______________________________________________________________________

  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 ()
          {
              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  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, 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_);

  --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 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 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.
  _over.call.object_ describes how overload resolution is  used  in  the
  third 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.

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 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     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.
  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_.

  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 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  argument7).
  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()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).
  _________________________
  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.
  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.

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_.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]

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_.

  _________________________
  10) 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.11)

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:

  _________________________
  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.

  --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  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-qualifi­
    cation than, cv1, 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_).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
  _________________________
  12) According to _dcl.fct.default_, parameters following the  (m+1)-st
  parameter must also have default arguments.

  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_, _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-formed13).

3 [Example:

  _________________________
  13) The algorithm for selecting the best viable function is linear  in
  the  number  of  viable  functions.  Run a simple tournament to find a
  function W that is not worse than any opponent it faced.  Although an­
  other function F that W did not face might be 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_14).  If that conversion sequence is not better
  _________________________
  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 (); };

  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 they
      yield types identical except for cv-qualifiers and S2 adds all the
      cv-qualifiers  that  S1  adds (and in the same places) and S2 adds
      yet more cv-qualifiers than S1, [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 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 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
    sions; 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.15) [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++

  _________________________
  15) 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 form16)
          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);