Re: [boost] Iterator facade example could use some changes/documentation adjustments

Subject: Re: [boost] Iterator facade example could use some changes/documentation adjustments
From: Jeffrey Hellrung (jhellrung_at_[hidden])
Date: 2010-02-10 14:33:13

Jacob McIntosh wrote:
> My example here:
> http://codepad.org/pH3pXAvb
> 
> As mentioned in the comment block at the codepad page:
> http://www.boost.org/doc/libs/1_41_0/libs/iterator/doc/iterator_facade.html#wrap-up
> 
> The above link demonstrates in a linked-list scenario, using
> is_convertible to handle
> the conversion from non-const iterators to const iterators.  This
> works fine in the
> example's linked list scenario, but it's likely than many users of
> iterator_facade
> will more generally copy this usage to their own container, from the example.
> 
> However, as my example demonstrates, in the case where the data lives
> in contiguous
> memory locations, or perhaps other scenarios, using is_convertible is very bad
> for this purpose.  It lets you convert from a derived class pointer to
> a base class
> pointer, and this is problematic if sizeof(base) is less than sizeof(derived)
> 
> See the below, very minimal example to illustrate my point.
> 
> Look at CFixedTest for my proposed replacement for is_convertible to
> prevent this
> undesirable behavior.
> 
> 
> 
> Seems to me that noting such behavior, or simply changing the example
> code to not
> allow the conversion (as my example does), would remedy the
> misunderstanding/problem.
> 
> Just thought I'd contribute that - thanks,
> nacitar

I think this is a valid concern, though I'm guessing this would cause 
problems at most infrequently.  I'm having difficulty coming up with a 
use case where this would be a problem.  Did you stumble upon this while 
writing "real" code?

Would this motivate the existence of an is_iterator_convertible 
metafunction, which is identical to is_convertible except for pointers?

- Jeff