Solved

pointer to objects as parameter of function

Posted on 1998-02-25
3
315 Views
Last Modified: 2011-09-20
Hi,

Is it possible to pass an object of any type as a var parameter of a function?

I am wanting to wrap up the two ideas of:
  Object.Free;
  Object := nil;

I've tried a few incarnations of this, and nothing seems to work out.

I did the obvious:

function FreeObject(var Thing: TObject);
begin
  ...
  Thing.Free;
  Thing := nil;
end;

That fails... It only lets me pass pure TObjects as a parameter.

Then I tried:
 
function FreeObject(var Thing: Pointer);
begin
  ...
  TObject(Thing^).Free;
  Thing := nil;
end;

This fails with the same compiler warning as the first one...

Then I tried:

function FreeObject(var Thing);
begin
  ...
  if Pointer(Thing) = nil then exit;  
  TObject(Thing).Free;
end;

This last one shows the best hope.. but no matter what I put in as a parameter for this, objects, records, integers, etc, it is always nil.. or that is what is reported to me.  Maybe I don't know how to de-reference these correctly.

Is there some other way that I'm missing?

Thanks for your help.

-joe
0
Comment
Question by:greendot
3 Comments
 
LVL 5

Expert Comment

by:inter
ID: 1359801
What about typecasting friend?

procedure FreeIt(var T :Pointer);
begin
  TObject(T).Free;
  T := nil;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  L : TList;
begin
  L := TList.Create;
  FreeIt(Pointer(L));
end;

Igor
0
 
LVL 5

Expert Comment

by:JimBob091197
ID: 1359802
Hi greendot

Greendot, your example of FreeObject comes close to the correct solution:
function FreeObject(var Thing: Pointer);
begin
  TObject(Thing^).Free;
  Thing := nil;
end;

The problem is you need to remove the ^
I.e.  "TObject(Thing^).Free;" becomes "TObject(Thing).Free;"


Here is a routine I wrote for the purpose.
It has the following differences from inter's:
  1. You can call FreeSafe for a nil pointer or a freed object.
  2. You don't need to typecast when calling the procedure.  You just pass in the address of the object.  (See example below.)  This makes coding that little bit faster.  :)

procedure FreeSafe(AObj: Pointer);
begin
      if (Assigned(AObj)) and (Assigned(Pointer(AObj^))) then begin
            TObject(AObj^).Free;
            Pointer(AObj^) := nil;
      end;
end;


To call it:
  FreeSafe(@MyObject);

It won't crash if you do the following:
  FreeSafe(@MyObject);
  FreeSafe(@MyObject);

JB
0
 

Accepted Solution

by:
keyquotes earned 100 total points
ID: 1359803
Try this,

FreeObject(TList as TObject);

function FreeObject(var Thing: TObject);
begin
  ...
  Thing.Free;
  Thing := nil;
end;


0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Creating an auto free TStringList The TStringList is a basic and frequently used object in Delphi. On many occasions, you may want to create a temporary list, process some items in the list and be done with the list. In such cases, you have to…
Have you ever had your Delphi form/application just hanging while waiting for data to load? This is the article to read if you want to learn some things about adding threads for data loading in the background. First, I'll setup a general applica…
This Micro Tutorial hows how you can integrate  Mac OSX to a Windows Active Directory Domain. Apple has made it easy to allow users to bind their macs to a windows domain with relative ease. The following video show how to bind OSX Mavericks to …
Internet Business Fax to Email Made Easy - With eFax Corporate (http://www.enterprise.efax.com), you'll receive a dedicated online fax number, which is used the same way as a typical analog fax number. You'll receive secure faxes in your email, fr…

920 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

16 Experts available now in Live!

Get 1:1 Help Now