Re: [boost] Re: [Formal review] Singleton library

From: Sam Partington (sam.partington_at_[hidden])
Date: 2005-05-10 10:23:50

On 5/10/05, Pavel Vozenilek <pavel_vozenilek_at_[hidden]> wrote:
> 
> "Dan Rosen" wrote:
(snip)
> > 6. The exists(), is_null() and operator bool() methods fall prey to a
> > well known C++ pitfall. Have a look at Bjorn Karlsson's article, "The
> > Safe Bool Idiom": http://www.artima.com/cppsource/safebool.html.
> >
> Looks so. It would be nice to have a common support for this
> in Boost, though.

I suggested this as an addition to the operators library sometime ago,
and patches to the code/docs/tests were applied, but they were later
removed. I'll to summarise why.

The first suggestion, which was based on Peters pointer to member
function conversion :
http://lists.boost.org/MailArchives/boost/msg44416.php
(the patches)
http://lists.boost.org/MailArchives/boost/msg44529.php

But then Daniel Frey found a problem with the implementation for
classes with integral conversion operators of their own. The integral
conversion operator was a better match and so the safe-bool conversion
was never called :

http://lists.boost.org/MailArchives/boost/msg44894.php

So then we tried an implementation based on using a private integral
conversion operator and a public operator bool :

http://lists.boost.org/MailArchives/boost/msg45089.php

This worked well in itself, but caused problems with the type traits
is_convertible (sorry, don't seem  to be able to find a link to any of
those messages).

Finally the whole topic was dropped after a long thread, and the code
was removed from the CVS tree.  A shame I think though, as there were
still several open designs which had possiblities.  The end of the
discussion is here :

http://aspn.activestate.com/ASPN/Mail/Message/boost/1900317

I'd like to see the solution back in boost, we've been using it here
for a couple of years now without problems, and we find it very
useful.  I'd be willing to put more work into it if there is interest.

Sam