[odb-users] reuse inheritance with templated base class

[Boris Kolpackov]

Hi Marcel,

Marcel Nehring <mne at qosmotec.com> writes:

> So I would have to split all my persistent template classes into two?
> A template part and a none template part? Actually I would like to 
> avoid that since it would mean a blow up in my number of classes.
> However, if I understand you correctly this would only be a possible
> optimization to reduce code size and to avoid the need to respecify
> pragmas for every instantiation?

Yes, that's correct. For example, if your id member is in the template,
then you will have to specify:

#pragma db member(some_base:::m_id) id auto

for every instantiation. However, if the type of m_id does not depend
on the template argument, then you can put it into a non-template base
class and specify the id pragma only once.

You also don't have to have a separate non-template base for every
template base. I would imagine in a typical situation you would have
the same types for the id and version members for all (or most) of
your classes. So you could place them into a single non-template base
and use it for all of your templates.


> However, yes, it would be great if you could build a pre-release binary. 

Here is the 2.4.0.a3 pre-release:

http://codesynthesis.com/~boris/tmp/odb/pre-release/

The NEWS file entry:

 * Support for defining persistent objects as instantiations of C++ class
   templates, similar to composite value types. For details, refer to
   Section 15.2, "Persistent Class Template Instantiations" in the ODB
   manual.

Let me know if there are any problems.

Boris