______________________________________________________________________

  13   Overloading                                      [over]

  ______________________________________________________________________

1 When two or more different declarations are  specified  for  a  single
  name in the same scope, that name is said to be overloaded.  By exten­
  sion, two declarations in the same scope that declare  the  same  name
  but  with  different  types  are called overloaded declarations.  Only
  function declarations can be overloaded; object and type  declarations
  cannot be overloaded.

2 When  an  overloaded function name is used in a call, which overloaded
  function declaration is being referenced is  determined  by  comparing
  the  types  of the arguments at the point of use with the types of the
  parameters in the overloaded declarations  that  are  visible  at  the
  point of use.  This function selection process is called overload res­
  olution and is defined in _over.match_.  [Example:
          double abs(double);
          int abs(int);
          abs(1);       // call abs(int);
          abs(1.0);     // call abs(double);
   --end example]

  13.1  Overloadable declarations                            [over.load]

1 Not all function declarations can be overloaded.  Those that cannot be
  overloaded are specified here.  A program is ill-formed if it contains
  two such non-overloadable declarations in the same scope.

2 Certain function declarations cannot be overloaded:

  --Function declarations that differ only in the return type cannot  be
    overloaded.

  --Member function declarations with the same name and the same parame­
    ter types cannot be overloaded if any of them  is  a  static  member
    function  declaration  (_class.static_).   The types of the implicit
    object parameters constructed for the member functions for the  pur­
    pose  of overload resolution (_over.match.funcs_) are not considered
    when comparing parameter types for enforcement  of  this  rule.   In
    contrast,  if there is no static member function declaration among a
    set of member function declarations with the same name and the  same
    parameter  types,  then  these  member  function declarations can be
    overloaded if they differ in  the  type  of  their  implicit  object
    parameter.  [Example: the following illustrates this distinction:

              class X {
                  static void f();
                  void f();                  // ill-formed
                  void f() const;            // ill-formed
                  void f() const volatile;   // ill-formed
                  void g();
                  void g() const;            // Ok: no static g
                  void g() const volatile;   // Ok: no static g
              };
     --end example]

3 [Note:  as  specified  in  _dcl.fct_,  function declarations that have
  equivalent parameter declarations declare the same function and there­
  fore cannot be overloaded:

  --Parameter  declarations  that  differ  only in the use of equivalent
    typedef "types" are equivalent.  A typedef is not a  separate  type,
    but only a synonym for another type (_dcl.typedef_).  [Example:
              typedef int Int;

              void f(int i);
              void f(Int i);                  // OK: redeclaration of f(int)
              void f(int i) { /* ... */ }
              void f(Int i) { /* ... */ }     // error: redefinition of f(int)
     --end example]

    Enumerations,  on the other hand, are distinct types and can be used
    to distinguish overloaded function declarations.  [Example:
              enum E { a };

              void f(int i) { /* ... */ }
              void f(E i)   { /* ... */ }
     --end example]

  --Parameter declarations that differ only in a  pointer  *  versus  an
    array [] are equivalent.  That is, the array declaration is adjusted
    to become a pointer declaration (_dcl.fct_).  Only  the  second  and
    subsequent  array  dimensions  are  significant  in  parameter types
    (_dcl.array_).  [Example:
              f(char*);
              f(char[]);      // same as f(char*);
              f(char[7]);     // same as f(char*);
              f(char[9]);     // same as f(char*);
              g(char(*)[10]);
              g(char[5][10]);  // same as g(char(*)[10]);
              g(char[7][10]);  // same as g(char(*)[10]);
              g(char(*)[20]);  // different from g(char(*)[10]);
     --end example]

  --Parameter declarations that differ only in the presence  or  absence
    of  const  and/or  volatile  are equivalent.  That is, the const and
    volatile type-specifiers for each parameter type  are  ignored  when
    determining  which  function  is being declared, defined, or called.

    [Example:
              typedef const int cInt;

              int f (int);
              int f (const int);      // redeclaration of f (int);
              int f (int) { ... }     // definition of f (int)
              int f (cInt) { ... }    // error: redefinition of f (int)
     --end example]

    Only the const and volatile type-specifiers at the  outermost  level
    of  the  parameter  type  specification are ignored in this fashion;
    const and volatile type-specifiers buried within  a  parameter  type
    specification  are  significant and can be used to distinguish over­
    loaded  function  declarations.1)  In  particular,  for  any type T,
    "pointer to T," "pointer to const T," and "pointer  to  volatile  T"
    are  considered  distinct  parameter types, as are "reference to T,"
    "reference to const T," and "reference to volatile T."

  --Two parameter declarations that differ only in their  default  argu­
    ments are equivalent.  [Example: consider the following:
              void f (int i, int j);
              void f (int i, int j = 99);         // Ok: redeclaration of f (int, int)
              void f (int i = 88, int j);         // Ok: redeclaration of f (int, int)
              void f ();                          // Ok: overloaded declaration of f

              void prog ()
              {
                  f (1, 2);  // Ok: call f (int, int)
                  f (1);     // Ok: call f (int, int)
                  f ();      // Error: f (int, int) or f ()?
              }
     --end example]  --end note]

  13.2  Declaration matching                                  [over.dcl]

1 Two  function declarations of the same name refer to the same function
  if they are in the same scope and have equivalent  parameter  declara­
  tions  (_over.load_).   A function member of a derived class is not in
  the same scope as a function member of the same name in a base  class.
  [Example:

  _________________________
  1) When a parameter type includes a function type, such as in the case
  of  a  parameter  type  that  is  a pointer to function, the const and
  volatile type-specifiers at the outermost level of the parameter  type
  specifications for the inner function type are also ignored.

          class B {
          public:
              int f(int);
          };

          class D : public B {
          public:
              int f(char*);
          };
  Here D::f(char*) hides B::f(int) rather than overloading it.
          void h(D* pd)
          {
              pd->f(1);       // error:
                              // D::f(char*) hides B::f(int)
              pd->B::f(1);    // ok
              pd->f("Ben");   // ok, calls D::f
          }
   --end example]

2 A  locally declared function is not in the same scope as a function in
  a containing scope.  [Example:
          int f(char*);
          void g()
          {
              extern f(int);
              f("asdf");  // error: f(int) hides f(char*)
                          // so there is no f(char*) in this scope
          }
          void caller ()
          {
              void callee (int, int);
              {
                  void callee (int);  // hides callee (int, int)
                  callee (88, 99);    // error: only callee (int) in scope
              }
          )
   --end example]

3 Different versions of an overloaded member function can be given  dif­
  ferent access rules.  [Example:
          class buffer {
          private:
              char* p;
              int size;
          protected:
              buffer(int s, char* store) { size = s; p = store; }
              // ...
          public:
              buffer(int s) { p = new char[size = s]; }
              // ...
          };
   --end example]

  13.3  Overload resolution                                 [over.match]

1 Overload  resolution is a mechanism for selecting the best function to
  call given a list of expressions that are to be the arguments  of  the
  call  and a set of candidate functions that can be called based on the
  context of the call.  The selection criteria for the best function are
  the number of arguments, how well the arguments match the types of the
  parameters of the candidate function, how well (for  nonstatic  member
  functions)  the  object matches the implied object parameter, and cer­
  tain other properties of the candidate function.  [Note: the  function
  selected  by  overload  resolution is not guaranteed to be appropriate
  for the context.  Other restrictions, such as the accessibility of the
  function, can make its use in the calling context ill-formed.  ]

2 Overload resolution selects the function to call in five distinct con­
  texts within the language:

  --invocation  of  a  function  named  in  the  function  call   syntax
    (_over.call.func_);

  --invocation  of  a function call operator, a pointer-to-function con­
    version function, or a reference-to-function conversion function  of
    a    class    object    named    in   the   function   call   syntax
    (_over.call.object_);

  --invocation   of   the   operator   referenced   in   an   expression
    (_over.match.oper_);

  --invocation  of  a constructor for direct-initialization (_dcl.init_)
    of a class object (_over.match.ctor_); and

  --invocation of  a  user-defined  conversion  for  copy-initialization
    (_dcl.init_)  of a class object, or initialization of an object of a
    built-in type from an expression of class type  (_over.match.user_).

3 Each  of these contexts defines the set of candidate functions and the
  list of arguments in its own unique  way.   But,  once  the  candidate
  functions  and  argument  lists have been identified, the selection of
  the best function is the same in all cases:

  --First, a subset of the  candidate  functions--those  that  have  the
    proper  number  of  arguments  and meet certain other conditions--is
    selected to form a set of viable functions.

