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 ]
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 a glvalue or class prvalue to which a reference ([dcl.init.ref]) will be directly bound ([over.match.ref]).
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.
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]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.
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.
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.
For non-static member functions, the type of the implicit object parameter is
“lvalue reference to cv X” for functions declared without a ref-qualifier or with the & ref-qualifier
“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 ([over.match.best]). — end note ]
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; and
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:
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 ([over.ics.rank]). — end note ]
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 ([over.match.best], [over.best.ics]). [ Example:
class T {
public:
T();
};
class C : T {
public:
C(int);
};
T a = 1;
— end example ]
In each case where a candidate is a function template, candidate function template specializations are generated using template argument deduction ([temp.over], [temp.deduct]). Those candidates are then handled as candidate functions in the usual way.125 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.
A defaulted move constructor or assignment operator ([class.copy]) that is defined as deleted is excluded from the set of candidate functions in all contexts.
13.3.1.1 Function call syntax [over.match.call]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 [over.over]. — end note ]
13.3.1.1.1 Call to named function [over.call.func]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 class126. Under this assumption, the id-expression in the call is looked up as a member function of T following the rules for looking up names in classes ([class.member.lookup]). 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 ([over.match.funcs]).
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]). 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 ([class.this]) 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 argument127. 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.
13.3.1.1.2 Call to object of class type [over.call.object]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().
In addition, for each non-explicit conversion function declared in T of the form
operator conversion-type-id ( ) cv-qualifier ref-qualifieropt exception-specificationopt attribute-specifier-seqopt ;
where cv-qualifier is the same cv-qualification as, or a greater cv-qualification than, cv, and where conversion-type-id denotes the type “pointer to function of (P1,...,Pn) returning R”, or the type “reference to pointer to function of (P1,...,Pn) returning R”, or the type “reference to function of (P1,...,Pn) returning R”, a surrogate call function with the unique name call-function and having the form
R call-function ( conversion-type-id F, P1 a1, ... ,Pn an) { return F (a1,... ,an); }
is also considered as a candidate function. Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration128.
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.
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);
13.3.1.2 Operators in expressions [over.match.oper]
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]). — end note ] [ Example:
struct String {
String (const String&);
String (const char*);
operator const char* ();
};
String operator + (const String&, const String&);
void f(void) {
const char* p= "one" + "two"; int I = 1 + 1; }
— end example ]
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 [tab:over.rel.op.func] (where @ denotes one of the operators covered in the specified subclause).
Table
11— Relationship between operator and function call notation
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 complete class type or a class currently being defined, 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.argdep]) 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-member candidate that is not a function template specialization.
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.
For all other operators, no such restrictions apply.
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].129 [ Example:
struct A {
operator int();
};
A operator+(const A&, const A&);
void m() {
A a, b;
a + b; }
— end example ]
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 ([over.ics.user]) is not applied. Then the operator is treated as the corresponding built-in operator and interpreted according to Clause [expr]. [ Example:
struct X {
operator double();
};
struct Y {
operator int*();
};
int *a = Y() + 100.0; int *b = Y() + X();
— end example ]
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.130
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].
[ 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); a + a; }
— end note ]
13.3.1.3 Initialization by constructor [over.match.ctor]When objects of class type are direct-initialized ([dcl.init]), or copy-initialized from an expression of the same or a derived class type ([dcl.init]), overload resolution selects the constructor. For direct-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 ([class.conv.ctor]) of that class. The argument list is the expression-list or assignment-expression of the initializer.
13.3.1.4 Copy-initialization of class by user-defined conversion [over.match.copy]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. [ Note: The conversion performed for indirect binding to a reference to a possibly cv-qualified class type is determined in terms of a corresponding non-reference copy-initialization. — end note ] 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 non-explicit conversion functions of S and its base classes are considered. When initializing a temporary to be bound to the first parameter of a constructor that takes a reference to possibly cv-qualified T as its first argument, called with a single argument in the context of direct-initialization of an object of type “cv2 T”, explicit conversion functions are also 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 X” return lvalues or xvalues, depending on the type of reference, of type X and are therefore considered to yield X for this process of selecting candidate functions.
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. — end note ]
13.3.1.5 Initialization by conversion function [over.match.conv]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 non-explicit conversion functions 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. For direct-initialization, those explicit conversion functions that are not hidden within S and yield type T or a type that can be converted to type T with a qualification conversion ([conv.qual]) are also 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 cv2 X” return lvalues or xvalues, depending on the type of reference, of type “cv2 X” and are therefore considered to yield X for this process of selecting candidate functions.
