______________________________________________________________________

  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.  [Note: this
  restriction  applies  to  explicit  declarations in a scope, including
  declarations made through a using-declaration (_namespace.udecl_).  It
  does  not  apply  to  sets of functions fabricated as a result of name
  lookup (e.g., because  of  using-directives)  or  overload  resolution
  (e.g., for operator functions).  ]

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 (_over.match.viable_).

  --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  of
  (P1,...,Pn) returning R" or type "reference to function of (P1,...,Pn)
  returning R", a surrogate 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 built-in 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.

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.

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)
  [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_.

  _________________________
  10) If the set of candidate functions is empty, overload resolution is
  unsuccessful.

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]

  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   converting   constructors
    (_class.conv.ctor_) of T are candidate functions.

  --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
  _________________________
  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.

    cv-qualification as, or lesser cv-qualification  than,  cv1,  via  a
    standard  conversion  sequence  (_over.ics.scs_) are candidate func­
    tions.  Conversion functions that return a nonclass type cv2  T  are
    considered  to  yield cv-unqualified T for this process of selecting
    candidate functions.  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
  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.,
  _________________________
  12)  According to _dcl.fct.default_, parameters following the (m+1)-st
  parameter must also have default arguments.

    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:
          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) 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 at least as good as W, F
  cannot be the best function because at some point in the tournament  F
  encountered  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.

  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.

  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]

  _________________________
  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).

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:
                  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
  _________________________
  15) See the definition of reference-compatible with  added  qualifica­
  tion in _dcl.init.ref_.

    conversion 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.

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_).  The operators new[], delete[], (), and [] are
  formed from more than one token.  ]

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 class and enumeration types by  defining  operator  functions
  that  implement these operators.  Operator functions are inherited the
  same as other functions, but because an instance of operator= is auto­
  matically constructed for each class (_class.copy_, _over.ass_), oper­
  ator= 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]

  _________________________
  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.

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

  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.
  Also note that the candidate operator functions given in this  section
  are  in  some  cases more permissive than the built-in operators them­
  selves.  As described in _over.match.oper_, after a built-in  operator
  is  selected  by  overload resolution the expression is subject to the
  requirements for the built-in operator given  in  clause  _expr_,  and
  therefore to any additional semantic constraints given there.  ]

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.   [Note: in all cases where a promoted integral
  type or promoted arithmetic type is required, an operand  of  enumera­
  tion type will be acceptable by way of the integral promotions.  ]

4 For  every  pair T, VQ), where T is an arithmetic or enumeration 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 enumeration type or 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 quintuple C1, C2, T, CV1, CV2), where C2 is a class type, C1
  is  the  same  type as C2 or is a derived class of C2, 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 C1*, CV2 T C2::*);
  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)
          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  or  enumeration
  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*);
  _________________________
  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
  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.

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
  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  or  enumeration
  type,  VQ  is  either  volatile or empty, and R is a promoted integral
  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);
          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);