From the February 2016 Issue of Prestwood eMag

Delphi Using Data: Delphi Instance Counter By Mike Prestwood

Posted 16 years ago on 12/17/2008 Take Away: Implement static member data in Delphi with variables declared in the implementation section (unit scope). Increment and decrement variable in constructor and destructor. Then use a class function to surface the variable's value publicly. KB101713

Introduction

For our instance counter, if no classes are currently created, the current value of your instance counter needs to be 0. For each class created, you add 1. For each destroyed, you subtract 1. This article will utilize static data and a class method to build an instance counter. This is not a generic instance counter but rather a simple build-it-per-class instance counter.

This article will utilize an initialized unit variable (a variable initialized to 0 in the implementation section of a unit) and a class method to build an instance counter for a specific class.

Static Data: Static data (a.k.a. static class member) is data of a class that retains state (it's value) whether or not there is an instance of a class. In Delphi, you implement static data using either global variables (declared in the interface section) or unit variables (declared in the implementation section).

Class Methods: A Delphi class method (a.k.a. static method) is a method you can call without instantiating the class. It's called a class method since it belongs to the class and not to a specific object created from the class. A static class is a class that contains only static members (class members). 

Build-It!

The following is a class that demonstrates initializing a global variable for use as an instance counter:

unit ObjectCounter;

interface

type
  TObjectCounter = class(TObject)
  public
    constructor Create; virtual;
    destructor Destroy; override;
    class function GetInstanceCount: Integer;
  end;

implementation

{ TObjectCounter } 

//Declared here rather than in interface section to limit the 

//scope to this unit. Otherwise, we would probably run into

//variable naming conflicts.

var
   TObjectCounterCount: Integer=0;

constructor TObjectCounter.Create;
begin
  inherited;
  TObjectCounterCount := TObjectCounterCount + 1;
end;

destructor TObjectCounter.Destroy;
begin
  TObjectCounterCount := TObjectCounterCount - 1;
  inherited;
end;

//Class function so you don't have to create

//the class to call this function. 

class function TObjectCounter.GetInstanceCount: Integer;
begin
  Result := TObjectCounterCount;
end;

end.

Usage

To use the class, create and destroy it's class in the normal way and anytime you wish to see how many instances are running, use the GetInstanceCount class method. For example:

TObjectCounter.GetInstanceCount

Test it out!

To test the above class, create a form with three buttons on it as follows:

Alter the form unit as follows:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ObjectCounter;

type
  TForm1 = class(TForm)
    Create1: TButton;
    Create2: TButton;
    ShowCount: TButton;
    procedure Create1Click(Sender: TObject);
    procedure Create2Click(Sender: TObject);
    procedure ShowCountClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Create1Click(Sender: TObject);
var
  ObjectCounter: TObjectCounter;
begin
  ObjectCounter := TObjectCounter.Create;
end;

 
procedure TForm1.Create2Click(Sender: TObject);
var
  ObjectCounter: TObjectCounter;
begin
  ObjectCounter := TObjectCounter.Create;
end;

procedure TForm1.ShowCountClick(Sender: TObject);
begin
  ShowMessage(IntToStr(TObjectCounter.GetInstanceCount));
end;

end.

When you run this form, click the Show Count button first to see that our instance counter is initially set to 0 even before we create an object. Then press the Create 1 and Create 2 buttons multiple times to create several instances of the class and then click the Show Count button again to see the count.

Note: In this simple demo, we never free the object which, of course, is bad practice. However, it is just a proof of concept (POC).

More Info

Code:	Delphi Member Field
Definition:	Static Class / Static Member

Comments

KB Post Contributed By Mike Prestwood:

Mike Prestwood is a drummer, an author, and creator of the PrestwoodBoards online community. He is the President & CEO of Prestwood IT Solutions. Prestwood IT provides Coding, Website, and Computer Tech services. Mike has authored 6 computer books and over 1,200 articles. As a drummer, he maintains play-drums.com and has authored 3 drum books. If you have a project you wish to discuss with Mike, you can send him a private message through his PrestwoodBoards home page or call him 9AM to 4PM PST at 916-726-5675 x205. Visit Profile