"Forward" compatibility means that it is possible to read old data files
with the new (probably modified) code. "Backward" compatibility means that
it is possible to read *new* data files with the old version of code.
Apparently ROOT support of schema evolution - with Streamer(..) methods
which could be written by a user - provides "forward" compatibility.
Let's now assume that we adopt the following convention: whenever a class
definition is modified, its Streamer method either is not being changed or is
only allowed to append new words to the saved (persistent) part of the object.
In this case it is logically possible to read the datafile written with the
new version of the code using the old version (in this case one can't
use appended information - but the old version of the code "doesn't know"
about it anyway).
The only thing needed is to increment byte counter in an I/O buffer in a way
independent on the user, when reading the next object - object in the buffer is
longer than its part read by an "old" Streamer() method.
Although this possibility is not something one needs urgently,
in a long time scale it might be important to have it implemented.
Regards, Pasha.