______________________________________________________________________

  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:
      int f(char*);
      int f(char[]);                  // same as f(char*);
      int f(char[7]);                 // same as f(char*);
      int f(char[9]);                 // same as f(char*);
      int g(char(*)[10]);
      int g(char[5][10]);             // same as g(char(*)[10]);
      int g(char[7][10]);             // same as g(char(*)[10]);
      int g(char(*)[20]);             // different from g(char(*)[10]);

     --end example]

  --Parameter  declarations  that  differ only in that one is a function
    type and the other is a pointer to the same function type are equiv-
    alent.   That  is, the function type is adjusted to become a pointer
    to function type (_dcl.fct_).  [Example:
      void h(int());
      void h(int (*)());              // redeclaration of h(int())
      void h(int x()) { }             // definition of h(int())
      void h(int (*x)()) { }          // ill-formed: redefinition of h(int())
    ]

  --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]
  _________________________
  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.

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

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

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

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

  public:
      buffer(int s) { p = new char[size = s]; }
      // ...
  };
   --end example]

  13.3  Overload resolution                                 [over.match]

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

2 Overload  resolution  selects  the  function to call in seven distinct
  contexts within the language:

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

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

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

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

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

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

  --invocation  of  a conversion function for conversion to an lvalue 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  (clause
  _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 subclauses of _over.match.funcs_ describe  the  set  of  candidate
  functions  and  the  argument list submitted to overload resolution in
  each of the seven contexts in which overload resolution is used.   The
  source  transformations  and constructions defined in these subclauses
  are only for the purpose of describing the  overload  resolution  pro-
  cess.   An  implementation 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).  [Note: no actual type is  established  for  the  implicit
  object  parameter  of a static member function, and no attempt will be
  made  to  determine  a  conversion   sequence   for   that   parameter
  (_over.match.best_).  ]

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.

  _________________________
  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.

  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.

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 _over.call.func_ 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  clause  _over_
  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,  clause _over_ 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
  _________________________
  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.

  function  is found, that function and its overloaded declarations con-
  stitute the set of candidate functions.   The  argument  list  is  the
  expression-list  in  the  call  augmented  by the addition of the left
  operand of the .  operator in the normalized member function  call  as
  the implied object argument (_over.match.funcs_).

3 In unqualified function calls, the name is not qualified by an -> or .
  operator and has the more general form of a  primary-expression.   The
  name  is  looked  up in the context of the function call following the
  normal    rules    for    name    lookup     in     function     calls
  (_basic.lookup.koenig_).   If  the name resolves to a non-member func-
  tion declaration, that function and its overloaded  declarations  con-
  stitute  the  set  of candidate functions5).  The argument list is the
  same as the expression-list in the call.  If the name  resolves  to  a
  nonstatic member function, then the function call is actually a member
  function call.  If the keyword this (_class.this_)  is  in  scope  and
  refers  to  the  class  of  that  member  function, or a derived class
  thereof, then the function call is transformed into a normalized qual-
  ified  function  call  using  (*this) as the postfix-expression to the
  left of the .  operator.  The candidate functions  and  argument  list
  are  as  described for qualified function calls above.  If the keyword
  this is not in scope or refers to another class, then name  resolution
  found  a  static member of some class T.  In this case, all overloaded
  declarations of the function name in T become candidate functions  and
  a contrived object of type T becomes the  implied  object  argument6).
  The call is ill-formed, however, if overload resolution selects one of
  the non-static member functions of T in this case.

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

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

2 In addition, for each conversion function declared in T of the form
  operator conversion-type-id () cv-qualifier;
  where cv-qualifier is the same cv-qualification as, or a  greater  cv-
  qualification  than, cv, and where conversion-type-id denotes the type
  "pointer to function of (P1,...,Pn) returning R", or the type  "refer-
  ence  to  pointer to function of (P1,...,Pn) returning R", or the type
  "reference to function of (P1,...,Pn) returning R", a  surrogate  call
  _________________________
  5) Because of the usual name hiding rules, these will be introduced by
  declarations or by using-directives all found in the same block or all
  found at namespace scope.
  6)  An  implied object argument must be contrived to correspond to the
  implicit object parameter attributed to member functions during  over-
  load resolution.  It is not used in the call to the selected function.
  Since the member functions all have the same implicit  object  parame-
  ter,  the contrived object will not be the cause to select or reject a
  function.

  function with the unique name call-function and having the form
  R call-function (conversion-type-id F, P1 a1,...,Pn an) { return F (a1,...,an); }
  is also considered as a candidate function.  Similarly, surrogate call
  functions are added to the set of candidate functions for each conver-
  sion  function declared in an accessible base class provided the func-
  tion is not hidden within T by another intervening declaration7).

