Practical differences

[ezlage]

Hi!

Which are the practical differences between the declaration ways below?

Code: Pascal  [Select][+][-]

1. DoSomething(var AStream: TStream);
2. DoSomething(const AStream: TStream);
3. DoSomething(AStream: TStream);

I know the theoretical difference between "var" and "const" only.

I would like to assign to the variable AStream all variants of the type (TMemoryStream, TFileStream, TBytesStream, etc.) without allocating unnecessary memory (without duplicating the data inside the stream).

Thank you.

[lainz]

Var: it passes an address not a copy of the value. In your case is the same since objects are always passed as reference.

Const: the value is a constant and should not be assigned or freed inside the procedure. Is the recommended for your case. You allocate the memory outside the procedure.

Normal: for normal types it passes a copy of the value except for objects as already mentioned.

[lainz]

http://www.marcocantu.com/epascal/English/ch06proc.htm

That website explains it better. It corrects something I said: objects are passed by value because are actually references.

[totya]

I would like to assign to the variable AStream all variants of the type (TMemoryStream, TFileStream, TBytesStream, etc.) without allocating unnecessary memory (without duplicating the data inside the stream).

Hi, no "duplication data" in this case, because TStream is a simple pointer.

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Memo1: TMemo;
16.     procedure FormCreate(Sender: TObject);
17.   private
18.     procedure DoSomethingV (var AStream: TStream);
19.     procedure DoSomethingC (const AStream: TStream);
20.     procedure DoSomethingN (AStream: TStream);
21.   end;
22.  
23. var
24.   Form1: TForm1;
25.  
26. implementation
27.  
28. {$R *.lfm}
29.  
30. { TForm1 }
31.  
32. procedure TForm1.DoSomethingV (var AStream: TStream);
33. begin
34.   Memo1.Lines.Add(IntToHex(PtrUInt(AStream), SizeOf(PtrUInt)*2));
35. end;
36. procedure TForm1.DoSomethingC (const AStream: TStream);
37. begin
38.   Memo1.Lines.Add(IntToHex(PtrUInt(AStream), SizeOf(PtrUInt)*2));
39. end;
40. procedure TForm1.DoSomethingN (AStream: TStream);
41. begin
42.   Memo1.Lines.Add(IntToHex(PtrUInt(AStream), SizeOf(PtrUInt)*2));
43. end;
44.  
45. procedure TForm1.FormCreate(Sender: TObject);
46. var
47.   MS: TMemoryStream;
48. begin
49.   MS:= nil;
50.   MS:= TMemoryStream.Create;
51.   if Assigned(MS) then
52.     try
53.       DoSomethingV(TStream(MS));
54.       DoSomethingC(TStream(MS));
55.       DoSomethingN(TStream(MS));
56.     finally
57.       MS.Free;
58.     end;
59. end;
60.  
61. end.
62.