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 extension, 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 resolution and is defined in over.match. [Example:

double abs(double);
int abs(int);

abs(1);                         //  call  abs(int);
abs(1.0);                       //  call  abs(double);

-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 fabricated as a result of name lookup (e.g., because of using-directives) or overload resolution (e.g., for operator functions). ]

-2- Certain function declarations cannot be overloaded:

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

• Member function declarations with the same name and the same parameter 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 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 therefore 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 equivalent. 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 overloaded function declarations.*

  [Footnote: 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. --- end foonote]

  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 arguments 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- Two function declarations of the same name refer to the same function if they are in the same scope and have equivalent parameter declarations (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*);
};

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
}

-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
    }
}

-3- Different versions of an overloaded member function can be given different 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]; }
    //  ...
};

-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 certain 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 conversion 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 conversion 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 succeeds, 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 process. 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 overload 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 argument.

-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 defining 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 indistinguishable from other arguments. The implicit object parameter, however, 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

-7- In each case where a candidate is a function template, candidate template functions are generated using template argument deduction (temp.over, temp.deduct). Those candidates are then handled as candidate functions in the usual way.*

[Footnote: 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. --- end foonote]

-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)

-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 functions*.

[Footnote: If F names a non-static member function, &F is a pointer-to-member, which cannot be used with the function call syntax. --- end foonote]

-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, perhaps nested arbitrarily deep in parentheses, has one of the following forms:

postfix-expression:
	postfix-expression . id-expression
	postfix-expression -> id-expression
	primary-expression

-2- In qualified function calls, the name to be resolved is an id-expression 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 . operator. Furthermore, clause over assumes that the postfix-expression that is the left operand of the . operator has type ``cv T'' where T denotes a class*.

[Footnote: Note that cv-qualifiers on the type of objects are significant in overload resolution for both lvalue and class rvalue objects. --- end foonote]

-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 function declaration, that function and its overloaded declarations constitute the set of candidate functions*.

[Footnote: 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. --- end foonote]

[Footnote: An implied object argument must be contrived to correspond to the implicit object parameter attributed to member functions during overload resolution. It is not used in the call to the selected function. Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function. --- end foonote]

-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;

R call-function (conversion-type-id F, P1 a1,...,Pn an) { return F (a1,...,an); }

[Footnote: Note that this construction can yield candidate call functions that cannot be differentiated one from the other by overload resolution because 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. --- end foonote]

-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 arguments 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 argument 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

-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 subclause are used to determine the conversions to be applied to the second and third operands when they have class or enumeration type (expr.cond). ] [Example:

class String {
public:
   String (const String&);
   String (char*);
	operator char* ();
};
String operator + (const String&, const String&);

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

-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 operator. Therefore, the operator notation is first transformed to the equivalent function-call notation as summarized in Table ?? (where @ denotes one of the operators covered in the specified subclause).

-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 candidates, are constructed as follows:

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

• The set of non-member candidates is the result of the unqualified lookup of operator@ in the context of the expression according to the usual rules for name lookup in unqualified function calls (basic.lookup.koenig) except that all member functions are ignored. 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 parameter 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 candidate.

-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.*

[Footnote: If the set of candidate functions is empty, overload resolution is unsuccessful. --- end foonote]

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)
}

-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 operator-> 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.*

[Footnote: 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. --- end foonote]

-9- If the operator is the operator ,, the unary operator &, or the operator ->, 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+
}

-1- When objects of class type are direct-initialized (dcl.init), overload 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-defined conversion [over.match.copy]

-1- Under the conditions specified in dcl.init, as part of a copy-initialization 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 candidate 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-unqualified version is the same type as T or is a derived class thereof are 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- In both cases, the argument list has one argument, which is the initializer 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 initialization 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 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 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-unqualified 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 expression. [Note: this argument will be compared against the implicit object parameter of the conversion functions. ]

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

-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 conversion 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 expression. [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 function 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).*

  [Footnote: According to dcl.fct.default, parameters following the (m+1)-st parameter must also have default arguments. --- end foonote]

  For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters.

