[odb-users] Re: Handling pimpl idiom in ODB

[Uwe Kindler]

Hi Boris,

I think your first approach, making only the 'impl' class an ODB object 
will not work. Because then I cannot use the public interface class in a 
relation because ODB does not know anything about the interface class. 
That means I cannot have another persistent class that has a one to many 
or one to one relation to my COdbPimplTest interface class. Exactly this 
was the reason I made the private impl class as well as the public 
interface class ODB objects.

When I first thought about this problem, I tried to solve it with a 
composite value type. That means I thought of the private impl class as 
a composite value type. But it was not possible to use a pointer to a 
composite value type so I decided to go the persistent object way.

Regarding the second approach I have a question. The keyword virtual 
does not mean, that it will be a virtual method, or does it? If it would 
be a virtual method then it would not work because virtual methods also 
break binary compatibility. So I assume the keyword virtual simply 
identifies virtual ODB members.

As long as the accessor and modifier expressions are non virtual private 
or protected C++ methods, I don't see any issues at the moment. The 
problem with the second approach is, that it would cause a lot of 
pragmas for the accessors and modifiers. At the moment one big plus for 
ODB is that I can make a class persistent with a minimum number of changes:

Often

#pragma db object
#pragma db id auto
friend class odb::access;

are the only thing you need to make a class persistent. An then I would 
need to write a pragma for each member. This won't make the more 
beautiful ;O). But if it works, I think it would be o.k.

What do you think about the composite value approach. Would it be 
possible to implement pointers to composite values?

Kind Regards,

Uwe