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:
  _________________________
  7) Note that this construction can yield candidate call functions that
  cannot be differentiated one from the other by overload resolution be-
  cause they have identical declarations or differ only in their  return
  type.  The call will be ambiguous if overload resolution cannot select
  a match to the call that is uniquely better than such undifferentiable
  functions.

  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 a type whose  cv-unqualified
  version  is  T1,  and for a binary operator @ with a left operand of a
  type whose cv-unqualified version is T1 and a right operand of a  type
  whose cv-unqualified version is T2, three sets of candidate functions,
  designated member candidates, non-member candidates and built-in  can-
  didates, 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.   However,  if no operand has a class type, only those non-
    member functions in the lookup set that have a  first  parameter  of
    type  T1 or "reference to (possibly cv-qualified) T1", when T1 is an
    enumeration type, or (if there is a right operand) a second  parame-
    ter of type T2 or "reference to (possibly cv-qualified) T2", when T2
    is an enumeration type, are candidate functions.

  --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_, and

    --do not have the same parameter type list as any non-template  non-
      member candidate.

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

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

  --no user-defined conversions are  applied  to  the  left  operand  to
    achieve a type match with the left-most parameter of a built-in can-
    didate.

5 For all other operators, no such restrictions apply.

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

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

  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,  the
  operands are converted to the types of the corresponding parameters of
  the selected operation function.  Then the operator is treated as  the
  corresponding  built-in  operator  and interpreted according to clause
  _expr_.

8 The second operand of operator -> is ignored in  selecting  an  opera-
  tor-> function, and is not an argument when the operator-> function is
  called.  When operator-> returns, the operator -> is  applied  to  the
  value returned, with the original second operand.9)

9 If the operator is the operator ,, the unary operator &, or the opera-
  tor  ->,  and  there  are  no  viable  functions, then the operator is
  assumed to be the  built-in  operator  and  interpreted  according  to
  clause _expr_.

10[Note:  the  lookup  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]

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

  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:

  --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 a type whose cv-unqual-
    ified version is the same type as T or is a  derived  class  thereof
    are  candidate  functions.  Conversion functions that return "refer-
    ence to T" return lvalues of type T and are therefore considered  to
    yield T for this process of selecting candidate functions.

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

  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 T or a type that
    can be converted to  type  T  via  a  standard  conversion  sequence
    (_over.ics.scs_) are candidate functions.  Conversion functions that
    return a cv-qualified type are considered to yield  the  cv-unquali-
    fied  version  of  that type for this process of selecting candidate
    functions.  Conversion functions that return "reference to T" return
    lvalues  of  type T and are therefore considered to yield T for this

    process of selecting candidate functions.

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

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

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

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

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

  13.3.2  Viable functions                           [over.match.viable]

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

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

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

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

  --A candidate function having more than m parameters is viable only if
    the    (m+1)-st     parameter     has     a     default     argument
    (_dcl.fct.default_).10) For the purposes of overload resolution, the
  _________________________
  10)  According to _dcl.fct.default_, parameters following the (m+1)-st
  parameter must also have default arguments.

    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 Define ICSi(F) as follows:

  --if  F  is  a  static  member  function, ICS1(F) is defined such that
    ICS1(F) is neither better nor worse than ICS1(G) for any function G,
    and,  symmetrically,  ICS1(G)  is  neither  better  nor  worse  than
    ICS1(F)11); otherwise,

  --let  ICSi(F)  denote  the implicit conversion sequence that converts
    the i-th argument in the list to the type of the i-th  parameter  of
    viable  function F.  _over.best.ics_ defines the implicit conversion
    sequences and _over.ics.rank_ defines what it means for one implicit
    conversion sequence to be a better conversion sequence or worse con-
    version sequence than another.

