[ROOT] Clarification needed

From: Laurent Aphecetche (aphecetc@in2p3.fr)
Date: Thu Oct 18 2001 - 17:59:37 MEST 

Hi,

A very generic (and very naive probably) question about ClassDef and
ClassImp. All my apologies if this is really trivial.

Assume I have a class A deriving from TObject :

class A : public TObject {
public:
  virtual void SomeMethod(void) = 0 ;
} ;

and a class B deriving from A : 

class B : public A 
{
public:
  B() : A() { }
  virtual ~B() { }

  virtual void SomeMethod(void) { cout << "fSize" << endl ; }

private:
  Int_t fSize ;

  ClassDef(B,1)
} ;

Class A is meant to be just an interface (and to provide a
"TObject"-type), no I/O intended, but I'd like the following to work
under the ROOT prompt :

root[0] A* a = new B() ;
root[1] a.SomeMethod() ;

i.e. my class A must be in the dictionnary.

Class B is meant to be a persistent object, so I put a ClassDef/ClassImp
in there.

So here are the questions then :

a) should I put a ClassDef(A,i) (i=0 ?) in class A ?

b) what should be the corresponding LinkDef.h file for those two classes
in order to i) have access to the A methods from the root prompt and ii)
being able to write B objects into a ROOT file.

I made some tests and would conclude that I must put a ClassDef(A,0) and
use 

#pragma link C++ class A;
#pragma link C++ class B; // or B+;

if I want my stuff to work. 

BTW, in real life, class A is not one I get much control on ; it does
not have a ClassDef, and uses 

#pragma link C++ class A-! ;

Is that wrong or not ?

Thanks for your help,

Best regards,

PS: I'm using ROOT 3.01/05 on a Linux box.
-- 
Dr. Laurent APHECETCHE (mailto:aphecetc@in2p3.fr) (IN2P3-CNRS) 
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TEL (+33/0) 2 51 85 84 17 - FAX (+33/0) 2 51 85 84 24 (France)
Collaborations PHENIX http://www.phenix.bnl.gov/~aphecetc et MEGAPIE.

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