  --Then the best viable function is selected based on the implicit con­
    version sequences (_over.best.ics_) needed to match each argument to
    the corresponding parameter of each viable function.

4 If a best viable function exists and is  unique,  overload  resolution
  succeeds and produces it as the result.  Otherwise overload resolution
  fails and the invocation is ill-formed.  When overload resolution suc­
  ceeds, and the best viable function is not accessible (_class.access_)
  in the context in which it is used, the program is ill-formed.

  13.3.1  Candidate functions and argument lists      [over.match.funcs]

1 The  following  subclauses describe the set of candidate functions and
  the argument list submitted to overload resolution in each of the five
  contexts in which overload resolution is used.  The source transforma­
  tions and constructions defined in these subclauses are only  for  the
  purpose of describing the overload resolution process.  An implementa­
  tion is not required to use such transformations and constructions.

2 The set of candidate functions can contain both member and  non-member
  functions  to  be  resolved  against  the same argument list.  So that
  argument and parameter lists are comparable within this  heterogeneous
  set,  a  member  function  is  considered  to have an extra parameter,
  called the implicit object parameter, which represents the object  for
  which  the member function has been called.  For the purposes of over­
  load resolution, both static and non-static member functions  have  an
  implicit object parameter, but constructors do not.

3 Similarly,  when  appropriate,  the  context can construct an argument
  list that contains an implied object argument to denote the object  to
  be  operated  on.   Since  arguments  and parameters are associated by
  position within their respective lists, the  convention  is  that  the
  implicit  object  parameter, if present, is always the first parameter
  and the implied object argument, if present, is always the first argu­
  ment.

4 For  non-static  member  functions,  the  type  of the implicit object
  parameter is "reference to cv X" where X is the  class  of  which  the
  function  is  a  member  and  cv is the cv-qualification on the member
  function declaration.  [Example: for a const member function of  class
  X, the extra parameter is assumed to have type "reference to const X".
  ] For static member functions, the implicit object parameter  is  con­
  sidered  to  match  any object (since if the function is selected, the
  object is discarded).

5 During overload resolution, the implied object argument  is  indistin­
  guishable  from  other arguments.  The implicit object parameter, how­
  ever, retains its identity  since  conversions  on  the  corresponding
  argument shall obey these additional rules:

  --no  temporary  object can be introduced to hold the argument for the
    implicit object parameter;

  --no user-defined conversions can be applied to achieve a  type  match
    with it; and

  --even  if  the  implicit  object parameter is not const-qualified, an
    rvalue temporary can be bound to the parameter as  long  as  in  all
    other  respects  the  temporary  can be converted to the type of the
    implicit object parameter.

6 In each case where a candidate is a function template, candidate  tem­
  plate  functions  are  generated  using  template  argument  deduction

  (_temp.over_, _temp.deduct_).  Those candidates are  then  handled  as
  candidate  functions in the usual way.2) A given name can refer to one
  or more function templates and  also  to  a  set  of  overloaded  non-
  template functions.  In such a case, the candidate functions generated
  from each function template are combined with the set of  non-template
  candidate functions.

  13.3.1.1  Function call syntax                       [over.match.call]

1 Recall from _expr.call_, that a function call is a postfix-expression,
  possibly nested  arbitrarily  deep  in  parentheses,  followed  by  an
  optional expression-list enclosed in parentheses:
          (...(opt postfix-expression )...)opt (expression-listopt)
  Overload  resolution is required if the postfix-expression is the name
  of a function, a function template (_temp.fct_), an  object  of  class
  type, or a set of pointers-to-function.

2 _over.call.func_  and  _over.call.object_,  respectively, describe how
  overload resolution is used in the first three cases to determine  the
  function to call.

3 The fourth case arises from a postfix-expression of the form &F, where
  F names a set of overloaded functions.  In the context of  a  function
  call,  the  set  of functions named by F shall contain only non-member
  functions and static member functions3).  And in this context using &F
  behaves  the  same  as  using   the   name   F   by   itself.    Thus,
  (&F)(expression-listopt)  is  simply (F)(expression-listopt), which is
  discussed in _over.call.func_.  (The resolution of &F  in  other  con­
  texts is described in _over.over_.)

  13.3.1.1.1  Call to named function                    [over.call.func]

1 Of  interest  in this subclause are only those function calls in which
  the postfix-expression ultimately contains a name that denotes one  or
  more  functions that might be called.  Such a postfix-expression, per­
  haps nested arbitrarily deep in parentheses, has one of the  following
  forms:
          postfix-expression:
                  postfix-expression . id-expression
                  postfix-expression -> id-expression
                  primary-expression
  These  represent two syntactic subcategories of function calls: quali­
  fied function calls and unqualified function calls.

  _________________________
  2)  The  process  of argument deduction fully determines the parameter
  types of the template functions,  i.e.,  the  parameters  of  template
  functions contain no template parameter types.  Therefore the template
  functions can be treated as normal (non-template)  functions  for  the
  remainder of overload resolution.
  3) If F names a non-static member function, &F is a pointer-to-member,
  which cannot be used with the function call syntax.

2 In qualified function calls,  the  name  to  be  resolved  is  an  id-
  expression  and  is  preceded by an -> or .  operator.  Since the con­
  struct A->B is generally equivalent to (*A).B, the rest of this clause
  assumes,  without  loss  of generality, that all member function calls
  have been normalized to the form that uses an object and the .  opera­
  tor.   Furthermore,  this  clause  assumes that the postfix-expression
  that is the left operand of the .  operator has type "cv  T"  where  T
  denotes a class4).  Under this assumption, the  id-expression  in  the
  call  is  looked  up as a member function of T following the rules for
  looking up names in  classes  (_class.member.lookup_).   If  a  member
  function  is found, that function and its overloaded declarations con­
  stitute the set of candidate functions5).  The argument  list  is  the
  expression-list  in  the  call  augmented  by the addition of the left
  operand of the .  operator in the normalized member function  call  as
  the implied object argument (_over.match.funcs_).

3 In unqualified function calls, the name is not qualified by an -> or .
  operator and has the more general form of a  primary-expression.   The
  name  is  looked  up in the context of the function call following the
  normal rules for name lookup  (_basic.lookup.unqual_).   If  the  name
  resolves  to  a non-member function declaration, that function and its
  overloaded declarations constitute the set of  candidate  functions6).
  The  argument list is the same as the expression-list in the call.  If
  the name resolves to a nonstatic member function,  then  the  function
  call  is  actually  a  member  function  call.   If  the  keyword this
  (_class.this_) is in scope and refers to  the  class  of  that  member
  function, or a derived class thereof, then the function call is trans­
  formed into a normalized qualified function call using (*this) as  the
  postfix-expression  to  the  left  of  the .  operator.  The candidate
  functions and argument list are as described  for  qualified  function
  calls above.  If the keyword this is not in scope or refers to another
  class, then name resolution found a static member of some class T.  In
  this  case,  all  overloaded  declarations  of  the function name in T
  become candidate functions and a contrived object of  type  T  becomes
  the implied object argument7).  The call is  ill-formed,  however,  if
  overload  resolution selects one of the non-static member functions of
  T in this case.
  _________________________
  4)  Note  that cv-qualifiers on the type of objects are significant in
  overload resolution for both lvalue and class rvalue objects.
  5) Because of the usual name hiding rules, these will all be  declared
  in  T  or  they  will all be declared in the same base class of T; see
  _class.member.lookup_.
  6) Because of the usual name hiding rules, these will be introduced by
  declarations or by using directives all found in the same block or all
  found at namespace scope.
  7)  An  implied object argument must be contrived to correspond to the
  implicit object parameter attributed to member functions during  over­
  load resolution.  It is not used in the call to the selected function.
  Since the member functions all have the same implicit  object  parame­
  ter,  the contrived object will not be the cause to select or reject a
  function.

  13.3.1.1.2  Call to object of class type            [over.call.object]

1 If the primary-expression E in the function call syntax evaluates to a
  class  object  of  type  "cv  T",  then the set of candidate functions
  includes at least the function call operators of T.  The function call
  operators  of T are obtained by ordinary lookup of the name operator()
  in the context of (E).operator()8).