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

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

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

  --F1 and F2 are template functions, and the function template  for  F1
    is  more  specialized than the template for F2 according to the par-
    tial 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 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:

  _________________________
  11) If a function is a static member function, this  definition  means
  that  the  first argument, the implied object parameter, has no effect
  in the determination of whether the function is better or  worse  than
  any other function.

      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;
  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 clause _conv_, which
  _________________________
  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 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.

  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.copy_, _over.match.conv_), a well-formed implicit conver-
  sion 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.copy_,  _over.match.conv_),  only standard conversion
  sequences and ellipsis 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  clause  _over_;  the
  actual initialization is defined in terms of constructors and is not a
  conversion.  ] Any difference in top-level  cv-qualification  is  sub-
  sumed  by  the initialization itself and does not constitute a conver-
  sion.  [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 has the same type, the implicit conversion sequence  is  an
  identity  conversion.   When  the  parameter  has a class type and the
  argument expression has 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.  ] 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
  associated with the parameter is defined to be the  unique  conversion
  sequence  designated  the ambiguous conversion sequence.  For the pur-
  pose  of  ranking  implicit  conversion  sequences  as  described   in
  _over.ics.rank_,  the  ambiguous  conversion  sequence is treated as a
  user-defined sequence that is indistinguishable from any  other  user-
  defined conversion sequence13).  If a function that uses the ambiguous
  conversion sequence is selected as the best viable function, the  call
  will  be  ill-formed because the conversion of one of the arguments in
  the call is ambiguous.

  _________________________
  13) The ambiguous conversion sequence is ranked with user-defined con-
  version sequences because multiple conversion sequences for  an  argu-
  ment  can  exist  only  if they involve different user-defined conver-
  sions.  The ambiguous conversion sequence  is  indistinguishable  from
  any  other  user-defined  conversion sequence because it represents at
  least two user-defined conversion sequences, each with a different us-
  er-defined  conversion, and any other user-defined conversion sequence
  must be indistinguishable from at least one of them.

  This rule prevents a function from becoming non-viable because  of  an
  ambiguous  conversion  sequence  for  one of its parameters.  Consider
  this example,
  class B;
  class A { A (B&); };
  class B { operator A (); };
  class C { C (B&); };
  void f(A) { }
  void f(C) { }
  B b;
  f(b);                           // ambiguous because 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).

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 clause _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  cate-
  gories.   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 or  Conversion,  Qualification
  Adjustment.   --end note]

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

                           Table 2--conversions

  +-------------------------------+--------------------------+-------------+-----------------+
  |Conversion                     |         Category         |    Rank     |    Subclause    |
  +-------------------------------+--------------------------+-------------+-----------------+
  +-------------------------------+--------------------------+-------------+-----------------+
  |No conversions required        |         Identity         |             |                 |
  +-------------------------------+--------------------------+             +-----------------+
  |Lvalue-to-rvalue conversion    |                          |             |   _conv.lval_   |
  +-------------------------------+                          |             +-----------------+
  |Array-to-pointer conversion    |  Lvalue Transformation   | Exact Match |  _conv.array_   |
  +-------------------------------+                          |             +-----------------+
  |Function-to-pointer conversion |                          |             |   _conv.func_   |
  +-------------------------------+--------------------------+             +-----------------+
  |Qualification conversions      | Qualification Adjustment |             |   _conv.qual_   |
  +-------------------------------+--------------------------+-------------+-----------------+
  |Integral promotions            |                          |             |   _conv.prom_   |
  +-------------------------------+        Promotion         |  Promotion  +-----------------+
  |Floating point promotion       |                          |             |  _conv.fpprom_  |
  +-------------------------------+--------------------------+-------------+-----------------+
  |Integral conversions           |                          |             | _conv.integral_ |
  +-------------------------------+                          |             +-----------------+
  |Floating point conversions     |                          |             |  _conv.double_  |
  +-------------------------------+                          |             +-----------------+
  |Floating-integral conversions  |                          |             |  _conv.fpint_   |
  +-------------------------------+        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 _over.ics.rank_ 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_, exclud-
      ing any Lvalue Transformation; the identity conversion sequence is
      considered  to  be  a  subsequence  of any non-identity conversion
      sequence) or, if not that,

