1 This document specifies requirements for implementations of the C++ programming language. The first such requirement is that they implement the language, so this document also defines C++ . Other requirements and relaxations of the first requirement appear at various places within this document.

2 C++ is a general purpose programming language based on the C programming language as described in ISO/IEC 9899:2011 Programming languages — C (hereinafter referred to as the C standard). In addition to the facilities provided by C, C ++ provides additional data types, classes, templates, exceptions, namespaces, operator overloading, function name overloading, references, free store management operators, and additional library facilities.

1 The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

• (1.1) Ecma International, ECMAScript Language Specification, Standard Ecma-262, third edition, 1999.
• (1.2) ISO/IEC 2382 (all parts), Information technology — Vocabulary
• (1.3) ISO/IEC 9899:2011, Programming languages — C
• (1.4) ISO/IEC 9899:2011/Cor.1:2012(E), Programming languages — C, Technical Corrigendum 1
• (1.5) ISO/IEC 9945:2003, Information Technology — Portable Operating System Interface (POSIX)
• (1.6) ISO/IEC 10646-1:1993, Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane
• (1.7) ISO/IEC 10967-1:2012, Information technology — Language independent arithmetic — Part 1: Integer and floating point arithmetic
• (1.8) ISO/IEC/IEEE 60559:2011, Information technology — Microprocessor Systems — Floating-Point arithmetic
• (1.9) ISO 80000-2:2009, Quantities and units — Part 2: Mathematical signs and symbols to be used in the natural sciences and technology

2 The library described in Clause 7 of ISO/IEC 9899:2011 is hereinafter called the C standard library¹.

3 The operating system interface described in ISO/IEC 9945:2003 is hereinafter called POSIX.

4 The ECMAScript Language Specification described in Standard Ecma-262 is hereinafter called ECMA-262.

5 The arithmetic specification described in ISO/IEC 10967-1:2012 is hereinafter called LIA-1.

footnote 1 With the qualifications noted in Clauses 21 through 33 and in C.5, the C standard library is a subset of the C ++ standard library.

1 For the purposes of this document, the terms and definitions given in ISO/IEC 2382-1:1993, the terms, definitions, and symbols given in ISO 80000-2:2009, and the following apply.

2 ISO and IEC maintain terminological databases for use in standardization at the following addresses:

• (2.1) IEC Electropedia: available at http://www.electropedia.org/
• (2.2) ISO Online browsing platform: available at http://www.iso.org/obp

3 20.3 defines additional terms that are used only in Clauses 20 through 33 and Annex D.

4 Terms that are used only in a small portion of this document are defined where they are used and italicized where they are defined.

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 and function template declarations can be overloaded; variable 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 16.3. [Example:

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

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

-- end example]

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 (10.3.3). 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). - end note ]

2 Certain function declarations cannot be overloaded:

• (2.1) Function declarations that differ only in the return type, the exception specification (18.4), or both cannot be overloaded.
• (2.2) Member function declarations with the same name and the same parameter-type-list (11.3.5) cannot be overloaded if any of them is a static member function declaration (12.2.3). Likewise, member function template declarations with the same name, the same parameter-type-list, 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 (16.3.1) are not considered when comparing parameter-type-lists 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-type-list, 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 ]
• (2.3) Member function declarations with the same name and the same parameter-type-list (11.3.5) as well as member function template declarations with the same name, the same parameter-type-list, and the same template parameter lists cannot be overloaded if any of them, but not all, have a ref-qualifier (11.3.5). [Example:

  class Y {
    void h() &;
    void h() const &; // OK
    void h() &&;      // OK, all declarations have a ref-qualifier
    void i() &;
    void i() const;   // ill-formed, prior declaration of i
                      // has a ref-qualifier
  };
  

  —end example ]

3 [Note: As specified in 11.3.5, function declarations that have equivalent parameter declarations declare the same function and therefore cannot be overloaded:

• (3.1) 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 (10.1.3). [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 ]
• (3.2) 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 (11.3.5). Only the second and subsequent array dimensions are significant in parameter types (11.3.4). [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 ]
• (3.3) 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 (11.3.5). [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())
  

  —end example ]
• (3.4) 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. 123 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”.
• (3.5) 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 ]

footnote 123 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.

