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Question about using pointers to fill TClonesArray [message #17672]

Thu, 18 December 2014 15:58

MartinJGaluska
Messages: 203
Registered: March 2010
Location: Germany

first-grade participant

From: *physik.uni-giessen.de

Hello,

I am trying to find and fix memory leaks in my own code and have used code similar to the following line

PndTrackCand* myCand = new ((*fTrackCandArray)[i])PndTrackCand(trackFinder.GetTrackCand(i));

from https://subversion.gsi.de/trac/fairroot/browser/pandaroot/trunk/mvd/MvdT racking/PndMvdRiemannTrackFinderTask.cxx line 162.

delete myCand;

is not present and I am wondering if it is necessary and if not, why.

Kind regards,
Martin

[Updated on: Thu, 18 December 2014 15:59]

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Re: Question about using pointers to fill TClonesArray [message #17673 is a reply to message #17672]

Thu, 18 December 2014 16:49

Klaus Götzen
Messages: 293
Registered: June 2006
Location: GSI

first-grade participant

From: *gsi.de

Hi Martin,

as far as I understand the new at that point is a so-called 'new with placement', i.e. no new memory is allocated, but you write the new object at memory location, which already was use by an object of the same type (and size). This is much faster than new, and you don't need to delete (must not I guess!). This is the general clue of TClonesArrays.

Best,
Klaus

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Re: Question about using pointers to fill TClonesArray [message #17674 is a reply to message #17673]

Thu, 18 December 2014 16:53

MartinJGaluska
Messages: 203
Registered: March 2010
Location: Germany

first-grade participant

From: *physik.uni-giessen.de

Thank you very much for the clarification!

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Re: Question about using pointers to fill TClonesArray [message #17679 is a reply to message #17672]

Thu, 18 December 2014 20:58

Tobias Stockmanns
Messages: 489
Registered: May 2007

first-grade participant

From: *netcologne.de

Dear Martin (and Klaus),

the new with placement creates a pointer at a position inside a TClonesArray. The owner of this object is the TClonesArray fTrackCandArray, myCand is a pointer pointing at the same memory position of the newly created PndTrackCand and is used to add some more information.

fTrackCandArray has to be deleted otherwise you will have in the next event still the PndTrackCands from the prvious event. This is done in the FinishEvent() method where the Delete() of the TClonesArrays is called.

I hope it gets a bit more clear but the TClonesArrays are a bit tricky to handle.

Cheers and a Merry Christmas,

Tobias

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Re: Question about using pointers to fill TClonesArray [message #17681 is a reply to message #17679]

Thu, 18 December 2014 21:26

Klaus Götzen
Messages: 293
Registered: June 2006
Location: GSI

first-grade participant

From: *adsl.alicedsl.de

Hi Tobias,

thanks for pointing out the issue a bit more detailed!

Just like to give one additonal remark: If the objects stored in the TClonesArray don't allocate memory dynamically (like TString, std::vector, ...), you don't even need to call TCA->Delete(), which calls the destructor for all objects, but only TCA->Clear() which doesn't do that and thus is much faster.

However, PndTrackCand owns a std::vector, therefore Delete() has to be called I think.

Btw this is the reason, why the daughters (the only 'dynamic' quantity) of RhoCandidate are stored in an array with fixed size. Although being somehow unelegant, the destructor of RhoCandidate has never to be called by RhoFactory making it really fast. There actually was a severe memory leak before I got aware of that issue ...

Best and merry x-mas,
Klaus

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Re: Question about using pointers to fill TClonesArray [message #17682 is a reply to message #17681]

Thu, 18 December 2014 21:46

Tobias Stockmanns
Messages: 489
Registered: May 2007

first-grade participant

From: *netcologne.de

Dear Klaus,

the TClonesArray operates as you said. They allocate a block of memory which is dynamically extended if more storage is needed but never freed. If the TClonesArray is cleared the memory is internally marked as free. New data is then written into the same position as the previous data without the need to allocate new memory.

This only works if all objects in the TClonesArray have the same size which does not work for objects with container classes inside. For this purpose the TClonesArray uses a trick. It stores a pointer to the container classes while the data of the container classes are stored outside the memory block of the TClonesArray.

If you call a clear() for a TClonesArray with containers inside you get a memory leak only delete() calls the destructor of the container class which frees the memory of the container.

As I said, it is a bit tricky and it took me a while to figure it out.

Cheers,

Tobias

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Re: Question about using pointers to fill TClonesArray [message #17685 is a reply to message #17682]

Fri, 19 December 2014 13:42

MartinJGaluska
Messages: 203
Registered: March 2010
Location: Germany

first-grade participant

From: *physik.uni-giessen.de

Thank you very much for the detailed explanation! It is indeed tricky to get the handling of the TClonesArrays right. The problem I was having with my code was connected with the storing of a class I implemented myself into a TClonesArray. In the end, I realized that I don't need to do that as I have other means of obtaining the output I need for parameter optimization and degugging and removed the corresponding code. This fixed the crashes I saw before. Smile

Kind regards and happy holidays,
Martin

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