I/O Schema Evolution: Automate conversion from Derived* to Base*

[pcanal]

See https://sft.its.cern.ch/jira/browse/ROOT-871?jql=project%20%3D%20ROOT%20AND%20status%20in%20(Open%2C%20%22In%20Progress%22%2C%20Reopened)%20ORDER%20BY%20created%20ASC

I had a class containing a pointer to one of our classes and had to turn this pointer into a pointer to one of its base-class, eg:

Old: class A { TNamed *a; }
New: class A { TObject *a; }{code}

Now I get the following message reading a root file in which I stored an object of the old class with the new class in the dictionary:
Warning in <TStreamerInfo:BuildOld>: Cannot convert A::a from type:TNamed* to type TObject, skip element.

It should be easy to detect that TObject is a base class of TNamed (and both are in the dictionary) so reading the object and setting the pointer correctly should not be very difficult. And would allow simple changes to your classes without loosing compatibility.

[pcanal]

Use derived-to-base.tgz as a reproducer.

[pcanal]

A work-around on case by case basis is to use an I/O customization rule, for example in this case:

#pragma read sourceClass="A" targetClass="A" versions="[1-]" source="TNamed *fValue" target="fValue" code="{ fValue = onfile.fValue; }";

where versions could be replaced by checksums with the value corresponding to the old schema/class-layout.

[pcanal]

Automating this would (probably) require to create a new TStreamerInfoAction that captures as part of its configuration the base class offset needed to adjust the pointer (e.g. for cases where there derived class inherited from multiple base classes). Special care would need to be given to the case of diamond and virtual inheritance.