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 (16.1). 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:

struct B {
  int f(int);
};
struct D : B {
  int f(const char*);
};

Here D::f(const char*) hides B::f(int) rather than overloading it.

void h(D* pd) {
  pd->f(1);     // error:
                // D::f(const 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:

void f(const char*);
void g() {
  extern void f(int);
  f("asdf");                  // error: f(int) hides f(const char*)
                              // so there is no f(const 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 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]; }
};

—end example ]

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 parameter-type-list of the candidate function, how well (for non-static member functions) the object matches the implicit 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. —end note ]

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

• (2.1) invocation of a function named in the function call syntax (16.3.1.1.1);
• (2.2) 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 (16.3.1.1.2);
• (2.3) invocation of the operator referenced in an expression (16.3.1.2);
• (2.4) invocation of a constructor for default- or direct-initialization (11.6) of a class object (16.3.1.3);
• (2.5) invocation of a user-defined conversion for copy-initialization (11.6) of a class object (16.3.1.4);
• (2.6) invocation of a conversion function for initialization of an object of a non-class type from an expression of class type (16.3.1.5); and
• (2.7) invocation of a conversion function for conversion to a glvalue or class prvalue to which a reference (11.6.3) will be directly bound (16.3.1.6).

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:

• (2.8) 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 (16.3.2).
• (2.9) Then the best viable function is selected based on the implicit conversion sequences (16.3.3.1) needed to match each argument to the corresponding parameter of each viable function.

3 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 14) in the context in which it is used, the program is ill-formed.

1 The subclauses of 16.3.1 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

• (4.1) “lvalue reference to cv X” for functions declared without a ref-qualifier or with the & ref-qualifier
• (4.2) “rvalue reference to cv X” for functions declared with the && ref-qualifier

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 ”. —end example ] For conversion functions, the function is considered to be a member of the class of the implied 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 (16.3.3). —end note ]

5 During overload resolution, the implied object argument is indistinguishable from other arguments. The implicit object parameter, however, retains its identity since no user-defined conversions can be applied to achieve a type match with it. For non-static member functions declared without a ref-qualifier, an additional rule applies:

• (5.1) even if the implicit object parameter is not const-qualified, an rvalue can be bound to the parameter as long as in all other respects the argument can be converted to the type of the implicit object parameter. [Note: The fact that such an argument is an rvalue does not affect the ranking of implicit conversion sequences (16.3.3.2). —end note ]

6 Because other than in list-initialization only one user-defined conversion is allowed in an implicit conversion sequence, special rules apply when selecting the best user-defined conversion (16.3.3, 16.3.3.1). [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 function template specializations are generated using template argument deduction (17.8.3, 17.8.2). Those candidates are then handled as candidate functions in the usual way. 124 A given name can refer to one or more function templates and also to a set of overloaded non-template functions. In such a case, the candidate functions generated from each function template are combined with the set of non-template candidate functions.

8 A defaulted move constructor or assignment operator (15.8) that is defined as deleted is excluded from the set of candidate functions in all contexts.

footnote 124 The process of argument deduction fully determines the parameter types of the function template specializations, i.e., the parameters of function template specializations contain no template parameter types. Therefore, except where specified otherwise, function template specializations and non-template functions (11.3.5) are treated equivalently for the remainder of overload resolution.

1 In a function call (8.2.2)

if the postfix-expression denotes a set of overloaded functions and/or function templates, overload resolution is applied as specified in 16.3.1.1.1. If the postfix-expression denotes an object of class type, overload resolution is applied as specified in 16.3.1.1.2.

