Languages Focus: Class..Object

In short, a class is a data type, and an object is an instance of a class type. A class has methods (routines), properties (member variables), and a constructor. The current values of the properties is the current state of the object. The UML is one of the diagraming disciplines that allows you to document the various changing states of a series of objects.

//Interface section:

TCyborg = class(TObject)
public
 procedure IntroduceYourself;
end;

//Implementation section;

procedure TCyborg.IntroduceYourself;
begin
 ShowMessage('Hi, I do not have a name yet.');
end;

//Some event like a button click:

var
  T1: TCyborg;
begin
  T1 := T1.Create;
  T1.IntroduceYourself;

  FreeAndNil(T1);      //Be sure to clean up!
end;

Complete Example

Here is a simple example of a Delphi class in it's own Person.pas unit. Notice how you define the class in the interface section and then fulfill it in the implementation section.

unit Person;

 

interface
 
type //Custom types including classes.
  TPerson = class(TObject) //Start of class.
  private
    FName: String;
    FAge: Integer;
  public
    constructor Create; virtual;
  property Name: String read FName write FName;
  property Age: Integer read FAge write FAge;
  end; //End of class.
 
implementation
 
constructor TPerson.Create;
begin
  inherited; //Call the parent Create method
  
  //Set defaults.
  FName := 'unknown';
  FAge := 0;
  end; //End of constructor method.

end. //End of unit.

To use your new class, include the unit in your uses clause and instantiate the object in your code:

uses
  Person;

Here is the code from a button click event:

procedure TForm2.Button1Click(Sender: TObject);
var
  Lisa: TPerson;  //de clare object variable.
begin
  Lisa := TPerson.Create;  //Create object from class.
   
  //Use it. Show defaults.
  ShowMessage(Lisa.Name + ' is ' + IntToStr(Lisa.Age));
   
  //Use it. Set member fields then show.
  Lisa.Name := 'Lisa';
  Lisa.Age := 34;
  ShowMessage(Lisa.Name + ' is ' + IntToStr(Lisa.Age));
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