2 In addition, for each conversion function declared in T of the form
          operator conversion-type-id () cv-qualifier;
  where  conversion-type-id  denotes  the type "pointer to function with
  parameters of type P1,...,Pn and returning R" or  type  "reference  to
  function  with parameters of type P1,...,Pn and returning R", a surro­
  gate call function with the unique name call-function and  having  the
  form
          R call-function (conversion-type-id F, P1 a1,...,Pn an) { return F (a1,...,an); }
  is also considered as a candidate function.  Similarly, surrogate call
  functions are added to the set of candidate functions for each conver­
  sion  function declared in an accessible base class provided the func­
  tion is not hidden within T by another intervening declaration9).

3 If  such a surrogate call function is selected by overload resolution,
  its body, as defined above, will be  executed  to  convert  E  to  the
  appropriate  function  and then to invoke that function with the argu­
  ments of the call.

4 The argument list submitted to overload  resolution  consists  of  the
  argument  expressions  present in the function call syntax preceded by
  the implied object argument  (E).   [Note:  when  comparing  the  call
  against  the  function  call operators, the implied object argument is
  compared against the implicit object parameter of  the  function  call
  operator.   When comparing the call against a surrogate call function,
  the implied object argument is compared against the first parameter of
  the  surrogate  call function.  The conversion function from which the
  surrogate call function was derived will be  used  in  the  conversion
  sequence for that parameter since it converts the implied object argu­
  ment to the appropriate function pointer or reference required by that
  first parameter.  ] [Example:

  _________________________
  8) Because of the usual name hiding rules, these will all be  declared
  in T or they will all be declared in the same base class of T.
  9) Note that this construction can yield candidate call functions that
  cannot be differentiated one from the other by overload resolution be­
  cause they have identical declarations or differ only in their  return
  type.  The call will be ambiguous if overload resolution cannot select
  a match to the call that is uniquely better than such undifferentiable
  functions.

          int f1(int);
          int f2(float);
          typedef int (*fp1)(int);
          typedef int (*fp2)(float);
          struct A {
              operator fp1() { return f1; }
              operator fp2() { return f2; }
          } a;
          int i = a(1);     // Calls f1 via pointer returned from
                            // conversion function
   --end example]

  13.3.1.2  Operators in expressions                   [over.match.oper]

1 If  no  operand  of  an operator in an expression has a type that is a
  class or an enumeration, the operator is  assumed  to  be  a  built-in
  operator  and  interpreted according to clause _expr_.  [Note: because
  ., .*, ::, and ?: cannot be overloaded,  these  operators  are  always
  built-in  operators  interpreted according to clause _expr_.  ] [Exam­
  ple:
          class String {
          public:
             String (const String&);
             String (char*);
                  operator char* ();
          };
          String operator + (const String&, const String&);

          void f(void)
          {
             char* p= "one" + "two"; // ill-formed because neither
                                     // operand has user defined type
             int I = 1 + 1;          // Always evaluates to 2 even if
                                     // user defined types exist which
                                     // would perform the operation.
          }
   --end example]

2 If either operand has a type that is a  class  or  an  enumeration,  a
  user-defined  operator function might be declared that implements this
  operator or a user-defined conversion can be necessary to convert  the
  operand  to  a  type  that is appropriate for a built-in operator.  In
  this case, overload resolution is used  to  determine  which  operator
  function  or builtin operator is to be invoked to implement the opera­
  tor.  Therefore, the operator notation is  first  transformed  to  the
  equivalent  function-call  notation  as summarized in Table 1 (where @
  denotes one of the operators covered in the specified subclause).

    Table 1--relationship between operator and function call notation

  +--------------+------------+--------------------+------------------------+
  |Subclause     | Expression | As member function | As non-member function |
  +--------------+------------+--------------------+------------------------+
  |_over.unary_  | @a         | (a).operator@ ()   | operator@ (a)          |
  |_over.binary_ | a@b        | (a).operator@ (b)  | operator@ (a, b)       |
  |_over.ass_    | a=b        | (a).operator= (b)  |                        |
  |_over.sub_    | a[b]       | (a).operator[](b)  |                        |
  |_over.ref_    | a->        | (a).operator-> ()  |                        |
  |_over.inc_    | a@         | (a).operator@ (0)  | operator@ (a, 0)       |
  +--------------+------------+--------------------+------------------------+

3 For     a     type     T     whose     fully-qualified     name     is
  ::N1::...::Nn::C1::...::Cm::T  where  each  Ni is a namespace name and
  each  Ci  is  a  class  name,  the  fully-qualified   namespace   name
  ::N1::...::Nn  is  called the "namespace of the type T."  To look up X
  in the "context of the namespace of the type T" means to  perform  the
  qualified  name  lookup of ::N1::...::Nn::X (_over.call.func_), except
  that only names actually declared in the namespace are visible.  Names
  made  visible  by using-directives (_namespace.udir_) in the namespace
  are not considered.

  +-------                 BEGIN BOX 1                -------+
  Change: This last sentence is an  editorial  proposal.   The  Monterey
  resolution  17  changed the name look up rules for qualified namespace
  members. This last sentence ensure that the rule above keeps its  pre-
  Monterey meaning. See email message core-6081.
  +-------                  END BOX 1                 -------+

4 For a type T whose fully-qualified name is ::C1::...::Cm::T where each
  Ci is a class name, to look up X in the "context of the  namespace  of
  the  type T" means to perform the qualified name lookup of ::X, except
  that only names actually declared in global scope are visible.   Names
  made  visible  by  using-directives (_namespace.udir_) in global scope
  are not considered.

  +-------                 BEGIN BOX 2                -------+
  Change: This previous paragraph is an editorial proposal.  This  para­
  graph  was  added to correctly handle classes defined in global scope.
  Otherwise, the new operator look up rules  will  not  allow  operators
  defined in global scope to be found if the type of one of the operands
  is a class from global scope. See email message core-6082.
  +-------                  END BOX 2                 -------+

5 For a unary operator @ with an operand of type T1 or reference  to  cv
  T1, and for a binary operator @ with a left operand of type T1 or ref­
  erence to cv T1 and a right operand of type T2 or reference to cv  T2,

  three  sets of candidate functions, designated member candidates, non-
  member candidates and built-in candidates, are constructed as follows:

  --If T1 is a class type, the set of member candidates is the result of
    the qualified lookup of T1::operator@ (_over.call.func_); otherwise,
    the set of member candidates is empty.

  --The set of non-member candidates is the union of the functions found
    in the following name lookups:

    --The unqualified operator@ is looked  up  in  the  context  of  the
      expression  according  to  the  usual rules for name lookup except
      that all member functions are ignored.

    --For each type Z, where Z is a class type representing either T1 or
      T2,  or  a  direct  or  indirect base class of one of these types,
      operator@ is looked up in the context of the namespace of  type  Z
      according to the usual rules for name lookup.

    --For each type Z, where Z is an enumeration type representing T1 or
      T2, operator@ is looked up in the context of the namespace of that
      type according to the usual rules for name lookup.

  --For  the  operator  ,, the unary operator &, or the operator ->, the
    built-in candidates set is empty.   For  all  other  operators,  the
    built-in  candidates include all of the candidate operator functions
    defined in _over.built_ that, compared to the given operator,

    --have the same operator name, and

    --accept the same number of operands, and

    --accept operand types to which the given operand or operands can be
      converted according to _over.best.ics_.

6 For the built-in assignment operators, conversions of the left operand
  are restricted as follows:

  --no temporaries are introduced to hold the left operand, and

  --no user-defined conversions are applied to achieve a type match with
    it.

7 For all other operators, no such restrictions apply.

8 The set of candidate functions for overload resolution is the union of
  the member candidates, the non-member  candidates,  and  the  built-in
  candidates.   The  argument  list  contains all of the operands of the
  operator.  The best function from the set of  candidate  functions  is
  selected according to  _over.match.viable_  and  _over.match.best_.10)
  _________________________
  10) If the set of candidate functions is empty, overload resolution is
  unsuccessful.

  [Example:
          struct A {
              operator int();
          };
          A operator+(const A&, const A&);
          void m() {
              A a, b;
              a + b;        // operator+(a,b) chosen over int(a) + int(b)
          }
   --end example]

9 If  a built-in candidate is selected by overload resolution, any class
  operands are first converted to the appropriate type for the operator.
  Then  the  operator  is treated as the corresponding built-in operator
  and interpreted according to clause _expr_.