2 If the postfix-expression denotes the address of a set of overloaded functions and/or function templates, overload resolution is applied using that set as described above. If the function selected by overload resolution is a non-static member function, the program is ill-formed. [Note: The resolution of the address of an overload set in other contexts is described in 16.4. —end note ]

1 Of interest in 16.3.1.1.1 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:

These represent two syntactic subcategories of function calls: qualified function calls and unqualified function calls.

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 16 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 16 assumes that the postfix-expression that is the left operand of the . operator has type “cv T ” where T denotes a class 125 . 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 (13.2). The function declarations found by that lookup constitute 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 (16.3.1).

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 (6.4). The function declarations found by that lookup constitute the set of candidate functions. Because of the rules for name lookup, the set of candidate functions consists (1) entirely of non-member functions or (2) entirely of member functions of some class T . In case (1), the argument list is the same as the expression-list in the call. In case (2), the argument list is the expression-list in the call augmented by the addition of an implied object argument as in a qualified function call. If the keyword this (12.2.2.1) is in scope and refers to class T , or a derived class of T , then the implied object argument is (*this) . If the keyword this is not in scope or refers to another class, then a contrived object of type T becomes the implied object argument 126 . If the argument list is augmented by a contrived object and overload resolution selects one of the non-static member functions of T, the call is ill-formed.

footnote 125 Note that cv-qualifiers on the type of objects are significant in overload resolution for both glvalue and class prvalue objects.

footnote 126 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.

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 non-explicit conversion function declared in T of the form

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 ( P 1 ,...,P n ) returning R ”, or the type “reference to pointer to function of ( P 1 ,...,P n ) returning R ”, or the type “reference to function of ( P 1 ,...,P n ) returning R ”, a surrogate call function with the unique name call-function and having the form

is also considered as a candidate function. Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration 127 .

3 If such a surrogate call function is selected by overload resolution, the corresponding conversion function will be called to convert E to the appropriate function pointer or reference, and the function will then be invoked with the arguments of the call. If the conversion function cannot be called (e.g., because of an ambiguity), the program is ill-formed.

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. —end note ] [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 ]

footnote 127 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.

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 8. [Note: Because . , .* , and :: cannot be overloaded, these operators are always built-in operators interpreted according to Clause 8. ?: 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 (8.16). —end note ] [Example:

struct String {
  String (const String&);
  String (const char*);
  operator const char* ();
};
String operator + (const String&, const String&);

void f() {
  const char* p= "one" + "two"; // ill-formed because neither operand has class or enumeration type
  int I = 1 + 1;                // always evaluates to 2 even if class or enumeration types exist
                                // that 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 operator. Therefore, the operator notation is first transformed to the equivalent function-call notation as summarized in Table 12 (where @ denotes one of the operators covered in the specified subclause). However, the operands are sequenced in the order prescribed for the built-in operator (Clause 8).

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:

• (3.1) If T1 is a complete class type or a class currently being defined, the set of member candidates is the result of the qualified lookup of T1::operator@ (16.3.1.1.1); otherwise, the set of member candidates is empty.
• (3.2) 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 (6.4.2) 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 cv T1 ”, when T1 is an enumeration type, or (if there is a right operand) a second parameter of type T2 or “reference to cv T2”, when T2 is an enumeration type, are candidate functions.
• (3.3) 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 16.6 that, compared to the given operator,
  • (3.3.1) have the same operator name, and
  • (3.3.2) accept the same number of operands, and
  • (3.3.3) accept operand types to which the given operand or operands can be converted according to 16.3.3.1, and
  • (3.3.4) do not have the same parameter-type-list as any non-member candidate that is not a function template specialization.

