Given something like

implementation
var     DBConnectionNearby: TSQLConnection= nil;
..
case MainForm.NearbyKind of
   dbkFirebird: DBConnectionNearby := TIBConnection.Create(nil);
   dbkPostgres: DBConnectionNearby := TPQConnection.Create(nil);

is there an elegant way of overriding the Connected property to point to code that can e.g. check that all prerequisite libraries are available before trying to activate the object?

In our previous episode, Mark Morgan Lloyd said:

Check the prequisite libraries yourself, and then if found, initialize the corresponding unit yourself:

e.g. in the case of postgres3dyn using

Function InitialisePostgres3(Const libpath : ansistring) : integer;

I'm doing that. What I'm checking is that I'm not overlooking some way of declaring/implementing DBConnectionNearby, which can be either a TIBConnection or a TPQConnection, with an extension to the Connected property. I suppose that in effect it would be some variant of multiple inheritance.