    --the rank of S1 is better than the rank of S2 (by the rules defined

      below), or, if not that,

    --S1 and S2 differ only in their qualification conversion and  yield
      similar  types  T1 and T2 (_conv.qual_), respectively, and the cv-
      qualification signature of type T1 is a proper subset of  the  cv-
      qualification signature of type T2, [Example:
          int f(const int *);
          int f(int *);
          int i;
          int j = f(&i);                  // Calls f(int *)
       --end example] or, if not that,

    --S1  and  S2 are reference bindings (_dcl.init.ref_), and the types
      to which the references refer are the same type  except  for  top-
      level  cv-qualifiers, and the type to which the reference initial-
      ized by S2 refers is more cv-qualified than the type to which  the
      reference initialized by S1 refers.  [Example:
          int f(const int &);
          int f(int &);
          int g(const int &);
          int g(int);

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

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

4 Standard  conversion  sequences  are  ordered by their ranks: an Exact
  Match is a better conversion than a Promotion, which is a better  con-
  version  than  a  Conversion.   Two conversion sequences with the same

  rank are indistinguishable unless one of the following rules applies:

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

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

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

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

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

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

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

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

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

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

    --conversion of B to A is better than conversion of C to A.
    [Note:  compared  conversion  sequences  will  have different source
    types only in the context of comparing the second  standard  conver-
    sion  sequence  of an initialization by user-defined conversion (see
    _over.match.best_); in all other contexts, the source types will  be
    the same and the target types will be different.  ]

  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.  A

  function template name is considered to name a set of overloaded func-
  tions in such contexts.  The function selected is the one  whose  type
  matches the target type required in the context.  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.funcaddr_), 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_.

4 If  more  than one function is selected, any template functions in the
  set are eliminated if the set also contains a  non-template  function,
  and  any  given  template function is eliminated if the set contains a
  second template function that  is  more  specialized  than  the  first
  according  to  the partial ordering rules of _temp.func.order_.  After
  such eliminations, if any, there shall  remain  exactly  one  selected
  function.

5 [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]

6 [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).  ]

7 [Note: there are  no  standard  conversions  (clause  _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

  subscripting (_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 ## (clause _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.

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.14) [Example:
  class X {
  public:
      X&   operator++();          // prefix ++a
      X    operator++(int);       // postfix a++
  };
  class Y { };
  Y&   operator++(Y&);            // prefix ++b
  Y    operator++(Y&, int);       // postfix b++
  void f(X a, Y b)
  {
      ++a;                        // a.operator++();
      a++;                        // a.operator++(0);
      ++b;                        // operator++(b);
      b++;                        // operator++(b, 0);
      a.operator++();             // explicit call: like ++a;
      a.operator++(0);            // explicit call: like a++;
      operator++(b);              // explicit call: like ++b;
      operator++(b, 0);           // explicit call: like b++;
  }
   --end example]

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

  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  can-
  didate  functions participate in the operator overload resolution pro-
  cess as described in _over.match.oper_ and are used for no other  pur-
  pose.

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.  If
  there is a user-written candidate with the  same  name  and  parameter
  _________________________
  14) Calling operator++ explicitly, as  in  expressions  like  a.opera-
  tor++(2),  has no special properties: The argument to operator++ is 2.

  types as a built-in candidate operator function, the built-in operator
  function  is  hidden and is not included in the set of candidate func-
  tions.  ]

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 form
  ptrdiff_t       operator-(T, T);

16For  every pointer or enumeration type T, there exist candidate opera-
  tor 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  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 an enumeration or pointer to member
  type and VQ is either volatile or empty, there exist candidate  opera-
  tor 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 type, VQ is either
  volatile or empty, and R is a promoted integral type, there exist can-
  didate operator functions of the form
  VQ L&   operator%=(VQ L&, R);
  VQ L&   operator<<=(VQ L&, R);
  VQ L&   operator>>=(VQ L&, R);
  VQ L&   operator&=(VQ L&, R);
  VQ L&   operator^=(VQ L&, R);
  VQ L&   operator|=(VQ L&, R);

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

25For  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.   [Note: as with all these descriptions of candidate
  functions, this declaration serves only to describe the built-in oper-
  ator  for purposes of overload resolution.  The operator ?"  cannot be
  overloaded.  ]

26For 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);