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

• (4.1) no temporaries are introduced to hold the left operand, and
• (4.2) 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 16.3.2 and 16.3.3. 128 [Example:

struct A {
  operator int();
};
A operator+(const A&, const A&);
void m() {
  A a, b;
  a + b;      // operator+(a, b) chosen over int(a) + int(b)
}

—end example ]

7 If a built-in candidate is selected by overload resolution, the operands of class type are converted to the types of the corresponding parameters of the selected operation function, except that the second standard conversion sequence of a user-defined conversion sequence (16.3.3.1.2) is not applied. Then the operator is treated as the corresponding built-in operator and interpreted according to Clause 8. [Example:

struct X {
  operator double();
};

struct Y {
  operator int*();
};

int *a = Y() + 100.0; // error: pointer arithmetic requires integral operand
int *b = Y() + X();   // error: pointer arithmetic requires integral operand

—end example ]

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. 129

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 8.

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 ]

footnote 128 If the set of candidate functions is empty, overload resolution is unsuccessful.

footnote 129 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.

1 When objects of class type are direct-initialized (11.6), copy-initialized from an expression of the same or a derived class type (11.6), or default-initialized (11.6), overload resolution selects the constructor. For direct-initialization or default-initialization that is not in the context of copy-initialization, the candidate functions are all the constructors of the class of the object being initialized. For copy-initialization, the candidate functions are all the converting constructors (15.3.1) of that class. The argument list is the expression-list or assignment-expression of the initializer.

1 From the set of candidate functions constructed for a given context (16.3.1), a set of viable functions is chosen, from which the best function will be selected by comparing argument conversion sequences for the best fit (16.3.3). 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.

• (2.1) If there are m arguments in the list, all candidate functions having exactly m parameters are viable.
• (2.2) A candidate function having fewer than m parameters is viable only if it has an ellipsis in its parameter list (11.3.5). For the purposes of overload resolution, any argument for which there is no corresponding parameter is considered to “match the ellipsis” (16.3.3.1.3) .
• (2.3) A candidate function having more than m parameters is viable only if the (m+1)-st parameter has a default argument (11.3.6). 130 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 se- quence (16.3.3.1) 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 an lvalue reference to non- const cannot be bound to an rvalue and that an rvalue reference cannot be bound to an lvalue can affect the viability of the function (see 16.3.3.1.4).

foonote 130 According to 11.3.6, parameters following the (m+1)-st parameter must also have default arguments.

1 Define ICSi(F) as follows:

• (1.1) 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 ); 131 otherwise,
• (1.2) 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 . 16.3.3.1 defines the implicit conversion sequences and 16.3.3.2 defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another.

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

• (1.3) for some argument j, ICSj(F1) is a better conversion sequence than ICSj(F2), or, if not that,
• (1.4) the context is an initialization by user-defined conversion (see 11.6, 16.3.1.5, and 16.3.1.6) 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 ] or, if not that,
• (1.5) the context is an initialization by conversion function for direct reference binding (16.3.1.6) of a reference to function type, the return type of F1 is the same kind of reference (i.e. lvalue or rvalue) as the reference being initialized, and the return type of F2 is not [Example:

  template <class T> struct A {
    operator T&();  // #1
    operator T&&(); // #2
  };
  typedef int Fn();
  A<Fn> a;
  Fn& lf = a;       // calls #1
  Fn&& rf = a;      // calls #2
  

  —end example ] or, if not that,
• (1.6) F1 is not a function template specialization and F2 is a function template specialization, or, if not that,
• (1.7) F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in 17.5.6.2, or, if not that,
• (1.8) F1 is generated from a deduction-guide (16.3.1.8) and F2 is not, or, if not that,
• (1.9) F1 is the copy deduction candidate (16.3.1.8) and F2 is not, or, if not that,
• (1.10) F1 is generated from a non-template constructor and F2 is generated from a constructor template. [Example:

  template <class T> struct A {
    using value_type = T;
    A(value_type);    // #1
    A(const A&);      // #2
    A(T, T, int);     // #3
    template<class U>
      A(int, T, U);   // #4
    // #5 is the copy deduction candidate, A(A)
  };
  
