• bearophile <bearophileHUGS lycos.com> Sep 05 2010

bearophile <bearophileHUGS lycos.com> writes:

Don:
 That's impossible. That would make interfacing to C a nightmare.


I see.


 By the way, enums which consist of flags frequently have values which 
 have more than one bit set. The case { A=1, B=2, C=4, D=8 } is only a 
 special case.


That indeed happens, but from the code that I have seen the most common case is
the one with powers of two flags. This is why I think it's useful to add the
flagset struct to Phobos, because it avoids/catches some common bugs.


 In the general case, it's not realistic to hope that the compiler could 
 determine which values are valid, and which are not.


Right. But a flagset helps because:
- It gives a very short definition syntax for the common case when flags are
power of two.
- In that case it removes the mistakes in power-of-two flag and the missing
dummy (0) flag.
- The struct invariant is able to catch few runtime bugs because it tests that
the unused bits in the underlying value are always zero.


 But, with my suggestion, simple enums (a list of mutually exclusive 
 values) would become strong enums.


This is better than the current situation. I'd like Walter to give a word on
this thread.

Bye,
bearophile