______________________________________________________________________

  13   Overloading                                      [over]

  ______________________________________________________________________

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

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

  13.1  Overloadable declarations                            [over.load]

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

2 Certain function declarations cannot be overloaded:

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

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

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

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

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

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

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

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

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

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

    [Example:
              typedef const int cInt;

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

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

  --Two parameter declarations that differ only in their  default  argu­
    ments are equivalent.  [Example: consider the following:
              void f (int i, int j);
              void f (int i, int j = 99);         // Ok: redeclaration of f (int, int)
              void f (int i = 88);                // 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  paramater  type  that  is  a pointer to function, the const and
  volatile type-specifiers at the outermost level of the parameter  type
  specifications for the inner function type are also ignored.

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

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

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

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

  13.3  Overload resolution                                 [over.match]

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

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

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

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

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

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

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

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

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

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

4 If a best viable function exists and is  unique,  overload  resolution
  succeeds and produces it as the result.  Otherwise overload resolution
  fails and the invocation is ill-formed.

  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 that defines the
  member  function and cv is the cv-qualification on the member function
  declaration.  [Example: for a const member function of  class  X,  the
  extra parameter is assumed to have type "reference to const X".  ] For
  static member functions, the implicit object parameter  is  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 In each case where a candidate is a function template, candidate  tem­
  plate  functions  are  generated  using  template  argument  deduction

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

  13.3.1.1  Function call syntax                       [over.match.call]

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

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

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

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

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

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

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

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.  If the name resolves  to  a  non-member
  function  declaration,  that  function and its overloaded declarations
  constitute the set of candidate functions6).  The argument list is the
  same  as  the  expression-list in the call.  If the name resolves to a
  nonstatic member function, then the function call is actually a member
  function  call.   If  the  keyword  this 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 qualified function call
  using (*this) as the postfix-expression to the left of the  .   opera­
  tor.   The  candidate functions and argument list are as described for
  qualified function calls above.  If the keyword this is not  in  scope
  or refers to another class, then name resolution found a static member
  of some class T.  In this case, all  overloaded  declarations  of  the
  function  name  in T become candidate functions and a contrived object
  of  type  T  becomes  the implied object argument7).  The call is ill-
  formed, however, if overload resolution selects one of the  non-static
  member functions of T in this case.

  _________________________
  4)  Note  that cv-qualifiers on the type of objects are significant in
  overload resolution for both lvalue and rvalue objects.
  5) Because of the usual name hiding rules, these will all be  declared
  in  T  or  they  will all be declared in the same base class of T; see
  _class.member.lookup_.
  6) Because of the usual name hiding rules, these will be introduced by
  declarations  or by using directives all found found in the same block
  or all found at namespace scope.
  7) An implied object argument must be contrived to correspond  to  the
  implicit  object parameter attributed to member functions during over­
  load resolution.  It is not used in the call to the selected function.
  Since  the  member functions all have the same implicit object parame­
  ter, the contrived object will not be the cause to select or reject  a
  function.

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

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

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

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

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

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

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

  13.3.1.2  Operators in expressions                   [over.match.oper]

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

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

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

    Table 1--relationship between operator and function call notation

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

3 For     a     type     T     whose     fully-qualified     name     is
  ::N1::...::Nn::C1::...::Cm::T where each Ni is a  namespace  name  and
  each   Ci   is  a  class  name,  the  fully-qualified  namespace  name
  ::N1::...::Nn is called the "namespace of the type T." To look up X in
  the  "context  of  the  namespace  of the type T" means to perform the
  qualified name lookup of ::N1::...::Nn::X (_over.call.func_).

4 For a unary operator @ with an operand of type T1 or reference  to  cv
  T1, and for a binary operator @ with a left operand of type T1 or ref­
  erence to cv T1 and a right operand of type T2 or reference to cv  T2,
  three  sets of candidate functions, designated member candidates, non-
  member candidates and built-in candidates, are constructed as follows:

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

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

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

    --For each type Z, where Z is either a Ti of class type or a  direct
      or  indirect base class of a Ti of class type, operator@ is looked
      up in the context of type Z according to the usual rules for  name
      lookup.

    --For  each  Ti  of  enumeration type, operator@ is looked up in the
      context of the namespace of that type according to the usual rules
      for name lookup.

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

    --have the same operator name, and

    --accept the same number of operands, and

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

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

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

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

6 For all other operators, no such restrictions apply.

7 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;        // a.operator+(b) chosen over int(a) + int(b)
          }
   --end example]

8 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_.

9 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 built-in operator -> is  applied
  to the value returned, with the original second operand.