10The second operand of operator -> is ignored in  selecting  an  opera­
  tor-> function, and is not an argument when the operator-> function is
  called.  When operator-> returns, the operator -> is  applied  to  the
  value returned, with the original second operand.11)

11If the operator is the operator ,, the unary operator &, or the opera­
  tor  ->, and overload resolution is unsuccessful, then the operator is
  assumed to be the  built-in  operator  and  interpreted  according  to
  clause _expr_.

12[Note:  the  look  up rules for operators in expressions are different
  than the lookup rules for operator function names in a function  call,
  as shown in the following example:
  struct A { };
  void operator + (A, A);

  struct B {
    void operator + (B);
    void f ();
  };

  A a;

  void B::f() {
    operator+ (a,a);      // ERROR - global operator hidden by member
    a + a;                // OK - calls global operator+
  }
   --end note]

  _________________________
  11)  If  the value returned by the operator-> function has class type,
  this may result in selecting and calling another operator->  function.
  The  process  repeats  until an operator-> function returns a value of
  non-class type.

  13.3.1.3  Initialization by user-defined             [over.match.user]
       conversions

1 Under the conditions specified in _dcl.init_  and  _dcl.init.ref_,  as
  part  of an initialization a user-defined conversion can be invoked to
  convert the initializer expression to the type of an object or  tempo­
  rary  being  initialized.   Overload  resolution is used to select the
  user-defined conversion to be invoked.  Assuming that "cv1 T"  is  the
  type of the object or temporary being initialized, the candidate func­
  tions are selected as follows:

  --When T is a class type, the constructors of T  are  candidate  func­
    tions.

  --When  the type of the initializer expression is a class type "cv S",
    the conversion functions of S and its base classes  are  considered.
    Those  that are not hidden within S and yield type "cv2 T" or a type
    that can be converted to type "cv2 T," for any cv2 that is the  same
    cv-qualification  as,  or  lesser  cv-qualification than, cv1, via a
    standard conversion sequence (_over.ics.scs_)  are  candidate  func­
    tions.   Conversions  functions  that return "reference to T" return
    lvalues of type T and are therefore considered to yield T  for  this
    process of selecting candidate functions.

2 In  both  cases, the argument list has one argument, which is the ini­
  tializer expression.  [Note: this argument will  be  compared  against
  the  first  parameter  of  the  constructors  and against the implicit
  object parameter of the conversion functions.  ]

3 If the result of the conversion is required to be an lvalue (as when a
  conversion  is  done  on the initializer expression for a reference to
  non-const), only conversion functions returning  reference  types  are
  considered.

4 Because  only  one  user-defined  conversion is allowed in an implicit
  conversion sequence, special rules apply when selecting the best user-
  defined conversion (_over.match.best_, _over.best.ics_).  [Example:
          class T {
          public:
                  T();
                  // ...
          };
          class C : T {
          public:
                  C(int);
                  // ...
          };
          T a = 1;                 // ill-formed: T(C(1)) not tried
   --end example]

  13.3.1.4  Initialization by constructor              [over.match.ctor]

1 When  objects of class type are direct-initialized (_dcl.init_), over­
  load resolution selects the constructor.  The candidate functions  are
  all  the  constructors  of  the class of the object being initialized.
  The argument list is the expression-list within the parentheses of the
  initializer.

  13.3.2  Viable functions                           [over.match.viable]

1 From  the  set  of candidate functions constructed for a given context
  (_over.match.funcs_), a set of viable functions is chosen, from  which
  the  best  function  will be selected by comparing argument conversion
  sequences for the best  fit  (_over.match.best_).   The  selection  of
  viable  functions  considers relationships between arguments and func­
  tion parameters other than the ranking of conversion sequences.

2 First, to be a viable function, a candidate function shall have enough
  parameters to agree in number with the arguments in the list.

  --If there are m arguments in the list, all candidate functions having
    exactly m parameters are viable.

  --A candidate function having fewer than m parameters is  viable  only
    if  it  has  an ellipsis in its parameter list (_dcl.fct_).  For the
    purposes of overload resolution, any argument for which there is  no
    corresponding  parameter  is  considered  to  ``match the ellipsis''
    (_over.ics.ellipsis_) .

  --A candidate function having more than m parameters is viable only if
    the     (m+1)-st     parameter     has     a     default    argument
    (_dcl.fct.default_).12) For the purposes of overload resolution, the
    parameter list is truncated on the right, so that there are  exactly
    m parameters.

3 Second,  for  F  to  be  a viable function, there shall exist for each
  argument an implicit conversion sequence (_over.best.ics_)  that  con­
  verts  that  argument  to  the  corresponding  parameter of F.  If the
  parameter  has  reference  type,  the  implicit  conversion   sequence
  includes  the  operation of binding the reference, and the fact that a
  reference to non-const cannot be bound to an  rvalue  can  affect  the
  viability of the function (see _over.ics.ref_).

  13.3.3  Best Viable Function                         [over.match.best]

1 Let  ICSi(F) denote the implicit conversion sequence that converts the
  i-th argument in the list to the type of the i-th parameter of  viable
  function F.  _over.best.ics_ defines the implicit conversion sequences
  and _over.ics.rank_ defines what it means for one implicit  conversion
  _________________________
  12) According to _dcl.fct.default_, parameters following the  (m+1)-st
  parameter must also have default arguments.

  sequence  to  be  a  better  conversion  sequence  or worse conversion
  sequence than another.  Given these definitions, a viable function  F1
  is  defined to be a better function than another viable function F2 if
  for all arguments i, ICSi(F1) is not a worse conversion sequence  than
  ICSi(F2), and then

  --for  some  argument j, ICSj(F1) is a better conversion sequence than
    ICSj(F2), or, if not that,

  --F1 is a non-template function and F2 is a template function, or,  if
    not that,

  --F1  and  F2  are template functions with the same signature, and the
    function template for F1 is more specialized than the  template  for
    F2   according   to   the   partial   ordering  rules  described  in
    _temp.func.order_, or, if not that,

  --the context is an initialization  by  user-defined  conversion  (see
    _dcl.init_   and  _over.match.user_)  and  the  standard  conversion
    sequence from the return type of F1 to the destination  type  (i.e.,
    the  type  of  the  entity being initialized) is a better conversion
    sequence than the standard conversion sequence from the return  type
    of F2 to the destination type.  [Example:
              struct A {
                  A();
                  operator int();
                  operator double();
              } a;
              int i = a;     // a.operator int() followed by no conversion is better
                             // than a.operator double() followed by a conversion
                             // to int
              float x = a;   // ambiguous: both possibilities require conversions,
                             // and neither is better than the other
     --end example]

2 If there is exactly one viable function that is a better function than
  all other viable functions, then it is the one  selected  by  overload
  resolution; otherwise the call is ill-formed13).

3 [Example:

  _________________________
  13)  The algorithm for selecting the best viable function is linear in
  the number of viable functions.  Run a simple  tournament  to  find  a
  function W that is not worse than any opponent it faced.  Although an­
  other function F that W did not face might be better than W, F  cannot
  be the best function because at some point in the tournament F encoun­
  tered another function G such that F was not better than G.  Hence,  W
  is  either the best function or there is no best function.  So, make a
  second pass over the viable functions to verify that W is better  than
  all other functions.

          void Fcn(const int*,  short);
          void Fcn(int*, int);

          int i;
          short s = 0;

          Fcn(&i, s);     // is ambiguous because
                          // &i -> int* is better than &i -> const int*
                          // but s -> short is also better than s -> int

          Fcn(&i, 1L);    // calls Fcn(int*, int), because
                          // &i -> int* is better than &i -> const int*
                          // and 1L -> short and 1L -> int are indistinguishable

          Fcn(&i,'c');    // calls Fcn(int*, int), because
                          // &i -> int* is better than &i -> const int*
                          // and 'c' -> int is better than 'c' -> short
   --end example]

  13.3.3.1  Implicit conversion sequences                [over.best.ics]

1 An  implicit  conversion sequence is a sequence of conversions used to
  convert an argument in a function call to the type of the  correspond­
  ing  parameter  of the function being called.  The sequence of conver­
  sions is an implicit conversion as defined in _conv_, which  means  it
  is  governed by the rules for initialization of an object or reference
  by a single expression (_dcl.init_, _dcl.init.ref_).

