16 Overloading [over]

16.3 Overload resolution [over.match]

16.3.3 Best viable function [over.match.best]

16.3.3.2 Ranking implicit conversion sequences [over.ics.rank]

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])
Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:
  • 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,
    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
    ]
  • 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 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,
    • S1 and S2 are reference bindings 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,
    • S1 and S2 differ only in their qualification conversion and yield similar types T1 and T2, 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,
    • S1 and S2 are reference bindings, 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
      ]
  • 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:
  • 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);                  // calls f(B*)
      
      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
    ]