  A x(1, 2, 3);       // uses #3, generated from a non-template constructor
  
  template <class T>
  A(T) -> A<T>;       // #6, less specialized than #5
  
  A a(42);            // uses #6 to deduce A<int> and #1 to initialize
  A b = a;            // uses #5 to deduce A<int> and #2 to initialize
  
  template <class T>
  A(A<T>) -> A<A<T>>; // #7, as specialized as #5
  
  A b2 = a;           // uses #7 to deduce A<A<int>> and #1 to initialize
  

  —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. ¹³² [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 ]

3 If the best viable function resolves to a function for which multiple declarations were found, and if at least two of these declarations — or the declarations they refer to in the case of using-declarations — specify a default argument that made the function viable, the program is ill-formed. [Example:

namespace A {
  extern "C" void f(int = 5);
}
namespace B {
  extern "C" void f(int = 5);
}

using A::f;
using B::f;

void use() {
  f(3); // OK, default argument was not used for viability
  f();  // Error: found default argument twice
}

—end example ]

footnote 131 If a function is a static member function, this definition means that the first argument, the implied object argument, has no effect in the determination of whether the function is better or worse than any other function.

footnote 132 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.

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 7, which means it is governed by the rules for initialization of an object or reference by a single expression (11.6, 11.6.3).

2 Implicit conversion sequences are concerned only with the type, cv-qualification, and value category 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, accessibility of the argument, whether the argument is a bit-field, and whether a function is deleted (11.4.3), 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 A well-formed implicit conversion sequence is one of the following forms:

• (3.1) a standard conversion sequence (16.3.3.1.1),
• (3.2) a user-defined conversion sequence (16.3.3.1.2), or
• (3.3) an ellipsis conversion sequence (16.3.3.1.3).

4 However, if the target is

• (4.1) the first parameter of a constructor or
• (4.2) the implicit object parameter of a user-defined conversion function

and the constructor or user-defined conversion function is a candidate by

• (4.3) 16.3.1.3, when the argument is the temporary in the second step of a class copy-initialization,
• (4.4)16.3.1.4, 16.3.1.5, or 16.3.1.6 (in all cases), or
• (4.5)the second phase of 16.3.1.7 when the initializer list has exactly one element that is itself an initializer list, and the target is the first parameter of a constructor of class X , and the conversion is to X or reference to cv X,

user-defined conversion sequences are not considered. [Note: These rules prevent more than one user-defined conversion from being applied during overload resolution, thereby avoiding infinite recursion. —end note ] [Example:

struct Y { Y(int); };
struct A { operator int(); };
Y y1 = A();   // error: A::operator int() is not a candidate

struct X { };
struct B { operator X(); };
B b;
X x({b});     // error: B::operator X() is not a candidate

—end example ]

5 For the case where the parameter type is a reference, see 16.3.3.1.4.

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 a prvalue 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 16; the actual initialization is defined in terms of constructors and is not a conversion. —end note ] 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 . —end example ] 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. —end note ] A derived-to-base Conversion has Conversion rank (16.3.3.1.1).

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 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 (16.3.3.1.1).

9 If no sequence of conversions can be found to convert an argument to a parameter type, 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 16.3.3.2, the ambiguous conversion sequence is treated as a user-defined conversion sequence that is indistinguishable from any other user-defined conversion sequence. [Note: This rule prevents a function from becoming non-viable because of an ambiguous conversion sequence for one of its parameters. [Example:

[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);         // ill-formed: ambiguous because there is a conversion b → C (via constructor)
              // and an (ambiguous) conversion b → A (via constructor or conversion function)
void f(B) { }
f(b);         // OK, unambiguous

—end example ] —end note ] 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.

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

1 This subclause 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 16.3.3.1)

• (2.1) a standard conversion sequence (16.3.3.1.1) is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and
• (2.2) a user-defined conversion sequence (16.3.3.1.2) is a better conversion sequence than an ellipsis conversion sequence (16.3.3.1.3).