2 Implicit conversion sequences are concerned only with  the  type,  cv-
  qualification,  and lvalue-ness of the argument and how these are con­
  verted to match the corresponding properties of the parameter.   Other
  properties,  such as the lifetime, storage class, alignment, or acces­
  sibility of the argument and whether or not the  argument  is  a  bit-
  field  are  ignored.  So, although an implicit conversion sequence can
  be defined for a given argument-parameter pair,  the  conversion  from
  the  argument  to the parameter might still be ill-formed in the final
  analysis.

3 Except in the context of an initialization by user-defined  conversion
  (_over.match.user_), a well-formed implicit conversion sequence is one
  of the following forms:

  --a standard conversion sequence (_over.ics.scs_),

  --a user-defined conversion sequence (_over.ics.user_), or

  --an ellipsis conversion sequence (_over.ics.ellipsis_).

4 In the context of an initialization by user-defined conversion  (i.e.,
  when  considering  the argument of a user-defined conversion function;
  see _over.match.user_), only standard conversion sequences and  ellip­
  sis conversion sequences are allowed.

5 When  initializing a reference, the operation of binding the reference
  to an object or temporary occurs after any  conversion.   The  binding
  operation  is  not  a conversion, but it is considered to be part of a
  standard conversion sequence, and it can affect the rank of  the  con­
  version sequence.  See _over.ics.ref_.

6 In  all  contexts, when converting to the implicit object parameter or
  when converting to the left operand of an  assignment  operation  only
  standard  conversion sequences that create no temporary object for the
  result are allowed.

7 If no conversions are required to match an  argument  to  a  parameter
  type,  the  implicit  conversion  sequence  is the standard conversion
  sequence consisting of the identity conversion (_over.ics.scs_).

8 If no sequence of conversions can be found to convert an argument to a
  parameter  type or the conversion is otherwise ill-formed, an implicit
  conversion sequence cannot be formed.

9 If several different sequences of conversions exist that each  convert
  the  argument  to the parameter type, the implicit conversion sequence
  is a sequence among these that is not worse than all the rest  accord­
  ing to _over.ics.rank_14).  If that conversion sequence is not  better
  than all the rest and a function that uses such an implicit conversion
  sequence is selected as the best viable function, then the  call  will
  be  ill-formed  because  the conversion of one of the arguments in the
  call is ambiguous.

10The three forms of implicit conversion sequences mentioned  above  are
  defined in the following subclauses.

  _________________________
  14)  This rule prevents a function from becoming non-viable because of
  an ambiguous conversion sequence for one of its parameters.   Consider
  this example,
          class B;
          class A { A (B&); };
          class B { operator A (); };
          class C { C (B&); };
          void f(A) { }
          void f(C) { }
          B b;
          f(b);   // ambiguous since b -> C via constructor and
                  // b -> A via constructor or conversion function.
  If  it  were  not  for this rule, f(A) would be eliminated as a viable
  function for the call f(b) causing overload resolution to select  f(C)
  as the function to call even though it is not clearly the best choice.
  On the other hand, if an f(B) were to be declared then f(b) would  re­
  solved  to  that f(B) because the exact match with f(B) is better than
  any of the sequences required to match f(A).

  13.3.3.1.1  Standard conversion sequences               [over.ics.scs]

1 Table 2 summarizes the conversions defined in clause _conv_ and parti­
  tions them into four disjoint categories: Lvalue Transformation, Qual­
  ification  Adjustment,  Promotion, and Conversion.  [Note: these cate­
  gories are orthogonal with respect to  lvalue-ness,  cv-qualification,
  and  data representation: the Lvalue Transformations do not change the
  cv-qualification or data representation of the type; the Qualification
  Adjustments  do  not  change the lvalue-ness or data representation of
  the type; and the Promotions and Conversions do not change the lvalue-
  ness or cv-qualification of the type.  ]

2 [Note:  As  described  in  _conv_,  a  standard conversion sequence is
  either the Identity conversion by itself (that is, no  conversion)  or
  consists  of  one to three conversions from the other four categories.
  At most one conversion from each category is allowed in a single stan­
  dard conversion sequence.  If there are two or more conversions in the
  sequence, the conversions are applied in the canonical  order:  Lvalue
  Transformation,  Promotion  or Conversion, Qualification Adjustment.
  --end note]

3 Each conversion in Table 2 also has an associated rank  (Exact  Match,
  Promotion, or Conversion).  These are used to rank standard conversion
  sequences (_over.ics.rank_).  The rank of  a  conversion  sequence  is
  determined  by considering the rank of each conversion in the sequence
  and the rank of any reference binding  (_over.ics.ref_).   If  any  of
  those  has  Conversion  rank, the sequence has Conversion rank; other­
  wise, if any of those has Promotion rank, the sequence  has  Promotion
  rank; otherwise, the sequence has Exact Match rank.

                           Table 2--conversions

  +-------------------------------+--------------------------+-------------+-----------------+
  |Conversion                     |         Category         |    Rank     |    Subclause    |
  +-------------------------------+--------------------------+-------------+-----------------+
  +-------------------------------+--------------------------+-------------+-----------------+
  |No conversions required        |         Identity         |             |                 |
  +-------------------------------+--------------------------+             +-----------------+
  |Lvalue-to-rvalue conversion    |                          |             |   _conv.lval_   |
  +-------------------------------+                          |             +-----------------+
  |Array-to-pointer conversion    |  Lvalue Transformation   | Exact Match |  _conv.array_   |
  +-------------------------------+                          |             +-----------------+
  |Function-to-pointer conversion |                          |             |   _conv.func_   |
  +-------------------------------+--------------------------+             +-----------------+
  |Qualification conversions      | Qualification Adjustment |             |   _conv.qual_   |
  +-------------------------------+--------------------------+-------------+-----------------+
  |Integral promotions            |                          |             |   _conv.prom_   |
  +-------------------------------+        Promotion         |  Promotion  +-----------------+
  |Floating point promotion       |                          |             |  _conv.fpprom_  |
  +-------------------------------+--------------------------+-------------+-----------------+
  |Integral conversions           |                          |             | _conv.integral_ |
  +-------------------------------+                          |             +-----------------+
  |Floating point conversions     |                          |             |  _conv.double_  |
  +-------------------------------+                          |             +-----------------+
  |Floating-integral conversions  |                          |             |  _conv.fpint_   |
  +-------------------------------+                          |             +-----------------+
  |Pointer conversions            |        Conversion        | Conversion  |   _conv.ptr_    |
  +-------------------------------+                          |             +-----------------+
  |Pointer to member conversions  |                          |             |   _conv.mem_    |
  +-------------------------------+                          |             +-----------------+
  |Base class conversion          |                          |             |  _conv.class_   |
  +-------------------------------+                          |             +-----------------+
  |Boolean conversions            |                          |             |   _conv.bool_   |
  +-------------------------------+--------------------------+-------------+-----------------+

  13.3.3.1.2  User-defined conversion sequences          [over.ics.user]

1 A  user-defined  conversion  sequence  consists of an initial standard
  conversion   sequence   followed   by   a   user-defined    conversion
  (_class.conv_)  followed by a second standard conversion sequence.  If
  the  user-defined   conversion   is   specified   by   a   constructor
  (_class.conv.ctor_), the initial standard conversion sequence converts
  the source type to the type required by the argument of the  construc­
  tor.   If  the  user-defined  conversion  is specified by a conversion
  function (_class.conv.fct_), the initial standard conversion  sequence
  converts  the source type to the implicit object parameter of the con­
  version function.

2 The second standard conversion sequence converts  the  result  of  the
  user-defined conversion to the target type for the sequence.  Since an

  implicit conversion sequence is an initialization, the  special  rules
  for initialization by user-defined conversion apply when selecting the
  best user-defined conversion for a  user-defined  conversion  sequence
  (see _over.match.best_ and _over.best.ics_).

3 If  the  user-defined conversion is specified by a template conversion
  function, the second standard  conversion  sequence  must  have  exact
  match rank.

4 A  conversion of an expression of class type to the same class type or
  to a base class of that type is a standard conversion  rather  than  a
  user-defined  conversion  in spite of the fact that a copy constructor
  (i.e., a user-defined conversion function) is called.

  13.3.3.1.3  Ellipsis conversion sequences          [over.ics.ellipsis]

1 An ellipsis conversion sequence occurs when an argument in a  function
  call is matched with the ellipsis parameter specification of the func­
  tion called.

  13.3.3.1.4  Reference binding                           [over.ics.ref]

1 The operation of binding a reference is not a conversion, but for  the
  purposes of overload resolution it is considered to be part of a stan­
  dard conversion sequence (specifically, it is the last step in such  a
  sequence).

