Re: TMinuit

Hi Navzat et al,

On Sun, 2006-01-29 at 11:15 +0100, Rene Brun wrote: > You have at least 3 C++ coding errors in your code. > In the attachement see lines marked with //<-----

Let me take this opportunity to point out a few things about the ROOT plug-in manager and TMinuit in particular.

In ROOT, there's a number `TVirtual' interfaces, like `TSQlServer', `TVirtualFitter', `TVirtualMC', `TVirtualAuth', `TGenerator', `TFile' and so on.

Most of these (if not all) are transparent interfaces to plug-ins. The concrete implementations implements these interfaces. However, it is advisable _not_ to instantise the concrete implementations in code directly. Instead one should use the static member functions that instantises the interface. For example:

   TFile* file  = TFile::Open("foo.root");
   TFile* file  = TFile::Open("root://127.0.0.1/foo.root");
   TFile* file  = TFile::Open("castor://cern.ch/castor/foo.root");

and so on. The point is, that `TFile::Open' looks for a plug-in that will handle a particular protocol ("", "root:", "castor:"), and instantise the proper concrete implementation. The end-user can customise the exact concrete implementation in `.rootrc'.

The same goes for TMinuit and TVirtualFitter. There's a line in the `.rootrc' which tells ROOT which concrete fitter the _end_user_ prefers:

  # Default Fitter (current choices are Minuit and Fumili).   Root.Fitter: Minuit

So in code, one should _not_ instantise TMinuit directly. So instead of doing

  gMinuit = new TMinuit(2);

one should do

  TVirtualFitter* fitter = TVirtualFitter::Fitter(0, 2);

In that way, the end user is free to choose exactly which fitter class to use. This of course means that one has to adapt the code to the `TVirtualFitter' interface rather than the `TMinuit' interface. However, that is easily done:

   TMinuit call -> TVirtualFitter call
   gMinuit->mnexcm("SET ERR", arglist ,1,ierflg);

-> ierflg = fitter->ExecuteCommand("SET ERR", arglist, 1);
   

   gMinuit->mnexcm("MIGRAD", arglist ,2,ierflg);

-> ierflg = fitter->ExecuteCommand("MIGRAD", arglist, 2);
   gMinuit->mnparm(0, "a1", vstart[0], step[0], 0,0,ierflg);

-> ierflg = fitter->SetParameter(0,"a1",vstart[0],step[0],0,0);
   gMinuit->mnstat(amin,edm,errdef,nvpar,nparx,icstat);

-> icstat = fitter->GetStats(amin, edm, errdef, nvpar, nparx);
   gMinuit->mnprin(3,amin);

-> fitter->PrintResults(3,amin);

Of course, it would be best if `TMinuit', `TFitter', and similar concrete implementations did not export any public constructors, copy constructors, or assignment operators.

Yours,

-- 
 ___  |  Christian Holm Christensen 
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