Re: [boost] [type traits] add a set of templates to propagate const etc from one type to another?

Subject: Re: [boost] [type traits] add a set of templates to propagate const etc from one type to another?
From: Daniel Larimer (dlarimer_at_[hidden])
Date: 2010-03-02 13:47:33

Along the same lines... I find I need a "remove_const_ref"  template all of the time.  For now I have been using
a macro RCR() that uses remove_const and remove_ref.  There is a balance between having orthogonal features and having convenience features.   I don't know where that line is.  Surely you do not want to enumerate every possible combination of add/remove.

Dan

On Mar 2, 2010, at 10:27 AM, Vasilis Vasaitis wrote:

>  Hello,
> 
>  I've been looking at the Type Traits library recently, and it looks
> wonderful overall. However, it's lacking a use case that I end up
> needing quite often, so I thought I'd suggest it in case the authors
> and/or other users of the library find it useful. Allow me to explain.
> 
>  So Type Traits has the add_const / remove_const / is_const triplet
> (and similar for a few other type properties). add_const is great when
> you always know that you want to add the const, but sometimes you only
> want to do it conditionally to another type being const. So recently
> I've ended up using something like this from quite a few places in my
> code:
> 
> template<class Source, class Target>
> struct propagate_const {
>    typedef Target type;
> };
> 
> template<class Source, class Target>
> struct propagate_const<const Source, Target> {
>    typedef const Target type;
> };
> 
>  That is, propagate_const<Source, Target> will only add const to
> Target if Source is const too.
> 
>  What are the use cases for this? Well, for example recently I was
> writing an iterator class, and I wanted to avoid duplicating the code
> for the const_iterator case. Not a big deal, as it turned out, as I
> could just pass a const type as the template argument and have it
> behave as the const iterator. The only problem was that I was doing
> some pointer arithmetic with reinterpret_casts from T* to char* and
> back (where T is the template argument), so the compiler would
> complain if T was const. So instead of char I defined this:
> 
> typedef typename propagate_const<T, char>::type char_type;
> 
>  Of course I could also have done this with a combination of is_const
> and add_const, but this would probably require more code than the
> propagate_const definition above.
> 
>  So what do people think? Would it be a useful addition alongside
> add_* and remove_*?
> 
> Thanks,
> Vasilis
> 
> P.S. Please CC me in your replies, as I've set the mailing lists to
> not send me emails (similar to not being subscribed).
> 
> -- 
> Vasilis Vasaitis
> "A man is well or woe as he thinks himself so."
> 
> 
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