2 A standard conversion sequence cannot be formed if it requires binding
  a reference to non-const to an rvalue (except when binding an implicit
  object   parameter;   see   the   special   rules  for  that  case  in
  _over.match.funcs_).  [Note: this means, for example, that a candidate
  function  cannot  be a viable function if it has a non-const reference
  parameter (other than the implicit object parameter)  and  the  corre­
  sponding argument is a temporary or would require one to be created to
  initialize the reference (see _dcl.init.ref_).  ]

3 Other restrictions on binding a reference to a particular argument  do
  not  affect  the formation of a standard conversion sequence, however.
  [Example: a function with a "reference to  int"  parameter  can  be  a
  viable  candidate  even  if  the corresponding argument is an int bit-
  field.  The formation of implicit conversion sequences treats the  int
  bit-field  as  an int lvalue and finds an exact match with the parame­
  ter.  If the function is selected by  overload  resolution,  the  call
  will nonetheless be ill-formed because of the prohibition on binding a
  non-const reference to a bit-field (_dcl.init.ref_).  ]

4 A reference binding in general has no effect on the rank of a standard
  conversion sequence, but there are two exceptions:

    --the  binding  of a reference to a (possibly cv-qualified) class to
      an expression of a (possibly cv-qualified) class derived from that
      class  gives  the  overall standard conversion sequence Conversion
      rank.  [Example:

                  struct A {};
                  struct B : public A {} b;
                  int f(A&);
                  int f(B&);
                  int i = f(b);     // Calls f(B&), an exact match, rather than
                                    // f(A&), a conversion
       --end example]

    --the binding of a reference to an  expression  that  is  reference-
      compatible with added qualification influences the rank of a stan­
      dard conversion; see _over.ics.rank_ and _dcl.init.ref_.

  13.3.3.2  Ranking implicit conversion sequences        [over.ics.rank]

1 This  clause  defines  a  partial  ordering  of  implicit   conversion
  sequences  based  on  the relationships better conversion sequence and
  better conversion.  If an implicit conversion sequence S1  is  defined
  by  these rules to be a better conversion sequence than S2, then it is
  also the case that S2 is a worse conversion sequence than S1.  If con­
  version  sequence  S1 is neither better than nor worse than conversion
  sequence S2, S1 and S2 are said  to  be  indistinguishable  conversion
  sequences.

2 When  comparing  the  basic forms of implicit conversion sequences (as
  defined in _over.best.ics_)

  --a standard conversion sequence (_over.ics.scs_) is a better  conver­
    sion sequence than a user-defined conversion sequence or an ellipsis
    conversion sequence, and

  --a user-defined conversion sequence  (_over.ics.user_)  is  a  better
    conversion   sequence   than   an   ellipsis   conversion   sequence
    (_over.ics.ellipsis_).

3 Two implicit conversion sequences of the same form are  indistinguish­
  able conversion sequences unless one of the following rules apply:

  --Standard conversion sequence S1 is a better conversion sequence than
    standard conversion sequence S2 if

    --S1 is a proper subsequence of S2, or, if not that,

    --the rank of S1 is better than the rank of S2 (by the rules defined
      below), or, if not that,

    --S1  and  S2 differ only in their qualification conversion and they
      yield types identical except for cv-qualifiers and S2 adds all the
      qualifiers  that  S1 adds (and in the same places) and S2 adds yet
      more cv-qualifiers than S1, or the  similar  case  with  reference
      binding15).  [Example:
  _________________________
  15)  See  the definition of reference-compatible with added qualifica­
  tion in _dcl.init.ref_.

                  int f(const int *);
                  int f(int *);
                  int g(const int &);
                  int g(int &);
                  int i;
                  int j = f(&i);    // Calls f(int *)
                  int k = g(i);     // Calls g(int &)

                  class X {
                  public:
                      void f() const;
                      void f();
                  };
                  void g(const X& a, X b)
                  {
                      a.f();        // Calls X::f() const
                      b.f();        // Calls X::f()
                  }
       --end example]

  --User-defined conversion sequence U1 is a better conversion  sequence
    than another user-defined conversion sequence U2 if they contain the
    same user-defined conversion operator or constructor and if the sec­
    ond  standard  conversion  sequence  of U1 is better than the second
    standard conversion sequence of U2.  [Example:
              struct A {
                  operator short();
              } a;
              int f(int);
              int f(float);
              int i = f(a);     // Calls f(int), because short -> int is
                                // better than short -> float.
     --end example]

4 Standard conversions are ordered by their ranks: an Exact Match  is  a
  better  conversion than a Promotion, which is a better conversion than
  a Conversion.  Two conversions with the same rank  are  indistinguish­
  able unless one of the following rules applies:

  --A  conversion  that  is not a conversion of a pointer, or pointer to
    member, to bool is better than another conversion  that  is  such  a
    conversion.

  --If  class  B is derived directly or indirectly from class A, conver­
    sion of B* to A* is better than conversion of B* to void*, and  con­
    version of A* to void* is better than conversion of B* to void*.

  --If  class B is derived directly or indirectly from class A and class
    C is derived directly or indirectly from B,

    --conversion of C* to B* is better than conversion of C* to A*,

    --binding of an expression of type C to a reference of  type  B&  is

      better than binding an expression of type C to a reference of type
      A&,

    --conversion of A::* to B::* is better than conversion  of  A::*  to
      C::*,

    --conversion of C to B is better than conversion of C to A,

    --conversion of B* to A* is better than conversion of C* to A*,

    --binding  of  an  expression of type B to a reference of type A& is
      better than binding an expression of type C to a reference of type
      A&,

    --conversion  of  B::*  to C::* is better than conversion of A::* to
      C::*, and

    --conversion of B to A is better than conversion of C to A.   [Exam­
      ple:
                  struct A {};
                  struct B : public A {};
                  struct C : public B {};
                  C *pc;
                  int f(A *);
                  int f(B *);
                  int i = f(pc);    // Calls f(B *)
       --end example]

  13.4  Address of overloaded function                       [over.over]

1 A  use of an overloaded function name without arguments is resolved in
  certain contexts to a function, a pointer to function or a pointer  to
  member  function  for  a specific function from the overload set.  The
  function selected is the  one  whose  type  matches  the  target  type
  required  in  the  context.   It is required that exactly one function
  matches the target type.  The target can be

  --an   object   or   reference    being    initialized    (_dcl.init_,
    _dcl.init.ref_),

  --the left side of an assignment (_expr.ass_),

  --a parameter of a function (_expr.call_),

  --a parameter of a user-defined operator (_over.oper_),

  --the  return  value  of  a function, operator function, or conversion
    (_stmt.return_), or

  --an explicit type conversion  (_expr.type.conv_,  _expr.static.cast_,
    _expr.cast_).

  The  overloaded  function  name can be preceded by the & operator.  An

  overloaded function name shall not be used without arguments  in  con­
  texts other than those listed.

  +-------                 BEGIN BOX 3                -------+
  Change: this subclause did not include reference initialization as one
  of the contexts that could disambiguate the use of an overloaded func­
  tion  name used without arguments.  This context was added to the list
  as an editorial proposal.  See email message core-6024.
  +-------                  END BOX 3                 -------+

2 If the name is a function template,  template  argument  deduction  is
  done  (_temp.deduct_),  and  if  the  argument deduction succeeds, the
  deduced template arguments are used  to  generate  a  single  template
  function,  which  is  added to the set of overloaded functions consid­
  ered.

