Question

I have an abstract class:

type   TInterfaceMethod = class abstract   public     destructor Destroy;         virtual; abstract;     function GetBasePlan: Word; virtual; abstract;     procedure CountBasePlan;    virtual; abstract;     procedure Calculate;        virtual; abstract;     procedure PrepareForWork;   virtual; abstract;   end; 

and a derived class:

type   TFogelMethod = class(TInterfaceMethod)   private     matrix: TFogelMatrix;     BasePlan: Word;   public     constructor Create(var matr: TFogelMatrix);     procedure Calculate;     function GetBasePlan: Word;     procedure CountBasePlan;     procedure PrepareForWork;     destructor Destroy;   end; 

The question is, can I place the implementation of GetBasePlan and CountBasePlan methods into base class, make them only virtual – not abstract as now – and also place member BasePlan there? So, can I do this:

type   TInterfaceMethod = class abstract   private    BasePlan: Word;   public     destructor Destroy;         virtual; abstract;     function GetBasePlan: Word; virtual;      procedure CountBasePlan;    virtual;      procedure Calculate;        virtual; abstract;     procedure PrepareForWork;   virtual; abstract;   end; 

In case I can do it, will it be good from the point of view of object-oriented design, and how can I exactly access this member from derived classes?

Answer 1

Yes you can. Abstract classes are classes and they can have implementations.

By adding the abstract keyword to a class, you prohibit the class to be instantiated. It does not require to have any abstract methods.

A class with absract methods can be instantiated, but this result in a warning at compile time and an exception if the method is called.

Interfaces have no implementation, they have to be implemented by classes (which can be abstract by the way).