Solved

dangerling object pointers

Posted on 1998-04-28
3
180 Views
Last Modified: 2010-04-06
Hi

I would like to know how I can detect dangerling object-pointers.

Ex:
Button1: TButton;
Button2: TButton;
...
Button1 := TButton.Create;
Button2 := Button1;
Button1.Free;
<< if I here try to get to any of Button2's attributes I of cause get af AccesError>>

I would like to be able to check i an object-reference points to a freed object. I cant use Assigned, cause Assigned only checks to see if the pointer is nil. My question is how do I detect if a object-reference points to somewhere that is no longer a object (It has been freed) - In the ex. above I would like to be able to do something like "Valid(Button2)" and get false, cause the object Button2 points to no longer exists.

Thanx
0
Comment
Question by:steff
[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 4

Accepted Solution

by:
d003303 earned 100 total points
ID: 1338137
Yo,
the easies way is to force an access violation and trap it with a silent exception handler. Like this :

function Valid(Instance : TObject): Boolean;
var CheckObj : IUnknown;
begin
  Result := true;
  try
    Instance.GetInterface(IUnknown, CheckObj);
  except
    Result := false;
  end;
end;

If the passes instance is not valid anymore, the function returns false.

Slash/d003303
0
 

Expert Comment

by:BigMadDrongo
ID: 1338138
For a quick and dirty solution, create a function like the following:

function Valid(Button: TButton): boolean
begin
  try
    if Button.Caption := '' then;
    Result := True;
  except
     Result := False;
  end;
end;

This is a really dirty way of doing it.  It access a property of the button, and if an exception is raised, the error handler returns false, else true is returned.

BigMadDrongo
0
 

Author Comment

by:steff
ID: 1338139
Thanx.

The solution from you is exactly how I have solved the problem until now, but I thought
that I was toooo dirty. But after getting the same solution for a "expert" I feel better about
it.

Per
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

Objective: - This article will help user in how to convert their numeric value become words. How to use 1. You can copy this code in your Unit as function 2. than you can perform your function by type this code The Code   (CODE) The Im…
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…
In this video, viewers will be given step by step instructions on adjusting mouse, pointer and cursor visibility in Microsoft Windows 10. The video seeks to educate those who are struggling with the new Windows 10 Graphical User Interface. Change Cu…
This tutorial will teach you the special effect of super speed similar to the fictional character Wally West aka "The Flash" After Shake : http://www.videocopilot.net/presets/after_shake/ All lightning effects with instructions : http://www.mediaf…
Suggested Courses
Course of the Month5 days, 2 hours left to enroll

635 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