TMemoryStream, manual memory management

[LemonParty]

How to create and use TMemoryStream where you manually specify the buffer and it's size?
I mean this thing from Delphi https://docwiki.embarcadero.com/Libraries/Sydney/en/System.Classes.TCustomMemoryStream.SetPointer

[marcov]

I use this, derived from Indy's memorystream:

Code: Pascal  [Select][+][-]

1. Type
2.  
3. // in a different (Indy)  Unit:
4.   TIdMemoryBufferStream = class(TCustomMemoryStream)
5.   public
6.     constructor Create(APtr: Pointer; ASize: TIdNativeInt);
7.     function Write(const Buffer; Count: Longint): Longint; override;
8.   end;
9.  
10.     TIdCandelaMemoryBufferStream = class(TIdMemoryBufferStream)
11.                                      procedure dosetpointer(aptr:pointer;asize:TIdNativeInt);
12.                                      procedure write(io:TIdIOHandler;aptr:pointer;asize:TIdNativeInt);
13.     end;
14.  
15. ....
16.  
17. constructor TIdMemoryBufferStream.Create(APtr: Pointer; ASize: TIdNativeInt);
18. begin
19.   inherited Create;
20.   SetPointer(APtr, ASize);
21. end;
22.  
23. function TIdMemoryBufferStream.Write(const Buffer; Count: Longint): Longint;
24. var
25.   LNumToCopy: Longint;
26. begin
27.   Result := 0;
28.   if (Position >= 0) and (Size > 0) and (Count > 0) then
29.   begin
30.     LNumToCopy := IndyMin(Size - Position, Count);
31.     if LNumToCopy > 0 then
32.     begin
33.       System.Move(Buffer, Pointer(PtrUInt(Memory) + Position)^, LNumToCopy);
34.       TIdStreamHelper.Seek(Self, LNumToCopy, soCurrent);
35.       Result := LNumToCopy;
36.     end;
37.   end;
38. end;
39.  
40. { TIdCandelaMemoryBufferStream }
41.  
42. procedure TIdCandelaMemoryBufferStream.dosetpointer(aptr: pointer;
43.   asize: TIdNativeInt);
44. begin
45.   SetPointer(aptr,asize);
46.   position:=0;
47. end;
48.  
49. procedure TIdCandelaMemoryBufferStream.write(io:TIdIOHandler;aptr:pointer;asize:TIdNativeInt);
50. begin
51.   SetPointer(aptr,asize);
52.   position:=0;
53.   if asize>0 then
54.     io.Write(self,asize);
55. end;
56.  
57.  

It's from Indy, but more the utility routines, and used by but not connected to the rest of Indy.

[Remy Lebeau]

How to create and use TMemoryStream where you manually specify the buffer and it's size?

TMemoryStream does not support that option. It manages its own buffer and size. To do what you ask, you need to use TCustomMemoryStream instead.

I mean this thing from Delphi https://docwiki.embarcadero.com/Libraries/en/System.Classes.TCustomMemoryStream.SetPointer

FPC has TCustomMemoryStream, too: https://www.freepascal.org/docs-html/rtl/classes/tcustommemorystream.html

In both Delphi and FPC, SetPointer() is a protected member, so you need to derive a new class from TCustomMemoryStream in order to access SetPointer(), eg:

Code: Pascal  [Select][+][-]

1. type
2.   TMyMemoryBufferStream = class(TCustomMemoryStream)
3.   public
4.     constructor Create(Ptr: Pointer; ASize: PtrInt);
5.   end;
6.  
7. constructor TMyMemoryBufferStream.Create(Ptr: Pointer; ASize: PtrInt);
8. begin
9.   inherited Create;
10.   SetPointer(APtr, ASize);
11. end;

[LemonParty]

marcov, what is TIdStreamHelper? Can you give a code?

[marcov]

marcov, what is TIdStreamHelper? Can you give a code?

Remy knows that better than me, but I think it is a simple wrapper for seek to abstract between standard VCL and other targets.  Just replace it with seek on the stream.

[Remy Lebeau]

marcov, what is TIdStreamHelper?

It is a utility class used internally by the Indy library to hide interface differences in TStream between Delphi/FPC and Delphi.NET.