I realize that I focused too much on the how, and not enough on
the why.
By "metadata" I mean the data that is "just there" in any object,
in addition to user defined fields.
An example of per-class metadata is the pointer to the the
virtual function table. It is installed by the compiler or the
runtime as part of object creation. It is the same for all the
instances of the same class.
Just like virtual functions, my openmethods library uses "method
tables" and needs a way of finding the method table relevant to
an object depending on its class. I want the library to work with
objects of any classes, without requiring modifications to
existing classes. Thus, there is a need to add that information
to any object, in an orthogonal manner. Openmethods has two ways
of doing this (one actually hijacks the deprecated 'deallocator'
field in ClassInfo) but could profit from the ability to plant
pointers right inside objects.
Examples of per-object metadata could be: a reference count, a
time stamp, an allocator, or the database an object was fetched
from.
