Class/Static Constants in Delphi

Question

In Delphi, I want to be able to create an private object that\'s associated with a class, and access it from all instances of that class. In Java, I\'d use:

pub         


        

Answer 1

Last year, Hallvard Vassbotn blogged about a Delphi-hack I had made for this, it became a two-part article:

1. Hack#17: Virtual class variables, Part I
2. Hack#17: Virtual class variables, Part II

Yeah, it's a long read, but very rewarding.

In summary, I've reused the (deprecated) VMT entry called vmtAutoTable as a variable. This slot in the VMT can be used to store any 4-byte value, but if you want to store, you could always allocate a record with all the fields you could wish for.

Answer 2

Before version 7, Delphi didn't have static variables, you'd have to use a global variable.

To make it as private as possible, put it in the implementation section of your unit.

Answer 3

Well, it's not beauty, but works fine in Delphi 7:

TMyObject = class
pulic
    class function MySharedObject: TMySharedObject; // I'm lazy so it will be read only
end;

implementation

...

class function MySharedObject: TMySharedObject;
{$J+} const MySharedObjectInstance: TMySharedObject = nil; {$J-} // {$J+} Makes the consts writable
begin
    // any conditional initialization ...
   if (not Assigned(MySharedObjectInstance)) then
       MySharedObjectInstance = TMySharedOject.Create(...);
  Result := MySharedObjectInstance;
end;

I'm curently using it to build singletons objects.

Answer 4

The keywords you are looking for are "class var" - this starts a block of class variables in your class declaration. You need to end the block with "var" if you wish to include other fields after it (otherwise the block may be ended by a "private", "public", "procedure" etc specifier). Eg

(Edit: I re-read the question and moved reference count into TMyClass - as you may not be able to edit the TMySharedObjectClass class you want to share, if it comes from someone else's library)

  TMyClass = class(TObject)
  strict private
    class var
      FMySharedObjectRefCount: integer;
      FMySharedObject: TMySharedObjectClass;
    var
    FOtherNonClassField1: integer;
    function GetMySharedObject: TMySharedObjectClass;
  public
    constructor Create;
    destructor Destroy; override;
    property MySharedObject: TMySharedObjectClass read GetMySharedObject;
  end;


{ TMyClass }
constructor TMyClass.Create;
begin
  if not Assigned(FMySharedObject) then
    FMySharedObject := TMySharedObjectClass.Create;
  Inc(FMySharedObjectRefCount);
end;

destructor TMyClass.Destroy;
begin
  Dec(FMySharedObjectRefCount);
  if (FMySharedObjectRefCount < 1) then
    FreeAndNil(FMySharedObject);

  inherited;
end;

function TMyClass.GetMySharedObject: TMySharedObjectClass;
begin
  Result := FMySharedObject;
end;

Please note the above is not thread-safe, and there may be better ways of reference-counting (such as using Interfaces), but this is a simple example which should get you started. Note the TMySharedObjectClass can be replaced by TLogLogger or whatever you like.

Answer 5

In Delphi static variables are implemented as variable types constants :)

This could be somewhat misleading.

procedure TForm1.Button1Click(Sender: TObject) ;
const
   clicks : Integer = 1; //not a true constant
begin
  Form1.Caption := IntToStr(clicks) ;
  clicks := clicks + 1;
end;

And yes, another possibility is using global variable in implementation part of your module.

This only works if the compiler switch "Assignable Consts" is turned on, globally or with {$J+} syntax (tnx Lars).

Answer 6

 TMyObject = class
    private
      class var FLogger : TLogLogger;
      procedure SetLogger(value:TLogLogger);
      property Logger : TLogLogger read FLogger write SetLogger;
    end;

procedure TMyObject.SetLogger(value:TLogLogger);
begin
  // sanity checks here
  FLogger := Value;
end;

Note that this class variable will be writable from any class instance, hence you can set it up somewhere else in the code, usually based on some condition (type of logger etc.).

Edit: It will also be the same in all descendants of the class. Change it in one of the children, and it changes for all descendant instances. You could also set up default instance handling.

 TMyObject = class
    private
      class var FLogger : TLogLogger;
      procedure SetLogger(value:TLogLogger);
      function GetLogger:TLogLogger;
      property Logger : TLogLogger read GetLogger write SetLogger;
    end;

function TMyObject.GetLogger:TLogLogger;
begin
  if not Assigned(FLogger)
   then FLogger := TSomeLogLoggerClass.Create;
  Result := FLogger;
end;

procedure TMyObject.SetLogger(value:TLogLogger);
begin
  // sanity checks here
  FLogger := Value;
end;