-3- Second, for F to be a viable function, there shall exist for each argument an implicit conversion sequence (over.best.ics) that converts 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)*;

  [Footnote: 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. --- end foonote]

  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 conversion sequence than another.

• 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 specialization, 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 partial ordering rules described in temp.func.order, or, if not that,

• the context is an initialization by user-defined conversion (see dcl.init, over.match.conv) and 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-formed*.

[Footnote: 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 another 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. --- end foonote]

-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
}

-1- An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called. The sequence of conversions is an implicit conversion as defined in clause conv, which means it is governed by the rules for initialization of an object or reference by a single expression (dcl.init, dcl.init.ref).

-2- Implicit conversion sequences are concerned only with the type, cv-qualification, and lvalue-ness of the argument and how these are converted to match the corresponding properties of the parameter. Other properties, such as the lifetime, storage class, alignment, or accessibility 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 conversion sequence is one of the following forms:

• a standard conversion sequence (over.ics.scs),

• a user-defined conversion sequence (over.ics.user), or

• an ellipsis conversion sequence (over.ics.ellipsis).

-4- In the context of an initialization by user-defined conversion (i.e., when considering the argument of a user-defined conversion function; see over.match.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 argument 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 conceptual 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 subsumed by the initialization itself and does not constitute a conversion. [Example: a parameter of type A can be initialized from an argument of type const A. The implicit conversion sequence for that case is the identity sequence; it contains no ``conversion'' from const A to A. ] When the parameter has a class type and the argument expression 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.

-10- If 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 purpose 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 sequence*.

[Footnote: The ambiguous conversion sequence is ranked with user-defined conversion sequences because multiple conversion sequences for an argument can exist only if they involve different user-defined conversions. 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 user-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 resolve to that f(B) because the exact match with f(B) is better than any of the sequences required to match f(A). --- end foonote]

-11- The 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 ?? summarizes the conversions defined in clause conv and partitions them into four disjoint categories: Lvalue Transformation, Qualification Adjustment, Promotion, and Conversion. [Note: these categories 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 categories. At most one conversion from each category is allowed in a single standard conversion sequence. If there are two or more conversions in the sequence, the conversions are applied in the canonical order: Lvalue Transformation, Promotion or Conversion, Qualification Adjustment.
--- end note]

-3- Each conversion in Table ?? 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; otherwise, if any of those has Promotion rank, the sequence has Promotion rank; otherwise, the sequence has Exact Match rank.

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 constructor. 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 conversion 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 function 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 identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion 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

-2- When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert 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-qualification is subsumed by the initialization itself and does not constitute 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 corresponding 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 parameter. 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-compatible with added qualification influences the rank of a standard conversion; 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 conversion 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 conversion 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 indistinguishable 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, excluding 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 initialized 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 second 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 conversion 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, conversion of B* to A* is better than conversion of B* to void*, and conversion of A* to void* is better than conversion of B* to void*.

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

• conversion of C* to B* is better than conversion of C* to A*, [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 conversion 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. ]

-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 functions 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).

-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 considered.

-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
}

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

-6- [Note: if f() and g() are both overloaded functions, the cross product of possibilities must be considered to resolve f(&g), or the equivalent 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

-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[]
	+    -    *    /    %    ^    &    |    ~
	!    =    <    >    +=   -=   *=   /=   %=
	^=   &=   |=   <<   >>   >>=  <<=  ==   !=
	<=   >=   &&   ||   ++   --   ,    ->*  ->
	()   []

-2- Both the unary and binary forms of

+    -    *     &

-3- The following operators cannot be overloaded:

 .    .*   ::    ?:

-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 operator-function-id as the name of the function in the function call syntax (expr.call). [Example:

complex z = a.operator+(b);     //   complex   z   =   a+b;
void* p = operator new(sizeof(int)*n);

-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 subclause 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 defined to be 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 in subclauses over.ass through over.inc act as ordinary unary and binary operators obeying the rules of 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 operator 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 operator 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&)
}

-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 )

-1- operator[] shall be a non-static member function with exactly one parameter. It implements the subscripting syntax

postfix-expression [ expression ]

-1- operator-> shall be a non-static member function taking no parameters. It implements class member access using ->

postfix-expression -> id-expression