16.3 Overload resolution [over.match]

16.3.3 Best viable function [over.match.best]

16.3.3.1 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, accessibility of the argument, whether the argument is a bit-field, and whether a function is deleted, 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.

A well-formed implicit conversion sequence is one of the following forms:

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

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]

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 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, 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 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:

```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.

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

16.3.3.1.1 Standard conversion sequences [over.ics.scs]

Table 13 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. 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 13 also has an associated rank (Exact Match, Promotion, or Conversion). These are used to rank standard conversion sequences. 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. 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.

Table 13 — Conversions
 Conversion Category Rank Subclause No conversions required Identity Lvalue-to-rvalue conversion [conv.lval] Array-to-pointer conversion Lvalue Transformation [conv.array] Function-to-pointer conversion Exact Match [conv.func] Qualification conversions [conv.qual] Function pointer conversion Qualification Adjustment [conv.fctptr] Integral promotions [conv.prom] Floating-point promotion Promotion Promotion [conv.fpprom] Integral conversions [conv.integral] Floating-point conversions [conv.double] Floating-integral conversions [conv.fpint] Pointer conversions Conversion Conversion [conv.ptr] Pointer to member conversions [conv.mem] Boolean conversions [conv.bool]

16.3.3.1.2 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, 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, 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.

16.3.3.1.3 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]).

16.3.3.1.4 Reference binding [over.ics.ref]

When a parameter of reference type binds directly to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion ([over.best.ics]). [Example:

```struct A {};
struct B : public A {} b;
int f(A&);
int f(B&);
int i = f(b);       // calls f(B&), an exact match, rather than f(A&), a conversion
```

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, 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 referenced type according to [over.best.ics]. Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the referenced 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 would require a temporary 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]

16.3.3.1.5 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 an aggregate class X and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence is the one required to convert the element to the parameter type.

Otherwise, if the parameter type is a character array133 and the initializer list has a single element that is an appropriately-typed string literal ([dcl.init.string]), the implicit conversion sequence is the identity conversion.

Otherwise, 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( {} );                // OK: f(initializer_­list<int>) identity conversion
f( {1,2,3} );           // OK: f(initializer_­list<int>) identity conversion
f( {'a','b'} );         // OK: f(initializer_­list<int>) integral promotion
f( {1.0} );             // error: narrowing

struct A {
A(std::initializer_list<double>);             // #1
A(std::initializer_list<complex<double>>);    // #2
A(std::initializer_list<std::string>);        // #3
};
A a{ 1.0,2.0 };         // OK, uses #1

void g(A);
g({ "foo", "bar" });    // OK, uses #3

typedef int IA[3];
void h(const IA&);
h({ 1, 2, 3 });         // OK: identity conversion
```

end example]

Otherwise, if the parameter type is “array of N X”, if there exists an implicit conversion sequence for each element of the array from the corresponding element of the initializer list (or from {} if there is no such element), the implicit conversion sequence is the worst such implicit conversion sequence.

Otherwise, if the parameter is a non-aggregate class X and overload resolution per [over.match.list] chooses a single best constructor C of X to perform the initialization of an object of type X from the argument initializer list:

• If C is not an initializer-list constructor and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence has Exact Match rank if U is X, or Conversion rank if U is derived from X.

• Otherwise, 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'} );        // OK: f(A(std​::​initializer_­list<int>)) user-defined conversion

struct B {
B(int, double);
};
void g(B);
g( {'a', 'b'} );        // OK: g(B(int, double)) user-defined conversion
g( {1.0, 1.0} );        // error: narrowing

void f(B);
f( {'a', 'b'} );        // error: ambiguous f(A) or f(B)

struct C {
C(std::string);
};
void h(C);
h({"foo"});             // OK: h(C(std​::​string("foo")))

struct D {
D(A, C);
};
void i(D);
i({ {1,2}, {"bar"} });  // OK: i(D(A(std​::​initializer_­list<int>{1,2}), C(std​::​string("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, 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'} );        // OK: f(A(int,double)) user-defined conversion
f( {1.0} );             // error: narrowing
```

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'} );        // OK: f(A(int,double)) user-defined conversion
f( {1.0} );             // error: narrowing

void g(const double &);
g({1});                 // same conversion as int to double
```

end example]

Otherwise, if the parameter type is not a class:

• if the initializer list has one element that is not itself an initializer list, the implicit conversion sequence is the one required to convert the element to the parameter type; [Example:

```void f(int);
f( {'a'} );             // OK: same conversion as char to int
f( {1.0} );             // error: narrowing
```

end example]

• if the initializer list has no elements, the implicit conversion sequence is the identity conversion. [Example:

```void f(int);
f( { } );               // OK: identity conversion
```

end example]

In all cases other than those enumerated above, no conversion is possible.

Since there are no parameters of array type, this will only occur as the referenced type of a reference parameter.