16 Overloading [over]

16.3 Overload resolution [over.match]

16.3.1 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 context 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 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 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.
    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;            // ill-formed: T(C(1)) not tried
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 special function ([class.copy]) that is defined as deleted is excluded from the set of candidate functions in all contexts.
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 ([dcl.fct]) are treated equivalently for the remainder of overload resolution.

16.3.1.1 Function call syntax [over.match.call]

postfix-expression ( expression-list )
if the postfix-expression denotes a set of overloaded functions and/or function templates, overload resolution is applied as specified in [over.call.func].
If the postfix-expression denotes an object of class type, overload resolution is applied as specified in [over.call.object].
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
]

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

Of interest in [over.call.func] are only those function calls in which the postfix-expression ultimately contains a name that denotes one or more functions that might be called.
Such a postfix-expression, perhaps nested arbitrarily deep in parentheses, has one of the following forms:
postfix-expression:
	postfix-expression . id-expression
	postfix-expression -> id-expression
	primary-expression
These represent two syntactic subcategories of function calls: qualified function calls and unqualified function calls.
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 [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, [over] assumes that the postfix-expression that is the left operand of the . operator has type “cv T” where T denotes a class.126
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.
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 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.127
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.
Note that cv-qualifiers on the type of objects are significant in overload resolution for both glvalue and class prvalue objects.
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.

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

If the postfix-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-qualifier noexcept-specifier attribute-specifier-seq ;
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 () returning R”, or the type “reference to pointer to function of () returning R”, or the type “reference to function of () returning R”, a surrogate call function with the unique name call-function and having the form
R call-function ( conversion-type-id  F, P a, , P a) { return F (a, , a); }
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.128
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);                   // calls f1 via pointer returned from conversion function
end example
]
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.

16.3.1.2 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 [expr.compound].
[Note
:
Because ., .*, and ​::​ cannot be overloaded, these operators are always built-in operators interpreted according to [expr.compound].
?: 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() {
 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
]
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 ([expr.compound]).
Table 12 — Relationship between operator and function call notation
Subclause
Expression
As member function
As non-member function
@a
(a).operator@ ()
operator@(a)
a@b
(a).operator@ (b)
operator@(a, b)
a=b
(a).operator= (b)
a[b]
(a).operator[](b)
a->
(a).operator->()
a@
(a).operator@ (0)
operator@(a, 0)
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 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.
  • 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 for some operator @ is the union of the member candidates, the non-member candidates, and the built-in candidates for that operator @.
If that operator is a relational ([expr.rel]) or equality ([expr.eq]) operator with operands x and y, then for each member, non-member, or built-in candidate for the operator <=>:
  • that operator is added to the set of candidate functions for overload resolution if x <=> y @ 0 is well-formed using that operator<=>; and
  • a synthesized candidate is added to the candidate set where the order of the two parameters is reversed if 0 @ y <=> x is well-formed using that operator<=>;
where in each case operator<=> candidates are not considered for the recursive lookup of operator @.
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;                        // operator+(a, b) chosen over int(a) + int(b)
}
end example
]
If an operator<=> candidate is selected by overload resolution for an operator @, but @ is not <=>, x @ y is interpreted as 0 @ y <=> x if the selected candidate is a synthesized candidate with reversed order of parameters, or x <=> y @ 0 otherwise, using the selected operator<=> candidate.
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 is not applied.
Then the operator is treated as the corresponding built-in operator and interpreted according to [expr.compound].
[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
]
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 [expr.compound].
[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
]
If the set of candidate functions is empty, overload resolution is unsuccessful.
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.

16.3.1.3 Initialization by constructor [over.match.ctor]

When objects of class type are direct-initialized, copy-initialized from an expression of the same or a derived class type ([dcl.init]), or default-initialized, 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 of that class.
The argument list is the expression-list or assignment-expression of the initializer.

16.3.1.4 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 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 object ([class.mem]) to be bound to the first parameter of a constructor where the parameter is of type “reference to possibly cv-qualified T” and the constructor is 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
]

16.3.1.5 Initialization by conversion function [over.match.conv]

Under the conditions specified in [dcl.init], as part of an initialization of an object of non-class 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 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 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
]

16.3.1.6 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 “reference to cv1 T” is the 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 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, 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
]

16.3.1.7 Initialization by list-initialization [over.match.list]

When objects of non-aggregate class type T are list-initialized such that [dcl.init.list] specifies that overload resolution is performed according to the rules in this subclause, 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
]

16.3.1.8 Class template argument deduction [over.match.class.deduct]

When resolving a placeholder for a deduced class type ([dcl.type.class.deduct]) where the template-name names a primary class template C, a set of functions and function templates is formed comprising:
  • If C is defined, for each constructor of C, a function template with the following properties:
    • The template parameters are the template parameters of C followed by the template parameters (including default template arguments) of the constructor, if any.
    • The types of the function parameters are those of the constructor.
    • The return type is the class template specialization designated by C and template arguments corresponding to the template parameters of C.
  • If C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C().
  • An additional function template derived as above from a hypothetical constructor C(C), called the copy deduction candidate.
  • For each deduction-guide, a function or function template with the following properties:
Initialization and overload resolution are performed as described in [dcl.init] and [over.match.ctor], [over.match.copy], or [over.match.list] (as appropriate for the type of initialization performed) for an object of a hypothetical class type, where the selected functions and function templates are considered to be the constructors of that class type for the purpose of forming an overload set, and the initializer is provided by the context in which class template argument deduction was performed.
As an exception, the first phase in [over.match.list] (considering initializer-list constructors) is omitted if the initializer list consists of a single expression of type cv U, where U is a specialization of C or a class derived from a specialization of C.
Each such notional constructor is considered to be explicit if the function or function template was generated from a constructor or deduction-guide that was declared explicit.
All such notional constructors are considered to be public members of the hypothetical class type.
[Example
:
template <class T> struct A {
  explicit A(const T&, ...) noexcept;               // #1
  A(T&&, ...);                                      // #2
};

int i;
A a1 = { i, i };    // error: explicit constructor #1 selected in copy-list-initialization during deduction,
                    // cannot deduce from non-forwarding rvalue reference in #2

A a2{i, i};         // OK, #1 deduces to A<int> and also initializes
A a3{0, i};         // OK, #2 deduces to A<int> and also initializes
A a4 = {0, i};      // OK, #2 deduces to A<int> and also initializes

template <class T> A(const T&, const T&) -> A<T&>;  // #3
template <class T> explicit A(T&&, T&&) -> A<T>;    // #4

A a5 = {0, 1};      // error: explicit deduction guide #4 selected in copy-list-initialization during deduction
A a6{0,1};          // OK, #4 deduces to A<int> and #2 initializes
A a7 = {0, i};      // error: #3 deduces to A<int&>, #1 and #2 declare same constructor
A a8{0,i};          // error: #3 deduces to A<int&>, #1 and #2 declare same constructor

template <class T> struct B {
  template <class U> using TA = T;
  template <class U> B(U, TA<U>);
};

B b{(int*)0, (char*)0};         // OK, deduces B<char*>
end example
]