3 Non-member functions and static member functions match targets of type
  "pointer-to-function"  or  "reference-to-function."   Nonstatic member
  functions match  targets  of  type  "pointer-to-member-function;"  the
  function  type  of  the pointer to member is used to select the member
  function from the set of overloaded member functions.  If a  nonstatic
  member  function is selected, the reference to the overloaded function
  name is required to have the form of a pointer to member as  described
  in _expr.unary.op_.  [Example:
          int f(double);
          int f(int);
          (int (*)(int))&f;           // cast expression as selector
          int (*pfd)(double) = &f;    // selects f(double)
          int (*pfi)(int) = &f;       // selects f(int)
          int (*pfe)(...) = &f;       // error: type mismatch
          void (&rfi)(int) = f;       // selects f(int)
          void (&rfd)(double) = f;    // selects f(double)
  The  initialization  of  pfe  is  ill-formed  because no f() with type
  int(...)  has been defined, and not because  of  any  ambiguity.   For
  another example,
          struct X {
              int f(int);
              static int f(long);
          };
          int (X::*p1)(int)  = &X::f;   // OK
          int    (*p2)(int)  = &X::f;   // error: mismatch
          int    (*p3)(long) = &X::f;   // OK
          int (X::*p4)(long) = &X::f;   // error: mismatch
          int (X::*p5)(int)  = &(X::f); // error: wrong syntax for
                                        // pointer to member
          int    (*p6)(long) = &(X::f); // OK
   --end example]

4 [Note: if f() and g() are both overloaded functions, the cross product
  of possibilities must be considered to resolve f(&g), or  the  equiva­
  lent expression f(g).  ]

5 [Note:  there  are no standard conversions (_conv_) of one pointer-to-
  function type into another.  In particular, even if B is a public base
  of D, we have
          D* f();
          B* (*p1)() = &f;       // error
          void g(D*);
          void (*p2)(B*) = &g;   // error
   --end note]

  13.5  Overloaded operators                                 [over.oper]

1 A  function declaration having one of the following operator-function-
  ids as its name declares an operator function.  An  operator  function
  is said to implement the operator named in its operator-function-id.
          operator-function-id:
                  operator operator
          operator: one of
                  new  delete    new[]     delete[]
                  +    -    *    /    %    ^    &    |    ~
                  !    =    <    >    +=   -=   *=   /=   %=
                  ^=   &=   |=   <<   >>   >>=  <<=  ==   !=
                  <=   >=   &&   ||   ++   --   ,    ->*  ->
                  ()   []
  [Note: the last two operators are function call (_expr.call_) and sub­
  scripting (_expr.sub_).  ]

2 Both the unary and binary forms of
                  +    -    *     &
  can be overloaded.

3 The following operators cannot be overloaded:
                  .    .*   ::    ?:
  nor can the preprocessing symbols # and ## (_cpp_).

4 Operator functions are usually not called directly; instead  they  are
  invoked  to  evaluate  the  operators  they  implement (_over.unary_ -
  _over.inc_).  They can be explicitly called, however, using the opera­
  tor-function-id  as the name of the function in the function call syn­
  tax (_expr.call_).  [Example:
          complex z = a.operator+(b);  // complex z = a+b;
          void* p = operator new(sizeof(int)*n);
   --end example]

5 The allocation and  deallocation  functions,  operator  new,  operator
  new[], operator delete and operator delete[], are described completely
  in _basic.stc.dynamic_.  The attributes and restrictions found in  the
  rest  of this section do not apply to them unless explicitly stated in
  _basic.stc.dynamic_.

6 An operator function shall either be a non-static member  function  or
  be a non-member function and have at least one parameter whose type is
  a class, a reference to a class, an enumeration, or a reference to  an
  enumeration.   It  is not possible to change the precedence, grouping,

  or number of operands of operators.  The meaning of the  operators  =,
  (unary) &, and , (comma), predefined for each type, can be changed for
  specific types by defining operator  functions  that  implement  these
  operators.   Operator  functions are inherited the same as other func­
  tions, but because an instance  of  operator=  is  automatically  con­
  structed for each class (_class.copy_, _over.ass_), operator= is never
  inherited by a class from its bases.

7 The identities among certain predefined  operators  applied  to  basic
  types (for example, ++a == a+=1) need not hold for operator functions.
  Some predefined operators, such as +=, require an  operand  to  be  an
  lvalue  when  applied to basic types; this is not required by operator
  functions.

8 An    operator    function    cannot    have     default     arguments
  (_dcl.fct.default_),  except  where explicitly stated below.  Operator
  functions cannot  have  more  or  fewer  parameters  than  the  number
  required  for  the corresponding operator, as described in the rest of
  this subclause.

9 Operators not mentioned explicitly below in _over.ass_  to  _over.inc_
  act  as  ordinary unary and binary operators obeying the rules of sec­
  tion _over.unary_ or _over.binary_.

  13.5.1  Unary operators                                   [over.unary]

1 A prefix unary operator shall be implemented by  a  non-static  member
  function  (_class.mfct_)  with  no parameters or a non-member function
  with one parameter.  Thus, for any prefix unary operator @, @x can  be
  interpreted as either x.operator@() or operator@(x).  If both forms of
  the   operator   function   have   been   declared,   the   rules   in
  _over.match.oper_  determine  which,  if  any, interpretation is used.
  See _over.inc_ for an explanation of the postfix  unary  operators  ++
  and --.

2 The unary and binary forms of the same operator are considered to have
  the same name.  [Note: consequently,  a  unary  operator  can  hide  a
  binary operator from an enclosing scope, and vice versa.  ]

  13.5.2  Binary operators                                 [over.binary]

1 A  binary  operator shall be implemented either by a non-static member
  function (_class.mfct_) with one parameter or by a non-member function
  with  two  parameters.   Thus,  for  any binary operator @, x@y can be
  interpreted as either x.operator@(y) or operator@(x,y).  If both forms
  of   the   operator   function   have  been  declared,  the  rules  in
  _over.match.oper_ determines which, if any, interpretation is used.

  13.5.3  Assignment                                          [over.ass]

1 An assignment operator shall be implemented  by  a  non-static  member
  function with exactly one parameter.  Because a copy assignment opera­
  tor operator= is implicitly declared for a class if  not  declared  by

  the  user  (_class.copy_),  a base class assignment operator is always
  hidden by the copy assignment operator of the derived class.

2 Any assignment operator, even the copy  assignment  operator,  can  be
  virtual.  [Note: for a derived class D with a base class B for which a
  virtual copy assignment has been declared, the copy assignment  opera­
  tor  in  D  does  not  override  B's virtual copy assignment operator.
  [Example:
          struct B {
                  virtual int operator= (int);
                  virtual B& operator= (const B&);
          };
          struct D : B {
                  virtual int operator= (int);
                  virtual D& operator= (const B&);
          };
          D dobj1;
          D dobj2;
          B* bptr = &dobj1;
          void f() {
                  bptr->operator=(99);    // calls D::operator(int)
                  *bptr = 99;             // ditto
                  bptr->operator=(dobj2); // calls D::operator(const B&)
                  *bptr = dobj2;          // ditto
                  dobj1 = dobj2;          // calls D::operator(const D&)
          }
   --end example]  --end note]

  13.5.4  Function call                                      [over.call]

1 operator() shall be a non-static member  function  with  an  arbitrary
  number  of  parameters.  It can have default arguments.  It implements
  the function call syntax
          postfix-expression ( expression-listopt )
  where the postfix-expression evaluates to a class object and the  pos­
  sibly  empty  expression-list  matches the parameter list of an opera­
  tor() member function of the class.   Thus,  a  call  x(arg1,...)   is
  interpreted  as x.operator()(arg1,...)  for a class object x of type T
  if T::operator()(T1, T2, T3) exists and if the operator is selected as
  the   best   match  function  by  the  overload  resolution  mechanism
  (_over.match.best_).

  13.5.5  Subscripting                                        [over.sub]

1 operator[] shall be a non-static  member  function  with  exactly  one
  parameter.  It implements the subscripting syntax
          postfix-expression [ expression ]
  Thus, a subscripting expression x[y] is interpreted as x.operator[](y)
  for a class object x of type T if T::operator[](T1) exists and if  the
  operator  is selected as the best match function by the overload reso­
  lution mechanism (_over.match.best_).

  13.5.6  Class member access                                 [over.ref]

1 operator-> shall be a non-static member function taking no parameters.
  It implements class member access using ->
          postfix-expression -> id-expression
  An  expression  x->m is interpreted as (x.operator->())->m for a class
  object x of type T if T::operator->() exists and if  the  operator  is
  selected  as the best match function by the overload resolution mecha­
  nism (_over.match_).

