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  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, and between
  such declarations and declarations made  through  a  using-declaration
  (_namespace.udecl_).   It  does  not apply to sets of functions fabri-
  cated 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_).   Likewise,  member  function
    template  declarations with the same name, the same parameter types,
    and the same template parameter lists cannot be overloaded if any of
    them is a static member function template declaration.  The types of
    the implicit object parameters constructed for the member  functions
    for  the purpose 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 declara-
    tion among a set of member function declarations with the same  name
    and  the  same  parameter types, then these member function declara-
    tions can be overloaded if they differ in the type of their implicit
    object parameter.  [Example: the following illustrates this distinc-
    tion:
              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 ()
          {
              extern void callee (int, int);
              {
                  extern 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  seven  distinct
  contexts 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,  a  reference-to-pointer-to-function  conversion
    function, or a reference-to-function conversion function on 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_);

  --invocation  of  a  user-defined  conversion  for copy-initialization
    (_dcl.init_) of a class object (_over.match.copy_);

  --invocation of a conversion function for initialization of an  object
    of   a   nonclass   type   from   an   expression   of   class  type
    (_over.match.conv_); and

  --invocation of a conversion function for initialization of  a  tempo-
    rary  to  which  a reference (_dcl.init.ref_) will be directly bound
    (_over.match.ref_).

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
  seven contexts in which  overload  resolution  is  used.   The  source
  transformations and constructions defined in these subclauses are only
  for the purpose of describing the  overload  resolution  process.   An
  implementation  is  not  required to use such transformations and con-
  structions.

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 conversion functions, the function is considered to be a  member
  of the class of the implicit object argument for the purpose of defin-
  ing the type of the implicit  object  parameter.   For  non-conversion
  functions  introduced by a using-declaration into a derived class, the
  function is considered to be a member of the  derived  class  for  the
  purpose  of  defining  the type of the implicit object parameter.  For
  static member functions, the implicit object parameter  is  considered
  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 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]

7 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-tem-
  plate  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_  describes  how  overload  resolution  is used in the
  first two of the above  cases  to  determine  the  function  to  call.
  _________________________
  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.

  _over.call.object_  describes  how  overload resolution is used in the
  third of the above 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 In  qualified function calls, the name to be resolved is an id-expres-
  sion and is preceded by an -> or .   operator.   Since  the  construct
  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  functions.  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    in    function    calls
  (_basic.lookup.koenig_).  If the name resolves to a  non-member  func-
  tion  declaration,  that  function  and  its  overloaded  declarations
  _________________________
  3) If F names a non-static member function, &F is a pointer-to-member,
  which cannot be used with the function call syntax.
  4) Note that cv-qualifiers on the type of objects are  significant  in
  overload resolution for both lvalue and class rvalue objects.

  constitute the set of candidate functions5).  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 transformed into a normalized qual-
  ified 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 argument6).
  The call is ill-formed, however, if overload resolution selects one of
  the non-static member functions of T in this case.

  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().

2 In addition, for each conversion function declared in T of the form
          operator conversion-type-id () cv-qualifier;
  where  cv-qualifier  is the same cv-qualification as, or a greater cv-
  qualification than, cv, and where conversion-type-id denotes the  type
  "pointer  to function of (P1,...,Pn) returning R", or the type "refer-
  ence to pointer to function of (P1,...,Pn) returning R", or  the  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 declaration7).
  _________________________
  5) 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.
  6) 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.
  7) 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.

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:
          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_.   ?:  cannot  be
  overloaded,  but  the  rules in this section are used to determine the
  conversions to be applied to the second and third operands  when  they
  have class or enumeration type (_expr.cond_).  ] [Example:

          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  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 result  of  the  unqualified
    lookup  of  operator@  in the context of the expression according to
    the usual rules  for  name  lookup  in  unqualified  function  calls
    (_basic.lookup.koenig_)   except   that  all  member  functions  are
    ignored.

  --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_.

4 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  the left operand to
    achieve a type match with the left-most parameter of a built-in can-
    didate.

5 For all other operators, no such restrictions apply.

6 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_.8)
  [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]

7 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_.

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

8 The 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.9)

9 If 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_.

10[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 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.

  13.3.1.4  Copy-initialization of class by user-      [over.match.copy]
       defined conversion

1 Under the conditions specified in _dcl.init_, as part of  a  copy-ini-
  tialization  of an object of class type, a user-defined conversion can
  be invoked to convert an initializer expression to  the  type  of  the
  object  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 being initialized, with T a class type, the candi-
  date functions are selected as follows:

  _________________________
  9)  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.

  --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 T2", where T2
    is the same type as T or is a derived class thereof, and  where  cv2
    is  the  same  cv-qualification as, or lesser cv-qualification than,
    cv1, are candidate functions.   Conversions  functions  that  return
    "reference  to T" return lvalues of type T and are therefore consid-
    ered 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.  ]

