Hi Fons,
I think the current implementation of copy constructor in TCollection
is correct. Only user can decide what does he want in copy.
Copy only pointers or copy objects. User can always inherit his collection
from TObjArray or TList and implement his own copy constructor.
Eight now, if user accidentally trying to copy collection, he has
error in linkig and aware, that he must do the decision.
And it is good.
So I think it is better to leave it as it is.
Victor
Fons Rademakers wrote:
>
> Hi George,
>
> I'll provide these missing copy ctor and assignment operators for the
> next release for most if not all containers (note TObjArray already has
> a copy ctor). So keep your nice scheme alive and it should work soon.
>
> Cheers, Fons.
>
> On Wed, Aug 30, 2000 at 05:12:12PM -0400, George Heintzelman wrote:
> >
> > Rooters,
> >
> > I was just trying to write a nice filtered collection template:
> >
> > template <class T, class Collection=TOrdCollection>
> > Collection FilteredList(const TCollection &l) {
> > Collection mylist; // Maybe should specialize with a preset capacity
> > // for classes which have that facility...
> > // but that's an optimization for later.
> > TIter Next(&l);
> > while (TObject *obj=Next()) {
> > if (T *filtered_obj = dynamic_cast<T *>(obj)) {
> > mylist.Add(filtered_obj);
> > }
> > }
> > return mylist;
> > }
> >
> > for use in routines that want to find the best object in a collection
> > satisfying certain criteria; and I didn't want to write the code over
> > and over in different filtering places. Aha, a template, I thought --
> > and a smart compiler will optimize out the copy constructor of the
> > return value (as its allowed to do) in most cases, so I don't feel too
> > bad about returning the object itself since I will only construct it
> > once, really.
> >
> > Then I compiled and discovered that TCollection::TCollection(TCollection
> > &) is *private* and not implemented, and thus this is true also for
> > all its descendants. I understand this idiom as preventing copying
> > (even in cases where the constructor could be optimized out) -- but
> > why? The comment says 'TCollections are too sensitive', but I don't see
> > how -- the collections don't own their contents, you're just copying a
> > bunch of pointers. Sure, it might lead to a few inefficiencies if
> > people use it incorrectly or without thinking about the copy
> > constructors... but that's not a reason to PREVENT the activity, and
> > even then with modern computing power the copying of a few, or a few
> > hundred, or even a few thousand pointers is of very little impact
> > except inside inner loops, where profiling ought to catch these kinds
> > of mistakes. Is there something terribly dangerous about copying
> > collections that I'm missing?
> >
> > Obviously, I can't use Clone() -- I certainly don't want to copy ALL
> > the contents, just the pointers -- and I don't want to return pointers,
> > since I don't want the user to be forced to delete the pointer
> > afterwards (despite the fact that a lot of ROOT code works this way, I
> > find this idiom odious and to be avoided if at all possible*). So what
> > should I do?
> >
> > N.B. I was trying to do this as a way to go around (in a slightly more
> > inefficient way) the problem I mentioned some weekes ago with TIter and
> > TIterator, which prevents you from defining TIterator classes not
> > connected 1-to-1 with a TCollection class. The combination of not being
> > able to do either, and being prevented by local policy from using STL
> > containers (which do supply copying) is producing a great deal of
> > frustration here.
> >
> > George Heintzelman
> > gah@bnl.gov
> >
> > * And besides, it makes this really nice syntax impossible:
> >
> > GetBestFrom(FilteredList<TSomeClass>(mylist));
> >
> > Instead, I have to make a local pointer, pass it, and delete it. Never
> > mind exception safety, of course...
> >
> > GAH
> >
> >
> >
> >
>
> --
> Org: CERN, European Laboratory for Particle Physics.
> Mail: 1211 Geneve 23, Switzerland
> E-Mail: Fons.Rademakers@cern.ch Phone: +41 22 7679248
> WWW: http://root.cern.ch/~rdm/ Fax: +41 22 7677910
--
Victor M. Perevoztchikov perev@bnl.gov perev@vxcern.cern.ch
Brookhaven National Laboratory MS 510A PO Box 5000 Upton NY 11973-5000
tel office : 631-344-7894; fax 631-344-4206; home 631-345-2690
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