[boost] Re: safe_bool alternative?

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2003-11-24 14:20:24

Douglas Gregor wrote:
> On Monday 24 November 2003 12:16 pm, Sam Partington wrote:
> 
>>I see two problems with the current implementation of bool_testable :
>>
>>1/ The is_convertible problem above.
>>
>>2/ Binary logical expressions with a bool_testable derived class on either
>>side are ambgious in VC6, VC7, and VC7.1.  Thus:
> 
> [snip]
> 
> I wouldn't hold this against bool_testable; it seems obvious to me that VC is 
> wrong here.

*nod* :)

>>Can we work around these problems?  The original ptr-to-member design
>>avoids both of them, but introduces a problem if the user provides their
>>own integer conversion operators.
> 
> There is no perfect solution, I'm afraid.

I guess the only solution is providing documentation for Peter's idiom 
and show why users should use it instead of any other "solution" :)

>>Can we use enable_if or is_convertible to provide a best fit solution for
>>the class in question?
> 
> Other than hacking is_convertible to first perform an is_base_and_derived 
> check to deal specifically with bool_testable, I don't see a good way around 
> this.

I also thought about this but it feels screwed. I don't want any 
dependency for is_convertible to the operators library. Even if 
is_convertible would offer an open plug-in architecture, this doesn't 
feels right, although further investigating might be an option... 
anyway, we have a better alternative...

>>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...

> extra work to make it pass. Assuming we can't find a clean is_convertible 
> workaround, at what point is it just easier to follow the ptr-to-member 
> idiom?
> 
> Another (minor?) issue with bool_testable: either the documentation (or, 
> better yet, the code) should note/enforce that bool_testable types should 
> have operator== or operator!= unless the user explicitly creates them (see, 
> e.g., the "poisoned" operator== and operator!= in the Function library). I'm 
> not sure if this would suffice:

Both idioms don't have a problem with operator== / operator!=, so they 
are equally good at that point. Two arbitrary bool_testables won't 
compare as the comparison is ambiguous (bool or int?). Or were you 
refering to a different example (or am I missing something)?

Regards, Daniel

PS: I will ask Thomas for the fast-track-review...

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