3 Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:

• (3.1) List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if
  • (3.1.1) L1 converts to std::initializer_list<X> for some X and L2 does not, or, if not that,
  • (3.1.2) L1 converts to type “array of N1 T ”, L2 converts to type “array of N2 T ”, and N1 is smaller than N2,
  even if one of the other rules in this paragraph would otherwise apply. [Example:

  void f1(int);                                 // #1
  void f1(std::initializer_list<long>);         // #2
  void g1() { f1({42}); }                       // chooses #2
  
  void f2(std::pair<const char*, const char*>); // #3
  void f2(std::initializer_list<std::string>);  // #4
  void g2() { f2({"foo","bar"}); }              // chooses #4
  

  —end example ]
• (3.2) Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if
  • (3.2.1) S1 is a proper subsequence of S2 (comparing the conversion sequences in the canonical form defined by 16.3.3.1.1, excluding any Lvalue Transformation; the identity conversion sequence is considered to be a subsequence of any non-identity conversion sequence) or, if not that,
  • (3.2.2) the rank of S1 is better than the rank of S2 , or S1 and S2 have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,
  • (3.2.3) S1 and S2 are reference bindings (11.6.3) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue reference [Example:

    int i;
    int f1();
    int&& f2();
    int g(const int&);
    int g(const int&&);
    int j = g(i); // calls g(const int&)
    int k = g(f1()); // calls g(const int&&)
    int l = g(f2()); // calls g(const int&&)
    
    struct A {
      A& operator<<(int);
      void p() &;
      void p() &&;
    };
    A& operator<<(A&&, char);
    A() << 1;     // calls A::operator<<(int)
    A() << 'c';   // calls operator<<(A&&, char)
    A a;
    a << 1;       // calls A::operator<<(int)
    a << 'c';     // calls A::operator<<(int)
    A().p();      // calls A::p()&&
    a.p();        // calls A::p()&
    

    —end example ] or, if not that,
  • (3.2.4) S1 and S2 are reference bindings (11.6.3) and S1 binds an lvalue reference to a function lvalue and S2 binds an rvalue reference to a function lvalue [Example:

    int f(void(&)());   // #1
    int f(void(&&)());  // #2
    void g();
    int i1 = f(g);      // calls #1
    

    —end example ] or, if not that,
  • (3.2.5) S1 and S2 differ only in their qualification conversion and yield similar types T1 and T2 (7.5), 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 volatile int *);
    int f(const int *);
    int i;
    int j = f(&i);  // calls f(const int*)
    

    —end example ] or, if not that,
  • (3.2.6) S1 and S2 are reference bindings (11.6.3), 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
    
    struct X {
      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 ]
• (3.3) 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 or they initialize the same class in an aggregate initialization and in either case 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:

• (4.1) A conversion that does not convert a pointer, a pointer to member, or std::nullptr_t to bool is better than one that does.
• (4.2) A conversion that promotes an enumeration whose underlying type is fixed to its underlying type is better than one that promotes to the promoted underlying type, if the two are different.
• (4.3) 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*.
• (4.4) If class B is derived directly or indirectly from class A and class C is derived directly or indirectly from B
  • (4.4.1) 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 ]
  • (4.4.2) binding of an expression of type C to a reference to type B is better than binding an expression of type C to a reference to type A,
  • (4.4.3) conversion of A::* to B::* is better than conversion of A::* to C::*,
  • (4.4.4) conversion of C to B is better than conversion of C to A,
  • (4.4.5) conversion of B* to A* is better than conversion of C* to A*,
  • (4.4.6) binding of an expression of type B to a reference to type A is better than binding an expression of type C to a reference to type A,
  • (4.4.7) conversion of B::* to C::* is better than conversion of A::* to C::*, and
  • (4.4.8) 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 16.3.3); in all other contexts, the source types will be the same and the target types will be different. —end note ]