Schema evolution of base class member

[uhlig]

Hi,

I have a situation where I want to change the value of the base class member fAddress when an old version of the derived class C is read from file. Class B and C also contain data members I don’t want to touch.

class C: public B {
};

class B: public A {
};

class A: public TObject {
  Int_t fAddress; 
};

I tried to add the pragma read statement

#pragma read sourceClass="C" version="[1]" targetClass="C" \
  source="Int_t fAddress" target="fAddress" \
  code="{ \
         fAddress = onfile.fAddress+100; \
         }"

in the corresponding LinkDef.h file but whatever I tried I always get a a warning that the data member specified as target does not exist.

Is such change of a base class member possible and in case it is what is the correct code to do it.

[pcanal]

The target indeed has to be in the target Class proper. You might be able to use:

#pragma read sourceClass="C" version="[1]" targetClass="C" \
  source="Int_t fAddress" target="" \
  code="{ \
         newObj->fAddress = onfile.fAddress+100; \
         }"

If this is not working, you might need to use an arbitrary data member of C as the target and set both that member and fAddress.

[jalopezg]

Thanks for replying, @pcanal.

[uhlig]

Unfortunately your proposal does not solve the issue completely.

With the proposed change I don’t get the warning any longer.
When accessing the variable in the #pragma read statement it does not have the proper value stored in the file but has the value from the default constructor, which in my case is -1 to indicate problems.

[pcanal]

Can you provide a complete running (and failing) test?

[uhlig]

I will try to provide some code which demonstrates the problem next week.