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

Subject: Re: [boost] [type traits] add a set of templates to propagate constetc from one type to another?
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-03-03 13:04:24

Jeffrey Hellrung wrote"
> Stewart, Robert wrote:
> > John Maddock wrote:
> >>> I too have needed and created propogate const (actually
> >>> propogate const
> >>> and/or volatile). I'd definitely appreciate such an addition to
> >>> Boost.TypeTraits.
> >> My gut feeling would be to call this "copy_cv" and it would
> >> do what it's name suggests, take a type and make it's
> >> cv-qualifiers the same as some other type.
> >
> > Nice name.  The "cv" part is right on target, and "copy" is
> > short and clear.
> >
> > If one wants only const or volatile, one need only combine
> > copy_cv with remove_volatile or remove_const, so copy_const
> > and copy_volatile are not required.  However, since there is
> > remove_const, remove_cv, and remove_volatile, wouldn't it be
> > less surprising to add copy_const, copy_cv, and copy_volatile?
>
> This depends on how you define copy_const.  I've used it
> where it simply
> adds (and never removes) const and volatile qualifiers on the
> "To" type,
> so remove_volatile< copy_cv< From, To >::type >::type isn't
> the same as
> copy_const< From, To >::type when To is volatile-qualified.

Note that John wrote about making To have the same cv-qualification as From, above.  Hence, he called it "copy_cv."  Your example, which is perhaps what John meant, is to make sure that To has at least as much cv-qualification as From, without removing any extra To already had.  That suggests "augment" or "append."

However, if From already is const, copying "const" from To to From won't make From any more const, so "copy" does capture the augmentation idea pretty well.

All that to say that I understand your point.  The question is whether we need both forms.  If so, then we need a way to distinguish the two.

> Also, I, too, use an implementation called "propagate_const" and
> "propagate_cv".  To me, "propagate" seems to better evoke the pure
> addition of qualifiers, whereas "copy" *could* (to some
> people) mean the
> overwriting of qualifiers (at least more so than "propagate"):

"Propagate" is probably not a good choice because it doesn't quite imply the addition to existing state you mean, at least to me.  Furthermore, "propagate" is frequently misspelled as "propogate."  "Augment" seems a bit stilted, but it does capture the idea well.  "Add" actually captures the idea pretty well, but we can't use that.

If we look at it from the other side, what John wrote above is about replacing From's cv-qualification with that of To, so "replace" works for that, which leaves "copy" for the augmenting form.

Therefore, copy_cv, copy_const, copy_volatile make From have at least To's cv-qualification, constness, or volatileness, respectively, while replace_cv, replace_const, and replace_volatile make From have To's cv-qualification, constness, or volatileness, respectively.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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