Re: FreeAndNil

"pri_sama" <pri_sama@xxxxxxxxx> schreef in bericht
news:1153281905.119034.68990@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
The Delphi 5 FreeAndNil procedure is as follows,
procedure FreeAndNil(var Obj);
var
P: TObject;
begin
P := TObject(Obj);
TObject(Obj) := nil; // clear the reference before destroying the
object
P.Free;
end;

Can someone tell me why hasn't it used,
procedure FreeAndNil(var Obj);
begin
TObject(Obj).Free;
TObject(Obj) := nil;
end;


You mean, why isn't it called NilAndFree ;-)
When you have a recursive process - such as freeing the nodes in an
intertwined tree - you must consider that the call to FreeAndNil will, via
its Tobject(Obj).free, call its own caller. To avoid an endless loop or AV
you'd use IF NOT assigned(obj) THEN FreeAndNil(obj);
But if the order of FREE and NIL would be as in your proposed procedure,
such a statement would not have the desired safe outcome.

Consider how the following program will loop forever if FreeAndNil is
replaced by your code.
type
Tnode = class
aNode : Tnode;
procedure free;
end;

procedure Tnode.free;
begin
if assigned(aNode)
then FreeAndNil(aNode)
end;

var
node : Tnode;

begin
node:=Tnode.create;
node.aNode:=node; // a trivial recursive tree
node.free; // this will loop if FreeAndNil has wrong order of things
end;

Tom


.

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