Re: [boost] Re: Proposed smart_handle library

From: John Torjo (john.lists_at_[hidden])
Date: 2003-08-05 13:02:45

>
> I might not have followed the discussion to deeply, but it does look to me
> like John is entirely right. Traits can fundamentally do one customization
> per type. That's not going to be enough if you have the same type
> representing multiple handles, as is the case with many C APIs. For
example,
> sockets and file descriptors might be both integers. Or, a variety of
> handles can come in the disguise of a void*. In Windows, if you #define
> STRICT, they use a trick to give different flavors of HANDLEs different
C++
> types (nothing fancy, it's all casts). Still, they forgot to do that for a
> couple of handle types (which were that? HBITMAP? HINTERNET? I can only
say
> HGEEITSBEENALONGTIME). So then after a team I was working with built a
very
> nice and sweet "smart handle" class using traits, we simply couldn't use
it
> because of this particular issue.

Indeed, but what about a wrapper for each time of handle, with a conversion
operator()?

I have used it a couple of times.

Something like:

struct hbitmap_wrapper {
    hbitmap_wrapper( HBITMAP h);
    operator HBITMAP() { ... }
...
};

Then in code, instead of HBITMAP, use hbitmap_wrappers.

Best,
John

>
> With a trait, you can only establish one way of dealing with int and one
way
> of dealing with void*. With a policy, you can define at the exact
> granularity that you want how exactly your handle maps to a type and how
it
> is manipulated - and that policy will be part of the object type. I'm not
> sure that the inheritance-based approach is superior or not, it sure would
> be worth some discussion.
>
> For more reference on traits, here are two articles that might be of
> relevance:
>
> http://www.moderncppdesign.com/publications/traits.html
> http://www.moderncppdesign.com/publications/traits_on_steroids.html
>
>
> Andrei
>
>
>
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