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

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

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

  A a;

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

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

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

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

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

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

3 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_).

  13.3.1.4  Initialization by constructor              [over.match.ctor]

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

2 [Note:  when  no  constructor  for  class T accepts the given type, no
  attempt is made to find other constructors to convert the  assignment-
  expression into a type that can be converted to T.  [Example:
          class T {
          public:
                  T();
                  // ...
          };
          class C : T {
          public:
                  C(int);
                  // ...
          };
          T a = 1;                 // ill-formed: T(C(1)) not tried
   --end example]  --end note]

  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_).11) For the purposes of overload resolution, the
    parameter list is truncated on the right, so that there are  exactly
  _________________________
  11) According to subclause _dcl.fct.default_, parameters following the
  (m+1)-st parameter must also have default arguments.

    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.  Subclause _over.best.ics_ defines the implicit conversion
  sequences and subclause _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.over.order_, or, if not that,

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

2 If there is exactly one viable function that is a better function than
  all other viable functions, then it is the one  selected  by  overload

  resolution; otherwise the call is ill-formed12).

3 [Example:
          void Fcn(const int*,  short);
          void Fcn(int*, int);

          int i;
          short s = 0;

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

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

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

  13.3.3.1  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 governed by the rules for initialization of an object or ref­
  erence by a single expression (_dcl.init_, _dcl.init.ref_).

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

3 Except  in the context of an initialization by user-defined conversion
  (_over.match.user_), a well-formed implicit conversion sequence is one
  _________________________
  12)  The algorithm for selecting the best viable function is linear in
  the number of viable functions.  Run a simple  tournament  to  find  a
  function W that is not worse than any opponent it faced.  Although an­
  other function F that W did not face might be better than W, F  cannot
  be the best function because at some point in the tournament F encoun­
  tered another function G such that F was not better than G.  Hence,  W
  is  either the best function or there is no best function.  So, make a
  second pass over the viable functions to verify that W is better  than
  all other functions.

  of the following forms:

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

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

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

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

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

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

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

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

9 If  several different sequences of conversions exist that each convert
  the argument to the parameter type, the implicit  conversion  sequence
  is  a sequence among these that is not worse than all the rest accord­
  ing  to _over.ics.rank_13).  If that conversion sequence in not better
  _________________________
  13) This rule prevents a function from becoming non-viable because  of
  an  ambiguous conversion sequence for one of its parameters.  Consider
  this example,
          class B;
          class A { A (B&); };
          class B { operator A (); };
          class C { C (B&); };
          f(A) { }
          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

  than all the rest and a function that uses such an implicit conversion
  sequence  is  selected as the best viable function, then the call will
  be ill-formed because the conversion of one of the  arguments  in  the
  call is ambiguous.

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

  13.3.3.1.1  Standard conversion sequences               [over.ics.scs]

1 Table 2 summarizes the conversions defined in clause _conv_ and parti­
  tions them into four disjoint categories: Lvalue Transformation, Qual­
  ification Adjustment, Promotion, and Conversion.  Note that these cat­
  egories  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 A standard conversion sequence is either the  Identity  conversion  by
  itself or consists of one to four conversions from the other four cat­
  egories.  At most one conversion from each category is  allowed  in  a
  single standard conversion sequence.  If there are two or more conver­
  sions in the sequence, the conversions are applied  in  the  canonical
  order:  Lvalue  Transformation,  Promotion,  Conversion, Qualification
  Adjustment.

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.

  _________________________
  solved 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_   |
  +-------------------------------+                          |             +-----------------+
  |Pointer conversions            |        Conversion        | Conversion  |   _conv.ptr_    |
  +-------------------------------+                          |             +-----------------+
  |Pointer to member conversions  |                          |             |   _conv.mem_    |
  +-------------------------------+                          |             +-----------------+
  |Base class conversion          |                          |             |  _conv.class_   |
  +-------------------------------+                          |             +-----------------+
  |Boolean conversions            |                          |             |   _conv.bool_   |
  +-------------------------------+--------------------------+-------------+-----------------+

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

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

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

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

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

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

  13.3.3.1.3  Ellipsis conversion sequences          [over.ics.ellipsis]

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

  13.3.3.1.4  Reference binding                           [over.ics.ref]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    --S1  and  S2 differ only in their qualification conversion and they
      yield types identical except for cv-qualifiers and S2 adds all the
      qualifiers  that  S1 adds (and in the same places) and S2 adds yet
      more cv-qualifiers than S1, or the  similar  case  with  reference
      binding14).  [Example:
  _________________________
  14)  See  the definition of reference-compatible with added qualifica­
  tion in _dcl.init.ref_.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  13.4  Address of overloaded function                       [over.over]

1 A  use of an overloaded function name without arguments is resolved in
  certain contexts to a pointer to function or pointer to  member  func­
  tion  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 being initialized (_dcl.init_),

  --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.cast_).

