom-
  plete type that cannot be completed.  It is used as  the  return  type
  for  functions  that  do  not  return  a value.  Any expression can be
  explicitly converted to type cv void (_expr.cast_).  An expression  of
  type void shall be used only as an expression statement (_stmt.expr_),
  as an operand of a comma expression (_expr.comma_), or as a second  or
  third operand of ?: (_expr.cond_).

10[Note:  even  if the implementation defines two or more basic types to
  have the same value representation, they  are  nevertheless  different
  types.  ]

  3.9.2  Compound types                                 [basic.compound]

1 Compound types can be constructed in the following ways:

  --arrays of objects of a given type, _dcl.array_;

  --functions,  which  have parameters of given types and return void or
    references or objects of a given type, _dcl.fct_;

  --pointers to void or objects or functions (including  static  members
  _________________________
  19) Therefore, enumerations (_dcl.enum_) are  not  integral;  however,
  enumerations  can  be promoted to int, unsigned int, long, or unsigned
  long, as specified in _conv.prom_.
  20) A positional representation for integers that uses the binary dig-
  its  0  and  1, in which the values represented by successive bits are
  additive, begin with 1, and are multiplied by successive integral pow-
  er  of  2,  except  perhaps  for  the  bit  with the highest position.
  (Adapted from the American National Dictionary  for  Information  Pro-
  cessing Systems.)

    of classes) of a given type, _dcl.ptr_;

  --references to objects or functions of a given type, _dcl.ref_;

  --classes containing a sequence of objects of various types (_class_),
    a set of types, enumerations and functions  for  manipulating  these
    objects  (_class.mfct_),  and a set of restrictions on the access to
    these entities, _class.access_;

  --unions, which are classes capable of containing objects of different
    types at different times, _class.union_;

  --enumerations,  which  comprise a set of named constant values.  Each
    distinct  enumeration  constitutes  a  different  enumerated   type,
    _dcl.enum_;

  --pointers to non-static21) class members, which identify members of a
    given type within objects of a given class, _dcl.mptr_.

2 These  methods  of  constructing  types  can  be  applied recursively;
  restrictions are mentioned in _dcl.ptr_, _dcl.array_,  _dcl.fct_,  and
  _dcl.ref_.

3 A  pointer  to  objects  of type T is referred to as a "pointer to T."
  [Example: a pointer to an  object  of  type  int  is  referred  to  as
  "pointer  to  int"  and  a pointer to an object of class X is called a
  "pointer to X."  ] Except for pointers to static members, text  refer-
  ring to "pointers" does not apply to pointers to members.  Pointers to
  incomplete types are allowed although there are restrictions  on  what
  can  be  done  with them (_basic.types_).  The value representation of
  pointer types is implementation-defined.  Pointers to cv-qualified and
  cv-unqualified  versions (_basic.type.qualifier_) of layout-compatible
  types shall have the same value representation and alignment  require-
  ments (_basic.types_).

4 Objects  of  cv-qualified  (_basic.type.qualifier_)  or cv-unqualified
  type void* (pointer to void), can be  used  to  point  to  objects  of
  unknown  type.   A  void* shall be able to hold any object pointer.  A
  cv-qualified or cv-unqualified  (_basic.type.qualifier_)  void*  shall
  have the same representation and alignment requirements as a cv-quali-
  fied or cv-unqualified char*.

  3.9.3  CV-qualifiers                            [basic.type.qualifier]

1 A type mentioned in _basic.fundamental_ and _basic.compound_ is a  cv-
  unqualified  type.   Each  type  which is a cv-unqualified complete or
  incomplete object type or is void  (_basic.types_)  has  three  corre-
  sponding cv-qualified versions of its type: a const-qualified version,
  a volatile-qualified version, and a const-volatile-qualified  version.
  The  term  object  type  (_intro.object_)  includes  the cv-qualifiers
  _________________________
  21)  Static  class  members  are objects or functions, and pointers to
  them are ordinary pointers to objects or functions.

  specified when the object is created.  The presence of a const  speci-
  fier  in  a  decl-specifier-seq  declares an object of const-qualified
  object type; such object is called a const object.  The presence of  a
  volatile  specifier  in  a  decl-specifier-seq  declares  an object of
  volatile-qualified object type;  such  object  is  called  a  volatile
  object.   The  presence  of both cv-qualifiers in a decl-specifier-seq
  declares an  object  of  const-volatile-qualified  object  type;  such
  object  is  called  a  const volatile object.  The cv-qualified or cv-
  unqualified versions of a type are distinct types; however, they shall
  have    the    same    representation   and   alignment   requirements
  (_basic.types_).22)

2 A compound type (_basic.compound_) is not cv-qualified by the cv-qual-
  ifiers  (if  any)  of  the types from which it is compounded.  Any cv-
  qualifiers applied to an array type affect the array element type, not
  the array type (_dcl.array_).

3 Each non-function, non-static, non-mutable member of a const-qualified
  class object is const-qualified, each non-function, non-static  member
  of  a  volatile-qualified class object is volatile-qualified and simi-
  larly for  members  of  a  const-volatile  class.  See  _dcl.fct_  and
  _class.this_ regarding cv-qualified function types.

