[boost] Re: safe_bool alternative?

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2003-11-25 04:11:19

David Abrahams wrote:
> Daniel Frey <daniel.frey_at_[hidden]> writes:
> 
>>>>My own feeling is that even if we can't find a solution to those two
>>>>problems then bool_testable is still a sufficiently useful addition to the
>>>>operators library for it to remain.  I think it seems an obvious omission
>>>
>>>>from the operators library - providing all operators except bool
>>>
>>>>comparison. Of course this problem should be reflected in the documentation
>>>>if we cannot solve it.
>>
>>As I said above, documenting Peter's idiom is an alternative. AFAICS
>>the best we have so far...
> 
> It seems to me it should be possible to make bool_testable use
> Peter's idiom.  Am I missing something?

I think Peter's idiom is perfect if applied manually. But if trying to 
use it in a base class implementation, it has some drawbacks. The 
current bool_testable allows the user to provide a plain and simple 
operator bool(), bool_testable adds safety (and operator!()). This 
interface is not possible with Peter's idiom, it needs some special 
callback. Also, the user can still add operator int() or other 
"ambiguous" casts (read: candidated for conversion to bool in absense of 
a real operator bool()), which is also not possible with Peter's idiom.

But instead of writing it down all here, this is probably what should go 
into the operators library's documentation. Or in a separate document. 
Peter? In this case the library would provide documentation, knowledge 
and background information, plus a comparison to alternatives. It cannot 
offer code to include, only help to implement Peter's technique in the 
right way with the right workarounds. In that sense, we could also say 
that the technique is already used in other parts of boost and IMHO a 
fast-track review is therefore justified. It's also a special situation 
as we are not discussing the inclusion of new code, but the removal of 
existing code from the CVS.

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial solutions & technology
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eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de