  13.3.1.5  Initialization by conversion function      [over.match.conv]

1 Under the conditions specified in _dcl.init_, as part of  an  initial-
  ization  of  an  object of nonclass type, a conversion function can be
  invoked to convert an initializer expression of class type to the type
  of  the  object  being  initialized.   Overload  resolution is used to
  select the conversion function to be invoked.  Assuming that  "cv1  T"
  is the type of the object being initialized, and "cv S" is the type of
  the initializer expression, with S a class type, the  candidate  func-
  tions are selected as follows:

  --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" via a standard conversion
    sequence (_over.ics.scs_), for any cv2 that is the same  cv-qualifi-
    cation as, or lesser cv-qualification than, cv1, 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 The argument list has one argument, which is the  initializer  expres-
  sion.   [Note:  this  argument  will  be compared against the implicit
  object parameter of the conversion functions.  ]

  13.3.1.6  Initialization by conversion function       [over.match.ref]
       for direct reference binding

1 Under  the  conditions specified in _dcl.init.ref_, a reference can be
  bound directly to an lvalue that is the result of applying  a  conver-
  sion  function  to  an initializer expression.  Overload resolution is
  used to select the conversion function to be invoked.   Assuming  that
  "cv1 T" is the underlying type of the reference being initialized, and
  "cv S" is the type of the initializer expression, with S a class type,
  the candidate functions are selected as follows:

  --The  conversion  functions of S and its base classes are considered.
    Those that are not hidden within S and yield type "reference to  cv2
    T2",  where  "cv1  T"  is reference-compatible (_dcl.init.ref_) with
    "cv2 T2", are candidate functions.

2 The argument list has one argument, which is the  initializer  expres-
  sion.   [Note:  this  argument  will  be compared against the implicit
  object parameter of the conversion functions.  ]

  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_).10) 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
  _________________________
  10)  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 special-
    ization, 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_,  _over.match.conv_,  and _over.match.ref_) and the stan-
    dard conversion sequence from the return type of F1 to the  destina-
    tion type (i.e., the type of the entity being initialized) is a bet-
    ter 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-formed11).

3 [Example:

  _________________________
  11)  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.

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

          int i;
          short s = 0;

          void f() {
            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.conv_), 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.conv_), only standard conversion sequences and  ellip-
  sis conversion sequences are allowed.

5 For   the   case   where  the  parameter  type  is  a  reference,  see
  _over.ics.ref_.

6 When the parameter type is not a reference,  the  implicit  conversion
  sequence  models a copy-initialization of the parameter from the argu-
  ment expression.  The implicit conversion sequence is the one required
  to  convert  the  argument  expression to an rvalue of the type of the
  parameter.  [Note: when the parameter has a class type, this is a con-
  ceptual conversion defined for the purposes of this clause; the actual
  initialization is defined in terms of constructors and is not  a  con-
  version.   ]  Any difference in top-level cv-qualification is subsumed
  by the initialization itself and does  not  constitute  a  conversion.
  [Example: a parameter of type A can be initialized from an argument of
  type const A.  The implicit conversion sequence for that case  is  the
  identity  sequence;  it contains no "conversion" from const A to A.  ]
  When the parameter has a class type and the argument expression is  an
  rvalue  of the same type, the implicit conversion sequence is an iden-
  tity conversion.  When the parameter has a class type and the argument
  expression  is  an  lvalue  of  the same type, the implicit conversion
  sequence is an lvalue-to-rvalue conversion.  When the parameter has  a
  class type and the argument expression is an rvalue of a derived class
  type, the implicit conversion sequence is a derived-to-base Conversion
  from  the  derived  class  to the base class.  [Note: there is no such
  standard conversion; this derived-to-base Conversion  exists  only  in
  the  description of implicit conversion sequences.  ] When the parame-
  ter has a class type and the argument expression is  an  lvalue  of  a
  derived  class type, the implicit conversion sequence is an lvalue-to-
  rvalue  conversion  followed  by  a  derived-to-base  Conversion.    A
  derived-to-base Conversion has Conversion rank (_over.ics.scs_).

7 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.

8 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_).

9 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.

10If 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_12).  If that conversion sequence is not  better
  _________________________
  12)  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 (); };

  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.

11The 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]

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.

  _________________________
          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-
  solve  to  that  f(B) because the exact match with f(B) is better than
  any of the sequences required to match f(A).

                           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_   |
  +-------------------------------+        Conversion        | Conversion  +-----------------+
  |Pointer conversions            |                          |             |   _conv.ptr_    |
  +-------------------------------+                          |             +-----------------+
  |Pointer to member conversions  |                          |             |   _conv.mem_    |
  +-------------------------------+                          |             +-----------------+
  |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 is
  given Exact Match rank, and a conversion of  an  expression  of  class
  type  to  a base class of that type is given Conversion rank, in spite
  of the fact that a copy constructor (i.e., a  user-defined  conversion
  function) is called for those cases.

  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 When  a parameter of reference type binds directly (_dcl.init.ref_) to
  an argument expression, the implicit conversion sequence is the  iden-
  tity  conversion,  unless the argument expression has a type that is a
  derived class of the parameter type, in which case the  implicit  con-
  version  sequence  is  a derived-to-base Conversion (_over.best.ics_).
  [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] If the parameter  binds  directly  to  the  result  of
  applying  a  conversion  function  to  the  argument  expression,  the
  implicit conversion sequence is  a  user-defined  conversion  sequence
  (_over.ics.user_), with the second standard conversion sequence either
  an identity conversion or,  if  the  conversion  function  returns  an
  entity  of  a  type  that  is a derived class of the parameter type, a
  derived-to-base Conversion.

