General Info: Inheritance

The concept of a class makes it possible to define subclasses that share some or all of the main class characteristics. This is called inheritance. Inheritance also allows you to reuse code more efficiently. In a class tree, inheritance is used to design classes vertically. (You can use Interfaces to design classes horizontally within a class tree.) With inheritance, you are defining an "is-a" relationship (i.e. a chow is-a dog). Analysts using UML call this generalization where you generalize specific classes into general parent classes.

Delphi Inheritance

In Delphi, you use the class keyword followed by the parent class in parens. If you leave out the parent class, your class inherits from TObject.

In the following example, a terminator T-600 is-an android. 

TAndroid = class

end;

T-600 = class(TAndroid)

end;

Now let's dive in...

Inheritance versus Interfaces

With inheritance, you design classes that have methods and attributes and then extend those methods and attributes in descendant classes adding functionality as the class hierarchy gets deeper and deeper. In a real sense, you design vertically top to bottom. Interfaces allow you to design horizontally througout your class tree. Interfaces define abstract methods and/or attributes in a psuedo abstract class that if used on a class must be implemented by the class.

Complete Example

More complete example...

//Declare class in Interface section.

TPerson = class(TObject)
private
  FName: String;
  FAge: Integer;
public
  constructor Create;
end;

//Implement class in Implementation section.

constructor TPerson.Create;
begin
  inherited;  //Call the parent Create method

  // Now set a default fruit name
  FName := 'unknown';
  FAge := 0;
end;

//Use class.

var
  Lisa: TPerson;
begin
  Lisa := TPerson.Create;

  //Defaults.
  ShowMessage(Lisa.FName + ' is ' + IntToStr(Lisa.FAge));

  //Use class.
  Lisa.FName := 'Lisa';
  Lisa.FAge := 34;
  ShowMessage(Lisa.FName + ' is ' + IntToStr(Lisa.FAge));
end;

Step by Step Example

The following is a step-by-step tutorial on creating your first Delphi class and does include an inheritance example. From this simple tutorial, you can then experiment with the various class inheritance features.

• Tutorial: A 10 Minute Your First Delphi Class Quick Start