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. — end note ]
13.3.1.6 Initialization by conversion function for direct reference binding [over.match.ref]Under the conditions specified in [dcl.init.ref], a reference can be bound directly to a glvalue or class prvalue 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 non-explicit conversion functions that are not hidden within S and yield type “lvalue reference to cv2 T2” (when initializing an lvalue reference or an rvalue reference to function) or “ cv2 T2” or “rvalue reference to cv2 T2” (when initializing an rvalue reference or an lvalue reference to function), where “cv1 T” is reference-compatible ([dcl.init.ref]) with “cv2 T2”, are candidate functions. For direct-initialization, those explicit conversion functions that are not hidden within S and yield type “lvalue reference to cv2 T2” or “cv2 T2” or “rvalue reference to cv2 T2,” respectively, where T2 is the same type as T or can be converted to type T with a qualification conversion ([conv.qual]), are also candidate functions.
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. — end note ]
13.3.1.7 Initialization by list-initialization [over.match.list]When objects of non-aggregate class type T are list-initialized ([dcl.init.list]), overload resolution selects the constructor in two phases:
Initially, the candidate functions are the initializer-list constructors ([dcl.init.list]) of the class T and the argument list consists of the initializer list as a single argument.
If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.
If the initializer list has no elements and T has a default constructor, the first phase is omitted. In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed. [ Note: This differs from other situations ([over.match.ctor], [over.match.copy]), where only converting constructors are considered for copy-initialization. This restriction only applies if this initialization is part of the final result of overload resolution. — end note ]
13.3.2 Viable functions [over.match.viable]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.
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]).131 For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters.
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 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 [over.ics.ref]).
13.3.3 Best viable function [over.match.best]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)132; 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.
Given these definitions, a viable function F1 is defined to be a better function than another viable function F2 if for all arguments i, ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then
for some argument j, ICSj(F1) is a better conversion sequence than ICSj(F2), or, if not that,
the context is an initialization by user-defined conversion (see [dcl.init], [over.match.conv], and [over.match.ref]) 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; float x = a;
— end example ] or, if not that,
the context is an initialization by conversion function for direct reference binding ([over.match.ref]) 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&(); operator T&&(); };
typedef int Fn();
A<Fn> a;
Fn& lf = a; Fn&& rf = a;
— end example ] or, if not that,
F1 is not a function template specialization and F2 is a function template specialization, or, if not that,
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 [temp.func.order].
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-formed133.
[ Example:
void Fcn(const int*, short);
void Fcn(int*, int);
int i;
short s = 0;
void f() {
Fcn(&i, s);
Fcn(&i, 1L);
Fcn(&i,'c'); }
— end example ]
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); f(); }
— end example ]
13.3.3.1 Implicit conversion sequences [over.best.ics]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]).
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, 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.
However, if the target is
the first parameter of a constructor or
the implicit object parameter of a user-defined conversion function
and the constructor or user-defined conversion function is a candidate by
[over.match.ctor], when the argument is the temporary in the second step of a class copy-initialization,
[over.match.copy], [over.match.conv], or [over.match.ref] (in all cases), or
the second phase of [over.match.list] when the initializer list has exactly one element, and the target is the first parameter of a constructor of class X, and the conversion is to X or reference to (possibly cv-qualified) 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();
struct X { };
struct B { operator X(); };
B b;
X x({b});
— end example ]
For the case where the parameter type is a reference, see [over.ics.ref].
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 [over]; 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 ([over.ics.scs]).
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.
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]).
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.
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 sequence134. 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.
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]Table [tab:over.conversions] 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 value category, 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 value category or data representation of the type; and the Promotions and Conversions do not change the value category or cv-qualification of the type. — end note ]
[ 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 ]
Each conversion in Table [tab:over.conversions] 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]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.
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]).
If the user-defined conversion is specified by a specialization of a conversion function template, the second standard conversion sequence shall have exact match rank.
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 constructor (i.e., a user-defined conversion function) is called for those cases.
13.3.3.1.3 Ellipsis conversion sequences [over.ics.ellipsis]An ellipsis conversion sequence occurs when an argument in a function call is matched with the ellipsis parameter specification of the function called (see [expr.call]).
13.3.3.1.4 Reference binding [over.ics.ref]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);
— end example ] If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence ([over.ics.user]), with the second standard conversion sequence either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base Conversion.
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.
