Data Hiding
If we use centralized control, only the control class needs access to the inserted data. In C++, this can be easily arranged by adding friend statements:
class Net {
friend class nets;
friend class blockNets;
ChildAggregate <Master,Net> nets;
TargetXtoX <Block,Terminal,Net> blockNets;
...
};
In Java, the inserted data must be public. If support for associations becomes a part of these languages, the compilers should take care of this problem.
Other Ways of Inserting Data
Until now, we have assumed that the additional data we have to insert to implant the association is inserted as members. We'll mention only briefly other approaches that we explored, which proved less practical for inserting this data.
Required data can be inserted through inheritance, but when a class participates in several associations we get multiple inheritance, and this approach cannot be used in Java or C#. For example:
class Net : ChildAggregate<Master,Net>,
TargetXtoX<Block,Terminal,Net>,
Parent1toX<Net,Pin>,
Parent1toX<Net,Connector> {
...
};
The Pattern Template Library (PTL; www.codefarms.com/products.htm) was built on this principle. The Data Object Library (DOL; www.codefarms.com/products.htm) provided automatically persistent associations based on ACC and inserted members. It was implemented with a code generator and C macros. Over 18 years, this library was successfully used on many large and complex projects.
Aspect Programming transparently inserts sections of code using a precompiler. However, AOP needs instruction about where and what should be inserted, and generating this instruction is more complicated than directly generating the members. We reached this conclusion after a discussion with Olaf Spinczyk, the author of AspectC++ (see www.codefarms.com/ aspects.doc).
