Variable destroyed

Hi

A embarrassing question how do I tell if a local variable has been destroyed

I thought one of
(assigned(var)), (@var <> nil)
would work but nope :-(
kogerAsked:
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intheConnect With a Mentor Commented:
i hope i am understanding the question properly and not filling it with rubbish ;-)

try this:

var
  Form1: TForm1;
sl : tstringlist;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
begin
if assigned(sl) then
 begin
  showmessage('sl is full');
  exit;
  end
else
 begin
  showmessage('sl is empty so i full it');
  sl := tstringlist.Create;
  sl.LoadFromFile('c:\autoexec.bat');
 end
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
if assigned(sl) then
begin
 sl.Destroy;
 sl := nil
 end
else
showmessage('sl is empty');
end;
0
 
intheCommented:
mm.not sure i understand ,is this what your after:

var
s: string;
begin
if assigned(var) then
shomessage('s equals: '+s)
else
showmessage('var is empty');
s := 'hello';
if assigned(var) then
shomessage('s equals: '+s)
else
showmessage('var is still empty');
//last line should never get shown
0
 
intheCommented:
how many typos ;-)

var
s: string;
begin
if assigned(s) then
showmessage('s equals: '+s)
else
showmessage('var is empty');
s := 'hello';
if assigned(s) then
showmessage('s equals: '+s)
else
//something went wrong if get here ..
0
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kogerAuthor Commented:
huh? :-)
well in this case it is a stringlist

u know

var
  strlst:tstringlist
begin
  create
  then free

  then check if strlst has been destoryed, ofcourse is it that here but in another function I can't tell in designtime :-)

end;
0
 
intheCommented:
well i have just made example for a form:
have a look at it while i make example for stringlist:


var
  Form1: TForm1;
formvar : tform;
implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
begin
if assigned(formvar) then
 begin
  showmessage('formvar is busy');
  exit;
  end
else
 begin
showmessage('formvar is destroyed so i create knew form');
  formvar := tform.Create(self);
  formvar.Left := 0;
  formvar.top := 0;
  formvar.Width := 150;
  formvar.Height := 100;
  formvar.color := clred;
  formvar.show;
  end

 end;

procedure TForm1.Button2Click(Sender: TObject);
begin
 formvar.Destroy;
 formvar := nil
end;
0
 
rwilson032697Commented:
You can free the item like this:

strlst.Free;
strlst := Nil;

or like this (with D5)

FreeAndNil(strLst).

Then you can do this:

if not assigned(strlst) then
  // strlst is destroyed.

Cheers,

Raymond.
0
 
kogerAuthor Commented:
ahh ok I get you must assign nil to the variable after having destroyed it to make assigned() work.

Just curious is there a way to make it work without assigning nil to the var
0
 
rwilson032697Commented:
Not really - you could try grovelling around in the low level memory manager stuff and examine the structures there to determine whether the pointer points to an area of used memory or not, but that is getting pretty nasty.

Assigning nil is simple and easy to do!

Cheers,

Raymond.
0
 
kogerAuthor Commented:
fine that's what I will do then :)
0
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