Except for an implicit object parameter, for which see [over.match.funcs], a standard conversion sequence cannot be formed if it requires binding an lvalue reference other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue other than a function lvalue. [ Note: This means, for example, that a candidate function cannot be a viable function if it has a non-const lvalue 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 lvalue reference (see [dcl.init.ref]). — end note ]
Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of a standard conversion sequence, however. [ Example: a function with an “lvalue 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 lvalue reference to a bit-field ([dcl.init.ref]). — end example ]
13.3.3.1.5 List-initialization sequence [over.ics.list]When an argument is an initializer list ([dcl.init.list]), it is not an expression and special rules apply for converting it to a parameter type.
If the parameter type is std::initializer_list<X> and all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X, or if the initializer list has no elements, the identity conversion. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor. [ Example:
void f(std::initializer_list<int>);
f( {} ); f( {1,2,3} ); f( {'a','b'} ); f( {1.0} );
struct A {
A(std::initializer_list<double>); A(std::initializer_list<complex<double>>); A(std::initializer_list<std::string>); };
A a{ 1.0,2.0 };
void g(A);
g({ "foo", "bar" });
typedef int IA[3];
void h(const IA&);
h({ 1, 2, 3 });
— end example ]
Otherwise, if the parameter type is “array of N X”135, if the initializer list has exactly N elements or if it has fewer than N elements and X is default-constructible, and if all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X.
Otherwise, if the parameter is a non-aggregate class X and overload resolution per [over.match.list] chooses a single best constructor of X to perform the initialization of an object of type X from the argument initializer list, the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion. If multiple constructors are viable but none is better than the others, the implicit conversion sequence is the ambiguous conversion sequence. User-defined conversions are allowed for conversion of the initializer list elements to the constructor parameter types except as noted in [over.best.ics]. [ Example:
struct A {
A(std::initializer_list<int>);
};
void f(A);
f( {'a', 'b'} );
struct B {
B(int, double);
};
void g(B);
g( {'a', 'b'} ); g( {1.0, 1.0} );
void f(B);
f( {'a', 'b'} );
struct C {
C(std::string);
};
void h(C);
h({"foo"});
struct D {
D(A, C);
};
void i(D);
i({ {1,2}, {"bar"} });
— end example ]
Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization ([dcl.init.aggr]), the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion. [ Example:
struct A {
int m1;
double m2;
};
void f(A);
f( {'a', 'b'} ); f( {1.0} );
— end example ]
Otherwise, if the parameter is a reference, see [over.ics.ref]. [ Note: The rules in this section will apply for initializing the underlying temporary for the reference. — end note ] [ Example:
struct A {
int m1;
double m2;
};
void f(const A&);
f( {'a', 'b'} ); f( {1.0} );
void g(const double &);
g({1});
— end example ]
Otherwise, if the parameter type is not a class:
if the initializer list has one element, the implicit conversion sequence is the one required to convert the element to the parameter type; [ Example:
void f(int);
f( {'a'} ); f( {1.0} );
— end example ]
if the initializer list has no elements, the implicit conversion sequence is the identity conversion. [ Example:
void f(int);
f( { } );
— end example ]
In all cases other than those enumerated above, no conversion is possible.
13.3.3.2 Ranking implicit conversion sequences [over.ics.rank]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]).
Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:
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, or S1 and S2 have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,
S1 and S2 are reference bindings ([dcl.init.ref]) 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); int k = g(f1()); int l = g(f2());
struct A {
A& operator<<(int);
void p() &;
void p() &&;
};
A& operator<<(A&&, char);
A() << 1; A() << 'c'; A a;
a << 1; a << 'c'; A().p(); a.p();
— end example ] or, if not that,
S1 and S2 are reference bindings ([dcl.init.ref]) and S1 binds an lvalue reference to a function lvalue and S2 binds an rvalue reference to a function lvalue. [ Example:
int f(void(&)()); int f(void(&&)()); void g(); int i1 = f(g);
— end example ] 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 volatile int *); int f(const int *); int i; int j = f(&i);
— 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); int k = g(i);
struct X {
void f() const;
void f();
};
void g(const X& a, X b) {
a.f(); b.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 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);
— end example ]
List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if
L1 converts to std::initializer_list<X> for some X and L2 does not, or, if not that,
L1 converts to type “array of N1 T”, L2 converts to type “array of N2 T”, and N1 is smaller than N2.
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 does not convert a pointer, a pointer to member, or std::nullptr_t to bool is better than one that does.
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.
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);
— end example ]
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,
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 to type A is better than binding an expression of type C to a reference to 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. — end note ]
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