4 There  is a (partial) ordering on cv-qualifiers, so that a type can be
  said to be more cv-qualified than another.  Table 1  shows  the  rela-
  tions that constitute this ordering.

                 Table 1--relations on const and volatile

                               +----------+
                      no cv-qualifier<const
                     no cv-qualifier<volatile
                  no cv-qualifier<const volatile
                       const<const volatile
                     volatile<const volatile
                               +----------+

5 In this International Standard, the notation cv (or cv1,  cv2,  etc.),
  used  in  the description of types, represents an arbitrary set of cv-
  qualifiers, i.e., one of {const}, {volatile},  {const,  volatile},  or
  the  empty  set.  Cv-qualifiers applied to an array type attach to the
  underlying element type, so the notation "cv T," where T is  an  array
  type,  refers to an array whose elements are so-qualified.  Such array
  types can be said to be more (or less) cv-qualified than  other  types
  based on the cv-qualification of the underlying element types.

  _________________________
  22) The same representation and alignment requirements  are  meant  to
  imply interchangeability as arguments to functions, return values from
  functions, and members of unions.

  3.10  Lvalues and rvalues                                 [basic.lval]

1 Every expression is either an lvalue or an rvalue.

2 An  lvalue refers to an object or function.  Some rvalue expressions--
  those of class or cv-qualified class type--also refer to objects.23)

3 [Note:  some  built-in  operators  and  function  calls yield lvalues.
  [Example: if E is an expression of pointer type, then *E is an  lvalue
  expression  referring to the object or function to which E points.  As
  another example, the function
          int& f();
  yields an lvalue, so the call f() is an lvalue expression.  ] ]

4 [Note: some built-in  operators  expect  lvalue  operands.   [Example:
  built-in  assignment  operators all expect their left hand operands to
  be lvalues.  ] Other built-in operators yield rvalues, and some expect
  them.   [Example: the unary and binary + operators expect rvalue argu-
  ments and yield rvalue results.  ] The  discussion  of  each  built-in
  operator in clause _expr_ indicates whether it expects lvalue operands
  and whether it yields an lvalue.  ]

5 The result of calling a functions that does not return a reference  is
  an  rvalue.   User  defined  operators are functions, and whether such
  operators expect or yield lvalues is determined by their parameter and
  return types.

6 An  expression which holds a temporary object resulting from a cast to
  a nonreference type is an rvalue (this includes the explicit  creation
  of an object using functional notation (_expr.type.conv_)).

7 Whenever  an  lvalue appears in a context where an rvalue is expected,
  the lvalue is converted to an rvalue; see  _conv.lval_,  _conv.array_,
  and _conv.func_.

8 The  discussion  of  reference initialization in _dcl.init.ref_ and of
  temporaries in _class.temporary_ indicates the behavior of lvalues and
  rvalues in other significant contexts.

9 Class  rvalues  can  have cv-qualified types; non-class rvalues always
  have cv-unqualified types.  Rvalues shall always have  complete  types
  or  the  void  type; in addition to these types, lvalues can also have
  incomplete types.

10An lvalue for an object is necessary in order  to  modify  the  object
  except  that  an  rvalue  of class type can also be used to modify its
  referent under certain circumstances.   [Example:  a  member  function
  called for an object (_class.mfct_) can modify the object.  ]
  _________________________
  23) Expressions such as invocations of constructors and  of  functions
  that  return a class type refer to objects, and the implementation can
  invoke a member function upon such objects, but  the  expressions  are
  not lvalues.

11Functions cannot be modified, but pointers to functions can be modifi-
  able.

12A pointer to an incomplete type can be modifiable.  At some  point  in
  the  program when the pointed to type is complete, the object at which
  the pointer points can also be modified.

13The referent of a const-qualified expression  shall  not  be  modified
  (through  that expression), except that if it is of class type and has
  a mutable component, that component can be modified (_dcl.type.cv_).

14If an expression can be used to modify the object to which it  refers,
  the  expression is called modifiable.  A program that attempts to mod-
  ify an object through a nonmodifiable lvalue or rvalue  expression  is
  ill-formed.

15If  a program attempts to access the stored value of an object through
  an lvalue of other than one of the following  types  the  behavior  is
  undefined24):

  --the dynamic type of the object,

  --a cv-qualified version of the dynamic type of the object,

  --a type that is the signed or  unsigned  type  corresponding  to  the
    dynamic type of the object,

  --a  type  that  is the signed or unsigned type corresponding to a cv-
    qualified version of the dynamic type of the object,

  --an aggregate or union type that includes one of  the  aforementioned
    types  among its members (including, recursively, a member of a sub-
    aggregate or contained union),

  --a type that is a (possibly cv-qualified)  base  class  type  of  the
    dynamic type of the object,

  --a char or unsigned char type.

  _________________________
  24) The intent of this list is to specify those circumstances in which
  an object may or may not be aliased.