  An  overloaded  function  name  shall not be used without arguments in
  contexts other than those listed.  The  reference  to  the  overloaded
  function name can be preceded by &.

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;" nonstatic member  functions  match  targets  of
  type  "pointer-to-member-function."  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_.

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 [Example:
          int f(double);
          int f(int);
          (int (*)(int))&f;           // cast expression as selector
          int (*pfd)(double) = &f;    // selects f(double)
          int (*pfi)(int) = &f;       // selects f (int)
          int (*pfe)(...) = &f;       // error: type mismatch
  The last  initialization  is  ill-formed  because  no  f()  with  type
  int(...)   has been defined, and not because of any ambiguity.   --end
  example]

6 Also note that 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

7 Note that if the target type is a  pointer  to  member  function,  the
  function  type  of  the pointer to member is used to select the member
  function from a set of overloaded member functions.  [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]  --end note]

  13.5  Overloaded operators                                 [over.oper]

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

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

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

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

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

6 An  operator  function shall either be a non-static member function or
  be a non-member function and have at least one parameter whose type is
  a  class, a reference to a class, an enumeration, or a reference to an
  enumeration.  It is not possible to change the  precedence,  grouping,
  or  number  of operands of operators.  The meaning of the operators =,
  (unary) &, and , (comma), predefined for each type, can be changed for
  specific  types  by  defining  operator functions that implement these
  operators.  Operator functions are inherited the same as  other  func­
  tions,  but  because  an  instance  of operator= is automatically con­
  structed for each class (_class.copy_, _over.ass_), operator= is never
  inherited by a class from its bases.

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

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

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

  13.5.1  Unary operators                                   [over.unary]

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

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

  13.5.2  Binary operators                                 [over.binary]

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

  13.5.3  Assignment                                          [over.ass]

1 An  assignment  operator  shall  be implemented by a non-static member
  function with exactly one parameter.  Because a copy assignment opera­
  tor  operator=  is  implicitly declared for a class if not declared by
  the user (_class.copy_), a base class assignment  operator  is  always
  hidden by the copy assignment operator of the derived class.

2 Any  assignment  operator,  even  the copy assignment operator, can be
  virtual.  [Note: for a derived class D with a base class B for which a
  virtual  copy assignment has been declared, the copy assignment opera­
  tor in D does not  override  B's  virtual  copy  assignment  operator.
  [Example:

          struct B {
                  virtual int operator= (int);
                  virtual B& operator= (const B&);
          };
          struct D : B {
                  virtual int operator= (int);
                  virtual D& operator= (const B&);
          };
          D dobj1;
          D dobj2;
          B* bptr = &dobj1;
          void f() {
                  bptr->operator=(99);    // calls D::operator(int)
                  *bptr = 99;             // ditto
                  bptr->operator=(dobj2); // calls D::operator(const B&)
                  *bptr = dobj2;          // ditto
                  dobj1 = dobj2;          // calls D::operator(const D&)
          }
   --end example]  --end note]

  13.5.4  Function call                                      [over.call]

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

  13.5.5  Subscripting                                        [over.sub]

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

  13.5.6  Class member access                                 [over.ref]

1 operator-> shall be a non-static member function taking no parameters.
  It implements class member access using ->
          postfix-expression -> primary-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_).   operator->  shall return either a pointer to a
  class or an object of or a reference to a class for  which  operator->

  is  defined,  except  in  some cases when it is a member of a template
  (see _temp.opref_).  T::operator-> shall not return an  object  of  or
  reference to its own class type T.

  13.5.7  Increment and decrement                             [over.inc]

1 The  prefix  and postfix increment operators shall be implemented by a
  function called operator++.  If this function  is  a  member  function
  with no parameters, or a non-member function with one class or enumer­
  ation parameter, 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 shall be of type int), it defines the post­
  fix increment operator ++ for objects of that type.  When the  postfix
  increment is called, the int argument will have value zero.  [Example:
          class X {
          public:
              const X&   operator++();     // prefix ++a
              const X&   operator++(int);  // postfix a++
          };
          class Y {
          public:
          };
          const Y&   operator++(Y&);       // prefix ++b
          const Y&   operator++(Y&, int);  // postfix b++
          void f(X a, Y b)
          {
              ++a;        // a.operator++();
              a++;        // a.operator++(0);
              ++b;        // operator++(b);
              b++;        // operator++(b, 0);
              a.operator++();    // explicit call: like ++a;
              a.operator++(0);   // explicit call: like a++;
              operator++(b);     // explicit call: like ++b;
              operator++(b, 0);  // explicit call: like b++;
          }
   --end example]

