Hi Victor,
of course that is exactly why I did what I did in the first place :-)
Do more people have an opinion on this subject?
Cheers, Fons.
Victor Perevoztchikov wrote:
>
> 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
--
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
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