The ABC's of Domain Modeling in Delphi [Delphi Features]

Created by zope. Last modified 2004-07-15 05:26:36.

Delphi Support for ABC's

Delphi allows the designer to declare methods virtual; abstract; -- virtual, to allow derived classes to override the method in place, and abstract, to require them to (a class which declares a method abstract cannot implement it at all)

Delphi's properties can be made "abstract virtual" by binding them to virtual Get/Set methods. Clients of the ABC thus get the best of both component programming (natural syntax for accessing/assigning attributes), and OO (reduction of coupling dependenies on other modules' implementations).

Delphi also provides a construct, called a class reference, for implementing several neat mechanisms around ABC's. Essentially, a class reference functions as a handle to the virtual function table of a class, allowing you to call methods which are declared virtual but which do not require access to a specific instance of a concrete class. This feature is especially useful for implementing object factories.

ABC's should consist of the following elements:

"Normal" public ABC methods, declared virtual; abstract;
Public and published properties, declared in terms of Get/Set methods (no data fields!)
The protected Get/Set methods, likewise declared virtual; abstract;
Public "template" methods, not declared virtual or abstract. These methods are implemented solely in terms of the "normal" ABC methods and the "virtual" properties, and serve to provide a controlled sequence of invocation of some part of the ABC interface.