Assigned() Issues

[speleomania]

Can anyone confirm/comment on this?

Program AccessViolation;

var
  c: TSomeClass;

begin
  if Assigned(c) then ShowMessage(c.name); // Access violation
end;

The problem I'm having is that for some reason Assigned seems to always return True unless I manually do

begin
  c := nil;
  if Assigned(c) then ShowMessage(c.name); // This works
end;

I wasn't expecting having to manually initialise my object vars...is this normal?
 

[howardpc]

I can't reproduce your issue. The following displays False, as one would expect:

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. {$Mode delphi}{$H+}
4.  
5. var
6.   obj: TObject;
7.  
8. begin
9.   WriteLn('Value of ''Assigned(obj)'' is ',Assigned(obj));
10. end.

[wp]

Some for me. But I would never rely on a variable being initialized automatically. Even if there seem to be rules (https://stackoverflow.com/questions/132725/are-delphi-variables-initialized-with-a-value-by-default), but I usually forget them (except that class members are always set to zero). Instead, it is easier to remember this more general rule:

Code: Pascal  [Select][+][-]

1. program Project1;
2.      
3. {$Mode delphi}{$H+}
4.      
5. var
6.   obj: TObject = nil;    // <--- initialization
7.      
8. begin
9.   WriteLn('Value of ''Assigned(obj)'' is ',Assigned(obj));
10. end.

[howardpc]

Surely you can rely on documented compiler initializations, such as all globals being set to zero/nil and all class fields/variables set likewise; and strings and dynamic arrays always having an initial zero length?
Not that it hurts to make the initial value explicit, as in your example (however, perhaps in some cases, the nil value is then actually assigned twice).

[speleomania]

My bad!

I'm ashamed to admit I completely forgot about a class variable with the same name which messed me up. You guys are correct and free pascal is not at fault here

Case closed!

[anfm]

With this example the error occurs.

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls;
9.  
10. type
11.  
12.   { TData }
13.  
14.   TData = class
15.   private
16.     fDateTime : TDateTime;
17.     fName: String;
18.   public
19.     constructor Create;
20.     procedure Clear;
21.  
22.     property DateTime: TDateTime
23.       read fDateTime write fDateTime;
24.     property Name     : String  read fName      write fName;
25.   end;
26.  
27.  
28.   { TForm1 }
29.  
30.   TForm1 = class(TForm)
31.     Button1: TButton;
32.     procedure Button1Click(Sender: TObject);
33.   private
34.  
35.   public
36.  
37.   end;
38.  
39. var
40.   Form1: TForm1;
41.  
42. implementation
43.  
44. {$R *.lfm}
45.  
46. { TForm1 }
47.  
48. procedure TForm1.Button1Click(Sender: TObject);
49. var
50.   aData: TData;
51. begin
52.   if Assigned(aData) then
53.     ShowMessage(aData.Name) // Access Violation
54.   else
55.     ShowMessage('False');
56. end;
57.  
58. { TData }
59.  
60. constructor TData.Create;
61. begin
62.   Clear;
63. end;
64.  
65. procedure TData.Clear;
66. begin
67.   fDateTime := Now;
68.   fName     := '';
69. end;
70.  
71. end.
72.  

[korba812]

Local variables are not automatically initialized and may contain random values.
https://www.freepascal.org/docs-html/ref/refse24.html

[wp]

The error occurs because the local variable "aData" is not initialized, i.e. is not nil. Simply declare it like this:

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. var
3.   aData: TData = nil;
4. begin
5.   if Assigned(aData) then
6.     ShowMessage(aData.Name) // Access Violation
7.   else
8.     ShowMessage('False');
9. end;
10.