thanks everybody. i got the stuff working, but with a lot of hassle. the biggest problem is that C++ doesn't clearly differentiate bool from int. Thus, a statement like while (len--) requires an operator bool(), but that really just introduces implicit conversions to int, which leads to huge numbers of ambiguities, major yuck. to avoid all the ambiguities and other problems, i had to end up declaring both the operations i wanted allowed [as public], and those i wanted disallowed [as private] -- and i mean *ALL* of them -- short, unsigned short, int, unsigned int, long, unsigned long, etc. etc. etc. to avoid these dreaded ambiguities. what a major hassle -- and clearly this needs to be rethought somewhat in the C++ design so that things like this are easier to create.

Jan Vroonhof wrote:

Ben Wing <ben@666.com> writes:

possible with an explicit cast, e.g. (Bytecount) x.  Problem is, the
"cast operator" that can be defined on a class seems to define
implicit casts, not explicit.  Is there any way to do what I want?


Yes you can.. C++ has an "explicit" keyword for this purpose.
The below example does it by defining an explicit copy constructor and
letting the compiler generate the conversion operators from that.

[Note that if you allow the uncommented conversions to int below, you
not strictly need the explicit copy constructors as]


struct charcount;
struct bytecount;

struct charcount {
  int count;

  charcount() { }
  charcount(int i) : count(i) { }
  // Allow type conversions
  explicit charcount(bytecount bc);
  int getcount() { return count;}
  // operator int() { return getcount(); }
};

struct bytecount {
  int count;

  bytecount() { }
  bytecount(int i) : count(i) { }
  // Allow explicit type conversions through copy
  explicit bytecount(charcount cc) : count(cc.getcount()) { }
  int getcount() { return count ;}
  // operator int() { return getcount(); }
}
;

inline charcount::charcount(bytecount bc) : count(bc.getcount()) { }

void foo(bytecount b);

int main()
{
  bytecount b1 = 50;
  bytecount b2;
  charcount c1 = 100;
  // Explit conversions (C style)
  c1 = (charcount)b1;
  b2 = (bytecount)c1;
  foo((bytecount)c1);
  // Explict conversion (C++ style)
  c1 = charcount(b1);
  b2 = bytecount(c1);
  // Conversion to int works if the convertor operators are defined above
  // int i1 = c1;
  // int i2 = b1;
  // These fail to compile because
  // 1. We made the copy contructor explicit
  // 2. Convertsion via int requires two conversions, if defined
  c1 = b1;
  b2 = c1;
  foo(c1);
}

cd /tmp/
g++-3.0 bytecount.cpp -o bytecount
bytecount.cpp: In function `int main()':
bytecount.cpp:49: no match for `charcount& = bytecount&' operator
bytecount.cpp:5: candidates are: charcount& charcount::operator=(const 
   charcount&
amp;)
bytecount.cpp:50: no match for `bytecount& = charcount&' operator
bytecount.cpp:16: candidates are: bytecount& bytecount::operator=(const 
   bytecount&)
bytecount.cpp:51: conversion from `charcount' to non-scalar type `bytecount' 
   requested

Compilation exited abnormally with code 1 at Sun Apr 14 16:28:06