Email to maus-devs on 09/06/2011

So there are a few places where we need or want global objects. Examples
are:

Iniitialise once per run per process:
MiceModules
Field maps
Virtual planes

Initialise once per process:
Geant4 (would like to be once per run, but not allowed)
Configuration cards
Logging
ErrorHandler

May be some stuff in recon?

At the moment they are implemented as singletons or just as some random
thing. So singleton is a C++ way of doing a global variable. Have a look
here:

http://www.devarticles.com/c/a/Cplusplus/C-plus-plus-In-Theory-The-Singleton-Pattern-Part-I/
http://www.devarticles.com/c/a/Cplusplus/C-plus-plus-In-Theory-The-Singleton-Pattern-Part-2/

The problem with all this singleton stuff is that memory allocation and
deallocation is a pain. Two problems:

• Memory deallocation order needs to be handled carefully (what if I
  want to use Logging while deallocating Geant4? Which one was deleted first?)
• Memory allocation can get screwed up in multithreading - two threads
  can attempt to allocate memory at the same time - leading to problems:

http://www.bombaydigital.com/arenared/2005/10/25/1

We live with this right now, but we will suffer if we try to
multithread, and we will leak memory per run per process.

My plan is to introduce two "configuration managers" - one that exists
for the duration of a run (say MAUSRun), one that exists for the
duration of a process (say MAUSProcess).

MAUSProcess has accessors to the per process objects and the MAUSRun
MAUSRun has accessors to the per run objects

If we multiprocess, we make multiple copies of this stuff.

Plan? Comments?