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Forum: Geometry
Re: Question Lifetime of touchables (Nicola Mori)
Re: Warning Re: Lifetime of touchables (Michael H. Kelsey)
Date: 04 Aug, 2014
From: John Apostolakis <John Apostolakis>

To address the original question, about the lifetime of touchables:

It is possible to get a smart pointer (handle) to a touchable using a very similar method:

G4TouchableHandle touchableHandle = aStep->GetPreStepPoint()->GetTouchableHandle();

When you get and keep an object of this type (handle), it acts as an anchor which maintains in memory not only the touchable to which it points, but also the necessary objects of which it is created.

One important caveat: for different tracks that encounter the same volume a different touchable will be created by the Navigator, so you will get a different handle which will point to a different touchable that contains the same information.  

There are some important consequences:
  - you cannot do a simple comparison of pointers (or handles) to identify whether you are referring to the same touchable volume.  A deep comparison is required to identify the equality of the touchables to which the handles point;
  - the amount of memory required to hold these smart pointers (handles) is likely significant. 

[ I mention that a different/revised version of the navigator which uses a single touchable for all tracks is under development by a colleague, who has created already a prototype implementation. Developments regarding its potential integration are under discussion. I note that this was *not* included in the last 10.1-beta development release. ] 

- John Apostolakis

On Aug 2, 2014, at 12:28 AM, "Michael H. Kelsey" <kelsey@slac.stanford.edu>
 wrote:

> 
> *** Discussion title: Geometry
> 
> On Fri, 01 Aug 2014 13:32:10 GMT, Nicola Mori wrote:
> 
>> Hi, to identify volumes where hits happen I'm using the address of the
>> corresponding touchable:
>> 
>>  const G4VTouchable *touchable = aStep->GetPreStepPoint()->GetTouchable();
>> 
>> In my hit object I record this address during
>> G4VSensitiveDetector::ProcessHits, but in my EndOfEventAction() it seems
>> that the memory location pointed by this pointer contains invalid data.
> 
> You can't do that. Steps and Tracks only have lifetime while they are
> actively involved. What's more, those objects, and some of their
> constituents, are allocated from fixed memory pools using the
> "G4Allocator" method, with the result that a cached address might point
> to a valid object of the type you care about, but have completely
> different content.
> 
> What you need to do in your ProcessHits() class, is make use of that
> Touchable to extract whatever information (data) you actually plan to
> store, rather than trying to defer that activity until later.
> 
>  -- Michael Kelsey
> 
> -------------------------------------------------------------
> Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or unsubscribe) at: 
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