THistSvc

The latest changes (2018-03-08) of the THistSvc provide two improvements:

1. Threadfriendly management of TObjetcs (histograms, graphs, trees)

1. Better memory management through communicating ownership with std::unique_ptrs in the interface

Parallel THistSvc

Parallel usage of the THistSvc is possible through either:

1. Use regHist() to register a clone for each involved thread that can be merged in the end. (e.g. during finalize of an Algorithm/Service ...)

1. Use regSharedHist() to register a single instance for all threads that is mutexed and accessed through a LockedHandle<>

Both approaches have their use cases, but for moderate histogram usage during a normal job of some kind, sharing a single instance is probably best.

In future ROOT versions (ROOT 7?) it might be possible to introduce further methods of parallelized histograms. Histograms with atomic bins looks very promising.

About ownership

You should register your TObejects to the THistSvc through moving std::unique_ptrs into the reg* method. The THistSvc manages these objects and is also responsible for writing these to ROOT files etc.

If your usage looked like this before:

TH1D* hist = new TH1D(...);
regHist( "name", hist );
hist->Fill(...);

you should replace it with:

std::unique_ptr<TH1D> hist = std::make_unique<TH1D>(...);
TH1D* histptr = hist.get();
regHist( "name", std::move(hist) );
histptr->Fill(...);

or even

std::unique_ptr<TH1D> hist = std::make_unique<TH1D>(...);
regHist( "name", std::move(hist) );
TH1D* histptr = getHist( "name" );
histptr->Fill(...);

This way it is clear that the THistSvc takes ownership of these objects. Working on the returned raw pointers is no issue as long as the THistSvc is still present (until finalize is called) and as long as the user does not do anything malicious with it (i.e. deleting it).