  13.5.7  Increment and decrement                             [over.inc]

1 The user-defined function called operator++ implements the prefix  and
  postfix  ++  operator.   If this function is a member function with no
  parameters, or a non-member function with one parameter  of  class  or
  enumeration  type,  it  defines  the  prefix increment operator ++ for
  objects of that type.  If the function is a member function  with  one
  parameter  (which  shall be of type int) or a non-member function with
  two parameters (the second of which shall be of type int), it  defines
  the  postfix increment operator ++ for objects of that type.  When the
  postfix increment is called as a result of using the ++ operator,  the
  int argument will have value zero.16) [Example:
          class X {
          public:
              const X&   operator++();     // prefix ++a
              const X&   operator++(int);  // postfix a++
          };
          class Y {
          public:
          };
          const Y&   operator++(Y&);       // prefix ++b
          const Y&   operator++(Y&, int);  // postfix b++
          void f(X a, Y b)
          {
              ++a;        // a.operator++();
              a++;        // a.operator++(0);
              ++b;        // operator++(b);
              b++;        // operator++(b, 0);
              a.operator++();    // explicit call: like ++a;
              a.operator++(0);   // explicit call: like a++;
              operator++(b);     // explicit call: like ++b;
              operator++(b, 0);  // explicit call: like b++;
          }
   --end example]

2 The prefix and postfix decrement operators -- are handled analogously.

  _________________________
  16)   Calling   operator++   explicitly,   as   in   expressions  like
  a.operator++(1,2), has no special properties: the arguments to  opera­
  tor++ are 1 and 2.

  13.6  Built-in operators                                  [over.built]

1 The candidate operator functions that represent the built-in operators
  defined in clause _expr_ are specified in this subclause. These candi­
  date functions participate in the operator overload resolution process
  as described in _over.match.oper_ and are used for no other purpose.

2 [Note:  since  built-in  operators  take  only operands with non-class
  type, and operator overload resolution occurs  only  when  an  operand
  expression originally has class or enumeration type, operator overload
  resolution can resolve to a built-in operator only when an operand has
  a  class  type  that has a user-defined conversion to a non-class type
  appropriate for the operator, or when an operand  has  an  enumeration
  type that can be converted to a type appropriate for the operator.  ]

3 In  this  section, the term promoted integral type is used to refer to
  those  integral  types  which  are  preserved  by  integral  promotion
  (including  e.g.   int and long but excluding e.g.  char).  Similarly,
  the term promoted arithmetic type refers to  promoted  integral  types
  plus floating types.

4 For every pair T, VQ), where T is an arithmetic type, and VQ is either
  volatile or empty, there exist candidate  operator  functions  of  the
  form
          VQ T&   operator++(VQ T&);
          T       operator++(VQ T&, int);

5 For  every pair T, VQ), where T is an arithmetic type other than bool,
  and VQ is either volatile or empty,  there  exist  candidate  operator
  functions of the form
          VQ T&   operator--(VQ T&);
          T       operator--(VQ T&, int);

6 For  every  pair  T,  VQ), where T is a cv-qualified or cv-unqualified
  complete object type, and VQ is either volatile or empty, there  exist
  candidate operator functions of the form
          T*VQ&   operator++(T*VQ&);
          T*VQ&   operator--(T*VQ&);
          T*      operator++(T*VQ&, int);
          T*      operator--(T*VQ&, int);

7 For every cv-qualified or cv-unqualified complete object type T, there
  exist candidate operator functions of the form
          T&      operator*(T*);

8 For every function type T, there exist candidate operator functions of
  the form
          T&      operator*(T*);

9 For every type T, there exist candidate operator functions of the form
          T*      operator+(T*);

10For every promoted arithmetic type T, there exist  candidate  operator
  functions of the form
          T       operator+(T);
          T       operator-(T);

11For  every  promoted  integral  type T, there exist candidate operator
  functions of the form
          T       operator~(T);

12For every quadruple C, T, CV1, CV2), where C is a class type, T  is  a
  complete  object  type  or  a  function  type, and CV1 and CV2 are cv-
  qualifier-seqs, there exist candidate operator functions of the form
          CV12 T& operator->*(CV1 C*, CV2 T C::*);
  where CV12 is the union of CV1 and CV2.

13For every pair of promoted arithmetic types L and R, there exist  can­
  didate operator functions of the form
          LR      operator*(L, R);
          LR      operator/(L, R);
          LR      operator+(L, R);
          LR      operator-(L, R);
          bool    operator<(L, R);
          bool    operator>(L, R);
          bool    operator<=(L, R);
          bool    operator>=(L, R);
          bool    operator==(L, R);
          bool    operator!=(L, R);
  where  LR  is  the  result of the usual arithmetic conversions between
  types L and R.

14For every pair of types T and I, where T  is  a  cv-qualified  or  cv-
  unqualified  complete  object  type and I is a promoted integral type,
  there exist candidate operator functions of the form
          T*      operator+(T*, I);
          T&      operator[](T*, I);
          T*      operator-(T*, I);
          T*      operator+(I, T*);
          T&      operator[](I, T*);

15For every triple T, CV1, CV2), where T is a complete object type,  and
  CV1  and  CV2  are  cv-qualifier-seqs,  there exist candidate operator
  functions of the form17)
          ptrdiff_t operator-(CV1 T*, CV2 T*);

16For  every  triple  T, CV1, CV2), where T is any type, and CV1 and CV2
  are cv-qualifier-seqs, there exist candidate operator functions of the
  form18)
  _________________________
  17) When T is itself a pointer type, the interior cv-qualifiers of the
  two  parameter types need not be identical.  The two pointer types are
  converted to a common type (which need not be the same as  either  pa­
  rameter type) by implicit pointer conversions.
  18) When T is itself a pointer type, the interior cv-qualifiers of the

          bool    operator<(CV1 T*, CV2 T*);
          bool    operator>(CV1 T*, CV2 T*);
          bool    operator<=(CV1 T*, CV2 T*);
          bool    operator>=(CV1 T*, CV2 T*);
          bool    operator==(CV1 T*, CV2 T*);
          bool    operator!=(CV1 T*, CV2 T*);

17For every quadruple C, T, CV1, CV2), where C is a class type, T is any
  type,  and  CV1  and  CV2 are cv-qualifier-seqs, there exist candidate
  operator functions of the form19)
          bool    operator==(CV1 T C::*, CV2 T C::*);
          bool    operator!=(CV1 T C::*, CV2 T C::*);

18For  every pair of promoted integral types L and R, there exist candi­
  date operator functions of the form
          LR      operator%(L, R);
          LR      operator&(L, R);
          LR      operator^(L, R);
          LR      operator|(L, R);
          L       operator<<(L, R);
          L       operator>>(L, R);
  where LR is the result of the  usual  arithmetic  conversions  between
  types L and R.

19For  every  triple  L,  VQ,  R),  where L is an arithmetic type, VQ is
  either volatile or empty, and R is a promoted arithmetic  type,  there
  exist candidate operator functions of the form
          VQ L&   operator=(VQ L&, R);
          VQ L&   operator*=(VQ L&, R);
          VQ L&   operator/=(VQ L&, R);
          VQ L&   operator+=(VQ L&, R);
          VQ L&   operator-=(VQ L&, R);

20For  every  pair T, VQ), where T is any type and VQ is either volatile
  or empty, there exist candidate operator functions of the form
          T*VQ&   operator=(T*VQ&, T*);

21For every pair T, VQ), where T is a pointer to member type and  VQ  is
  either  volatile or empty, there exist candidate operator functions of
  the form
          VQ T&   operator=(VQ T&, T);

22For every triple  T,  VQ,  I),  where  T  is  a  cv-qualified  or  cv-
  unqualified  complete object type, VQ is either volatile or empty, and
  _________________________
  two parameter types need not be identical.  The two pointer types  are
  converted  to  a common type (which need not be the same as either pa­
  rameter type) by implicit pointer conversions.
  19) When T is itself a pointer type, the interior cv-qualifiers of the
  two  parameter types need not be identical.  The two pointer types are
  converted to a common type (which need not be the same as  either  pa­
  rameter type) by implicit pointer conversions.

  I is a promoted integral type, there exist  candidate  operator  func­
  tions of the form
          T*VQ&   operator+=(T*VQ&, I);
          T*VQ&   operator-=(T*VQ&, I);

23For  every triple L, VQ, R), where L is an integral type, VQ is either
  volatile or empty, and R is a promoted integral type, there exist can­
  didate operator functions of the form
          VQ L&   operator%=(VQ L&, R);
          VQ L&   operator<<=(VQ L&, R);
          VQ L&   operator>>=(VQ L&, R);
          VQ L&   operator&=(VQ L&, R);
          VQ L&   operator^=(VQ L&, R);
          VQ L&   operator|=(VQ L&, R);

24There also exist candidate operator functions of the form
          bool    operator!(bool);
          bool    operator&&(bool, bool);
          bool    operator||(bool, bool);