• Daniel James (31/33) Aug 24 2004 The 'official' implementation of boost::is_convertible works by

Daniel James <daniel calamity.org.uk> writes:

The 'official' implementation of boost::is_convertible works by 
overloading on the ... operator. I managed to come up with a workaround 
which seems to work fine and has been accepted into boost, so this isn't 
particularly important.

Anyway, David Abrahams wrote:

 It's pretty simple.  a (...) parameter is always a worse match than
 anything else that matches, including template parameter types.

In the Digital Mars compiler it seems to be equal to certain 
user-defined conversions. All the overloads in the following code should 
not be ambiguous:

     #include <cassert>

     struct convertible_to_char { operator char() { return 0; } };
     struct convertible_from_char { convertible_from_char(char) {} };

     struct any_conversion
     {
         template <typename T> any_conversion(const volatile T&) {}
     };

     void check1(any_conversion ...) { assert(0); }
     void check1(float, int) {}
     void test1() { convertible_to_char x; check1(x, 0); }

     void check2(any_conversion ...) { assert(0); }
     void check2(convertible_from_char, int) {}
     void test2() { float x = 0; check2(x, 0); }

     void check3(...) { assert(0); }
     void check3(char) {}
     void test3() { convertible_to_char x; check3(x); }

     void check4(...) { assert(0); }
     void check4(convertible_from_char) {}
     void test4() { char x = 'a'; check4(x); }

     int main()
     {
         test1(); test2(); test3(); test4();
     }