13 Overloading [over]

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.

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 copy/move constructor (i.e., a user-defined conversion function) is called for those cases.

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]).

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

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

 — 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 ]

The binding of a reference to an expression that is reference-compatible with added qualification influences the rank of a standard conversion; see [over.ics.rank] and [dcl.init.ref].

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> or “array of X”135 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. 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( {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 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. 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 {
  C(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 ([dcl.init.aggr]), the implicit conversion sequence is a user-defined conversion sequence. [ 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:

• (6.1)

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

   — end example ]

• (6.2)

  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.