[Q] TClonesArray

ROOT
1

Hello all,

I’m new to root, and have some questions on TClonesArray impl details
(currently using v3.10/02, may not be easy to change version as I need to
match the version in our library system)

(1) user’s guide (latest version) and on-line documentation may not match?
on-line doc (generated by header file?) :
objects to be stored need to be same type (class)
user’s guide : same type and fixed same size <== may be out dated?
as I see there was some updates to allow TClonesArray can store not just
fixed size (but same type) in the release note some time ago?
(2) In user’s guide (or on-line doc), it’s not clear what requirements for objects to be
stored in the TClonesArray (or TCollection): e.g. Object Class need to define
default ctor(), …, but not necessarily dtor(), copy assignment, operator=()?
And it’s not clear what will happen when I store objects in my compiled application,
root interactive version … etc. Say
(a) In my compiled exe application to store objects (MainEvent.cc like), both
ctor() and dtor() are called (as I call Delete() for every event)
(b) It seems to me that if I loop my TClonesArray using TTree::GetEntry() method
only ctor() of objects is called, but not dtor(). Also, if I call Clear() before Delete()
dtor() not called. How memory will be free-ed?
© TTree::Draw() on TClonesArray – similar to T->Draw(“fTrack.fPX”) in user’s
guide example (for test/Event example) – ctor()/dtor() never called. I wonder
why and how memory will be handled?
(3) The primary reason why I use TClonesArray is to store array of objects (like Event/Track
example), but I still want to use TTree::Draw() on member of array. I tried TObjArray
but TTree::Draw() does not seem to support it ? (at least in v3.10/02)
Draw() may not call Clear(), but GetEntry() loop need to call Clear() … what’s the
difference? Memory will be properly free-ed in both case?
(4) I wonder why TClonesArray member (Track) in mother class(Event) are pointer to
TClonesArray? I made my member to have TClonesArray not a pointer to it to simplify
memory allocation process … etc. It seems to work: say
class Event : public TObject {
TClonesArray fTracks;
…
Do I need to make it “TClonesArray *fTracks”, as in the test/Event example? Then, why?
Also, SetOwner() shouldn’t be called for TClonesArray?
(5) What if I want to make Array of objects which have array of objects(say TRefArray or
another TClonesArray), are they supposed to be suppored in v3.10/02? I did some
test and it seem to be working but it’s not clear how memory alloc/de-alloc are done
to me.

Regards,
Chul Su

2

You are correct. The User’s Guide is outdate on this point.

This is the only absolutement requirement (but you also need to generate the rootcint dictionary).

quote It seems to me that if I loop my TClonesArray using TTree::GetEntry() method
only ctor() of objects is called, but not dtor(). Also, if I call Clear() before Delete()
dtor() not called. How memory will be free-ed? [/quote]
Do not call Delete on a TClonesArray. Instead call Clear(“C”) and implement the Clear method on your objects (which should delete the memory owned by your object but not your object themselves). The memory for the objects themself in NOT free (The whole purpose of TClonesArray is to minimize the new/delete that are needed!).

quote TTree::Draw() on TClonesArray – similar to T->Draw(“fTrack.fPX”) in user’s
guide example (for test/Event example) – ctor()/dtor() never called. I wonder
why and how memory will be handled? [/quote]The memory is allocated by the TClonesArray at its creation and deleted as its deletion.

[quote] I tried TObjArray
but TTree::Draw() does not seem to support it ? (at least in v3.10/02) [/quote]
TTree::Draw does not yet support TObjArray.

[quote] I made my member to have TClonesArray not a pointer to[/quote]That’s fine.

A TClonesArray is ALWAYS the owner of the memory for its content.

Yes it is. See the TClonesArray::Clear for details on the memory usage.

Cheers,
Philippe.