16 Overloading [over]

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:

• (3.1)

  a standard conversion sequence,

• (3.2)

  a user-defined conversion sequence, or

• (3.3)

  an ellipsis conversion sequence.

However, if the target is

• (4.1)

  the first parameter of a constructor or

• (4.2)

  the implicit object parameter of a user-defined conversion function

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

• (4.3)

  [over.match.ctor], when the argument is the temporary in the second step of a class copy-initialization,

• (4.4)

  [over.match.copy], [over.match.conv], or [over.match.ref] (in all cases), or

• (4.5)

  the second phase of [over.match.list] when the initializer list has exactly one element that is itself an initializer list, and the target is the first parameter of a constructor of class X, and the conversion is to X or reference to cv X,

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

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

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

 — end example ]

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.

This subclause defines a partial ordering of implicit conversion sequences based on the relationships better conversion sequence and better conversion. If an implicit conversion sequence S1 is defined by these rules to be a better conversion sequence than S2, then it is also the case that S2 is a worse conversion sequence than S1. If conversion sequence S1 is neither better than nor worse than conversion sequence S2, S1 and S2 are said to be indistinguishable conversion sequences.

When comparing the basic forms of implicit conversion sequences (as defined in [over.best.ics])

• (2.1)

  a standard conversion sequence is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and

• (2.2)

  a user-defined conversion sequence is a better conversion sequence than an ellipsis conversion sequence.

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

• (3.1)

  List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if

  • (3.1.1)

    L1 converts to std​::​initializer_­list<X> for some X and L2 does not, or, if not that,

  • (3.1.2)

    L1 converts to type “array of N1 T”, L2 converts to type “array of N2 T”, and N1 is smaller than N2,

  even if one of the other rules in this paragraph would otherwise apply. [ Example:

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

   — end example ]

• (3.2)

  Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if

  • (3.2.1)

    S1 is a proper subsequence of S2 (comparing the conversion sequences in the canonical form defined by [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,

  • (3.2.2)

    the rank of S1 is better than the rank of S2, or S1 and S2 have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,

  • (3.2.3)

    S1 and S2 are reference bindings ([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);                   // calls g(const int&)
    int k = g(f1());                // calls g(const int&&)
    int l = g(f2());                // calls g(const int&&)
    
    struct A {
      A& operator<<(int);
      void p() &;
      void p() &&;
    };
    A& operator<<(A&&, char);
    A() << 1;                       // calls A​::​operator<<(int)
    A() << 'c';                     // calls operator<<(A&&, char)
    A a;
    a << 1;                         // calls A​::​operator<<(int)
    a << 'c';                       // calls A​::​operator<<(int)
    A().p();                        // calls A​::​p()&&
    a.p();                          // calls A​::​p()&
    

     — end example ] or, if not that,

  • (3.2.4)

    S1 and S2 are reference bindings ([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(&)());               // #1
    int f(void(&&)());              // #2
    void g();
    int i1 = f(g);                  // calls #1
    

     — end example ] or, if not that,

  • (3.2.5)

    S1 and S2 differ only in their qualification conversion and yield similar types T1 and T2 ([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);                  // calls f(const int*)
    

     — end example ] or, if not that,

  • (3.2.6)

    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);                   // calls f(int &)
    int k = g(i);                   // ambiguous
    
    struct X {
      void f() const;
      void f();
    };
    void g(const X& a, X b) {
      a.f();                        // calls X​::​f() const
      b.f();                        // calls X​::​f()
    }

     — end example ]

• (3.3)

  User-defined conversion sequence U1 is a better conversion sequence than another user-defined conversion sequence U2 if they contain the same user-defined conversion function or constructor or they initialize the same class in an aggregate initialization and in either case the second standard conversion sequence of U1 is better than the second standard conversion sequence of U2. [ Example:

  struct A {
    operator short();
  } a;
  int f(int);
  int f(float);
  int i = f(a);                   // calls f(int), because short → int is
                                  // better than short → float.
  

   — end example ]

Standard conversion sequences are ordered by their ranks: an Exact Match is a better conversion than a Promotion, which is a better conversion than a Conversion. Two conversion sequences with the same rank are indistinguishable unless one of the following rules applies:

• (4.1)

  A conversion that does not convert a pointer, a pointer to member, or std​::​nullptr_­t to bool is better than one that does.

• (4.2)

  A conversion that promotes an enumeration whose underlying type is fixed to its underlying type is better than one that promotes to the promoted underlying type, if the two are different.

• (4.3)

  If class B is derived directly or indirectly from class A, conversion of B* to A* is better than conversion of B* to void*, and conversion of A* to void* is better than conversion of B* to void*.

• (4.4)

  If class B is derived directly or indirectly from class A and class C is derived directly or indirectly from B,

  • (4.4.1)

    conversion of C* to B* is better than conversion of C* to A*, [ Example:

    struct A {};
    struct B : public A {};
    struct C : public B {};
    C* pc;
    int f(A*);
    int f(B*);
    int i = f(pc);                  // calls f(B*)
    

     — end example ]

  • (4.4.2)

    binding of an expression of type C to a reference to type B is better than binding an expression of type C to a reference to type A,

  • (4.4.3)

    conversion of A​::​* to B​::​* is better than conversion of A​::​* to C​::​*,

  • (4.4.4)

    conversion of C to B is better than conversion of C to A,

  • (4.4.5)

    conversion of B* to A* is better than conversion of C* to A*,

  • (4.4.6)

    binding of an expression of type B to a reference to type A is better than binding an expression of type C to a reference to type A,

  • (4.4.7)

    conversion of B​::​* to C​::​* is better than conversion of A​::​* to C​::​*, and

  • (4.4.8)

    conversion of B to A is better than conversion of C to A.

  [ Note: Compared conversion sequences will have different source types only in the context of comparing the second standard conversion sequence of an initialization by user-defined conversion (see [over.match.best]); in all other contexts, the source types will be the same and the target types will be different.  — end note ]