2 When a parameter of reference type is not bound directly to  an  argu-
  ment  expression,  the conversion sequence is the one required to con-
  vert the argument expression to the underlying type of  the  reference
  according  to _over.best.ics_.  Conceptually, this conversion sequence
  corresponds to copy-initializing a temporary of  the  underlying  type
  with the argument expression.  Any difference in top-level cv-qualifi-
  cation is subsumed by the initialization itself and does  not  consti-
  tute a conversion.

3 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_).  ]

4 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_).  ]

5 The binding of a reference to an expression that is reference-compati-
  ble with added qualification influences the rank of a standard conver-
  sion; 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  (comparing  the conversion
      sequences in the canonical form  defined  by  _over.ics.scs_;  the
      identity  conversion sequence is considered to be a subsequence of
      any non-identity conversion sequence) 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  yield
      similar  types  T1 and T2 (_conv.qual_), respectively, and the cv-
      qualification signature of type T1 is a proper subset of  the  cv-
      qualification signature of type T2, [Example:
                  int f(const int *);
                  int f(int *);
                  int i;
                  int j = f(&i);    // Calls f(int *)
       --end example] or, if not that,

    --S1  and  S2 are reference bindings (_dcl.init.ref_), and the types
      to which the references refer are the same type  except  for  top-
      level  cv-qualifiers, and the type to which the reference initial-
      ized by S2 refers is more cv-qualified than the type to which  the
      reference initialized by S1 refers.  [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);     // ambiguous
                  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 function 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  conversion  sequences  are  ordered by their ranks: an Exact
  Match is a better conversion than a Promotion, which is a better  con-
  version  than  a  Conversion.   Two conversion sequences with the same
  rank are indistinguishable 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*,
      [Example:
                  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]

    --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.
    [Note:  it is necessary to compare conversions with different target
    types in the context of an initialization  by  user-defined  conver-
    sion; see _over.match.best_.  ]

  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.  [Note: any redundant set of  parenthe-
  ses surrounding the overloaded function name is ignored (_expr.prim_).
  ]

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 (*pfd)(double) = &f;    // selects f(double)
          int (*pfi)(int) = &f;       // selects f(int)
          int (*pfe)(...) = &f;       // error: type mismatch
          int (&rfi)(int) = f;       // selects f(int)
          int (&rfd)(double) = f;    // selects f(double)
          void g() {
            (int (*)(int))&f;           // cast expression as selector
          }
  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  in
  the same manner as other base class functions, but because an instance
  of   operator=   is   automatically   constructed   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 implicitly-declared
                                          // 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.13) [Example:
          class X {
          public:
              X&   operator++();     // prefix ++a
              X    operator++(int);  // postfix a++
          };
          class Y {
          public:
          };
          Y&   operator++(Y&);       // prefix ++b
          Y    operator++(Y&, int);  // postfix b++

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

          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.

  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 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
  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 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 an 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 cv-qualified or cv-unqualified object  type  T  there  exist
  candidate operator functions of the form
          T*      operator+(T*, ptrdiff_t);
          T&      operator[](T*, ptrdiff_t);
          T*      operator-(T*, ptrdiff_t);
          T*      operator+(ptrdiff_t, T*);
          T&      operator[](ptrdiff_t, T*);

15For  every  T, where T is a pointer to object type, there exist candi-
  date operator functions of the form14)
          ptrdiff_t operator-(T, T);

16For every pointer type T, there exist candidate operator functions  of
  the form
          bool    operator<(T, T);
          bool    operator>(T, T);
          bool    operator<=(T, T);
          bool    operator>=(T, T);
          bool    operator==(T, T);
          bool    operator!=(T, T);

17For  every  pointer  to  member type T, there exist candidate operator
  functions of the form
          bool    operator==(T, T);
          bool    operator!=(T, T);

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*);

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

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);

25For every pair T, CVQ), where T is a scalar, array, function, or class
  type,  and  CVQ  is  const,  volatile, const volatile, or empty, there
  exist candidate operator functions of the form
          CVQ T&  operator?(bool, CVQ T&, CVQ T&);
  [Note: as with all these descriptions  of  candidate  functions,  this
  declaration serves only to describe the built-in operator for purposes
  of overload resolution.  The operator ?"  cannot be overloaded.  ]

26For every pair of promoted arithmetic types L and R, there exist  can-
  didate operator functions of the form
          LR      operator?(bool, L, R);
  where  LR  is  the  result of the usual arithmetic conversions between
  types L and R.

27For every type T, where T is  a  pointer  or  pointer-to-member  type,
  there exist candidate operator functions of the form
          T       operator?(bool, T, T);

28For  every  pair  T,  CVQ),  where T is a class type and CVQ is const,
  volatile, const volatile, or empty,  there  exist  candidate  operator
  functions of the form
          CVQ T   operator?(bool, CVQ T, CVQ T);