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

  13.6  Built-in operators                                  [over.built]

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

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

  type that can be converted to a type appropriate for the operator.  ]

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

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

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

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

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

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

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

10For  every  promoted  integral  type T, there exist candidate operator
  functions of the form
          T       operator~(T);

11For every quadruple C, T, CV1, CV2), where C is a class type, T  is  a
  complete  object  type  or  a  function  type, and CV1 and CV2 are cv-
  qualifier-seqs, there exist candidate operator functions of the form
          CV12 T& operator->*(CV1 C*, CV2 T C::*);
  where CV12 is the union of CV1 and CV2.

12For 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.

13For every pair of types T and I, where T  is  a  cv-qualified  or  cv-
  unqualified  complete  object  type and I is a promoted integral type,
  there exist candidate operator functions of the form
          T*      operator+(T*, I);
          T&      operator[](T*, I);
          T*      operator-(T*, I);
          T*      operator+(I, T*);
          T&      operator[](I, T*);

14For every triple T, CV1, CV2), where T is a complete object type,  and
  CV1  and  CV2  are  cv-qualifier-seqs,  there exist candidate operator
  functions of the form15)
          ptrdiff_t operator-(CV1 T*, CV2 T*);

15For  every  triple  T, CV1, CV2), where T is any type, and CV1 and CV2
  are cv-qualifier-seqs, there exist candidate operator functions of the
  form16)
          bool    operator<(CV1 T*, CV2 T*);
          bool    operator>(CV1 T*, CV2 T*);
          bool    operator<=(CV1 T*, CV2 T*);
          bool    operator>=(CV1 T*, CV2 T*);
          bool    operator==(CV1 T*, CV2 T*);
          bool    operator!=(CV1 T*, CV2 T*);

16For every quadruple C, T, CV1, CV2), where C is a class type, T is any
  type,  and  CV1  and  CV2 are cv-qualifier-seqs, there exist candidate
  operator functions of the form17)
  _________________________
  15) When T is itself a pointer type, the interior cv-qualifiers of the
  two  parameter types need not be identical.  The two pointer types are
  converted to a common type (which need not be the same as  either  pa­
  rameter type) by implicit pointer conversions.
  16) When T is itself a pointer type, the interior cv-qualifiers of the
  two parameter types need not be identical.  The two pointer types  are
  converted  to  a common type (which need not be the same as either pa­
  rameter type) by implicit pointer conversions.
  17) When T is itself a pointer type, the interior cv-qualifiers of the
  two  parameter types need not be identical.  The two pointer types are
  converted to a common type (which need not be the same as  either  pa­
  rameter type) by implicit pointer conversions.

          bool    operator==(CV1 T C::*, CV2 T C::*);
          bool    operator!=(CV1 T C::*, CV2 T C::*);

17For  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.

18For  every  triple  L,  VQ,  R),  where L is an arithmetic type, VQ is
  either volatile or empty, and R is a promoted arithmetic  type,  there
  exist candidate operator functions of the form
          VQ L&   operator=(VQ L&, R);
          VQ L&   operator*=(VQ L&, R);
          VQ L&   operator/=(VQ L&, R);
          VQ L&   operator+=(VQ L&, R);
          VQ L&   operator-=(VQ L&, R);

19For  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*);

20For every triple  T,  VQ,  I),  where  T  is  a  cv-qualified  or  cv-
  unqualified  complete object type, VQ is either volatile or empty, and
  I is a promoted integral type, there exist  candidate  operator  func­
  tions of the form
          T*VQ&   operator+=(T*VQ&, I);
          T*VQ&   operator-=(T*VQ&, I);

21For  every triple L, VQ, R), where L is an integral type, VQ is either
  volatile or empty, and R is a promoted integral type, there exist can­
  didate operator functions of the form
          VQ L&   operator%=(VQ L&, R);
          VQ L&   operator<<=(VQ L&, R);
          VQ L&   operator>>=(VQ L&, R);
          VQ L&   operator&=(VQ L&, R);
          VQ L&   operator^=(VQ L&, R);
          VQ L&   operator|=(VQ L&, R);

22There also exist candidate operator functions of the form
          bool    operator!(bool);
          bool    operator&&(bool, bool);
          bool    operator||(bool, bool);