What is the goal of assigned() ?

[Fred vS]

Hello.

Refering to this, I would like to know what is the utility of assigned() if the object must be assigned by something, ( nil for nothing )?

I understood that assigned() is true if the object <> nil.
But if the result is not assigned(), in fact he must be assigned as nil.

So what must be used to know if something was not assigned by something ( even not nil )?

Thanks.

Fre;D

[Fred vS]

its just a fancy way of using NIL as a compare value..

One is verbose and the other is binbose ! 

And this one tribose ? :

Code: Pascal  [Select][+][-]

1.  
2.    procedure my_maybe_assigned_bitmap();
3.     var   abitmap : TBGRABitmap;
4.       begin
5.         if assigned(abitmap) then abitmap.free;
6.       end;

result:
 

   000000000418768 : EAccessViolation : Access violation

?

[mercurhyo]

compatibility with Old pascal from the 90's where "assigned" checked if a FILE was or not ready to open or create (FCB file control block totally filled)

[mercurhyo]

its just a fancy way of using NIL as a compare value..

One is verbose and the other is binbose ! 

And this one tribose ? :

Code: Pascal  [Select][+][-]

1.  
2.   var
3.     abitmap : TBGRABitmap;
4. ...
5.   procedure my_maybe_assigned_bitmap();
6.       begin
7.         if assigned(abitmap) then abitmap.free;
8.       end;

result:
 

   000000000418768 : EAccessViolation : Access violation

?

NO NO AND NO

if assigned(reftomyclass) then FREEANDNIL(reftomyclass)

otherwise myclass.free do not set the reference to nil and if you pass the code again a fatal error is NORMAL

[mercurhyo]

Code: Pascal  [Select][+][-]

1. if assigned(P) then begin
2.   P.free;
3.  P:=nil;
4. end;
5.  

is equivalent to

if assigned(P) then FreeAndNil(P);

[Fred vS]

Code: Pascal  [Select][+][-]

1. if assigned(P) then begin
2.   P.free;
3.  P:=nil;
4. end;
5.  

is equivalent to

if assigned(P) then FreeAndNil(P);

Many thanks, but I know that.

Huh, did you read my post?

With your code if P was not assigned by something included 'nil', your program will crash after if assigned(P)

And the question is: what to use if the object was assigned by nothing.
Using assigned() does not work for that.

Fre;D

[Fred vS]

I'll give up if you don't understand..

Come on Jamie...

Maybe you did not understand what I asked.

I dont ask how to free something, the problem is not there, it was just used for the code.

I ask how to know if a object was not assigned nor initialized.

Fre;D

[engkin]

I am a Windows person, sooo here I go:
Check that you have a valid range of addresses that you can read using IsBadReadPtr(pointer(abitmap), abitmap.InstanceSize)

We still don't know what's in these addresses. Repeat the same process based on the type you are checking.

For classes, the first address is a PPVMT, check the address it points at using IsBadReadPtr

Code: Pascal  [Select][+][-]

1.  pp := PPVMT(abitmap);

and again for

Code: Pascal  [Select][+][-]

1. . p := pp^;

and finally for the class name pointer. Then compare the class name.

[mercurhyo]

hm just read the "this" link... remember Fred... variables on stack are random filled; variables on memory are zeroed at startup

when compiler builds assembler code the begin of routines is standardized the stack pointer is decreased to make room for locals withouth initializing them to nil (zero) so they are random bytes (not nil until you set them to, manually)

[mercurhyo]

addendum :::
variables on ram for basic types and classes are zeroed, OBJECT are random filled like stack variables (locals) and its up to you to assign them to nil or initialize them properly

[Fred vS]

@ Engkin and mercurhyo.

OK, many thanks and perfectly clear.

Clear that each object must be defined at declaration, otherwise only random decides.

So to resume, to do very good practice, when you create a new project with Lazarus, for example, the layout self-created is:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     procedure FormCreate(Sender: TObject);
16.   private
17.  
18.   public
19.  
20.   end;
21.  
22. var
23.   Form1: TForm1; // VERY BAD PRACTICE
24.  
25. implementation
26.  
27. {$R *.lfm}
28.  
29. { TForm1 }
30.  
31. procedure TForm1.FormCreate(Sender: TObject);
32. begin
33.  
34. end;
35.  
36. end.

Should be for good Pascalian:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     procedure FormCreate(Sender: TObject);
16.   private
17.  
18.   public
19.  
20.   end;
21.  
22. var
23.   Form1: TForm1 = nil;  // GOOD BOY
24.  
25. implementation
26.  
27. {$R *.lfm}
28.  
29. { TForm1 }
30.  
31. procedure TForm1.FormCreate(Sender: TObject);
32. begin
33.  
34. end;
35.  
36. end.

 

Fre;D

[mercurhyo]

form1: TForm;

is good practice LOL
1) it declares global variable form1 as type TForm
2) the declaration is global so the compiler takes care to make it NIL

if you do the same inside a procedure locals THEN you must set it yourself to nil because the variable does not reside in global mem but onto the stack

ex.
procedure blabla;
  var form1:TForm=nil;
...
the var inside a procedure/function is local to it and the compiler does not init locals to NIL it just makes room on stack and leaves random bytes inside

[engkin]

Global variables are given the value nil. You only need to take care of local variables.

I over complicated my previous post. you simple do the first check, and then make sure it is of the expected type. But I think you got the idea.

[Fred vS]

form1: TForm

is good practice LOL
1) it declares global variable form1 as type TForm
2) the declaration is global so the compiler takes care to make it NIL

if you do the same inside a procedure locals THEN you must set it yourself to nil because the variable does not reside in global mem but onto the stack

ex.
procedure blabla;
  var form1:TForm=nil;
...
the var inside a procedure is local to it and the compiler does not init locals

Hum, and what about ( ouch, they will shout ) to propose to always assign to nil if nothing was assigned.

So, even in a local proc, if somebody does:

Code: Pascal  [Select][+][-]

1. procedure blabla;
2.   var form1:TForm;

the compiler will takes care to make form1 to NIL too?

Why this difference?

[mercurhyo]

Im sorry .. I've explained it carefully, if you can not understand between stack memory (locals) and heap memory (globals) its 2 different kinds of approach for all comilerS comprising C, Fortran, etc etc

its a standard from decades

local variables are ON STACK and never initialized , any compiler!
global variables are on HEAP and often initialized on modern compilers
by past nothing was initialized so you can feel lucky