Solved

pointer to objects as parameter of function

Posted on 1998-02-25
3
322 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
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
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

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

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

Suggested Solutions

A lot of questions regard threads in Delphi.   One of the more specific questions is how to show progress of the thread.   Updating a progressbar from inside a thread is a mistake. A solution to this would be to send a synchronized message to the…
Introduction The parallel port is a very commonly known port, it was widely used to connect a printer to the PC, if you look at the back of your computer, for those who don't have newer computers, there will be a port with 25 pins and a small print…
Are you ready to implement Active Directory best practices without reading 300+ pages? You're in luck. In this webinar hosted by Skyport Systems, you gain insight into Microsoft's latest comprehensive guide, with tips on the best and easiest way…

739 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