Can I convert a buffer + size to TBytes?

Question

Given a buffer and its size in bytes, is there a way to convert this to TBytes without copying it?

Example:

procedure HandleBuffer(_Buffer: PByte; _BufSize: integer);
var
  Arr: TBytes;
  i: Integer;
begin
  // some clever code here to get contents of the buffer into the Array
  for i := 0 to Length(Arr)-1 do begin
    HandleByte(Arr[i]);
  end;
end;

I could of course copy the data:

procedure HandleBuffer(_Buffer: PByte; _BufSize: integer);
var
  Arr: TBytes;
  i: Integer;
begin
  // this works but is very inefficient
  SetLength(Arr, _BufSize);
  Move(PByte(_Buffer)^, Arr[0], _BufSize);
  //
  for i := 0 to Length(Arr)-1 do begin
    HandleByte(Arr[i]);
  end;
end;

But for a large buffer (about a hundred megabytes) this would mean I have double the memory requirement and also spend a lot of time unnecessarily copying data.

I am aware that I could simply use a PByte to process each byte in the buffer, I'm only interested in a solution to use a TBytes instead.

I think it's not possible, but I have been wrong before.

Answer 1

No, this is not possible (without unreasonable hacks).

The problem is that TBytes = TArray<Byte> = array of Byte is a dynamic array and the heap object for a non-empty dynamic array has a header containing the array's reference count and length.

A function that accepts a TBytes parameter, when given a plain pointer to an array of bytes, might (rightfully) attempt to read the (non-existing) header, and then you are in serious trouble.

Also, dynamic arrays are managed types (as indicated by the reference count I mentioned), so you might have problems with that as well.

---

However, in your particular example code, you don't actually use the dynamic array nature of the data at all, so you can work directly with the buffer:

procedure HandleBuffer(_Buffer: PByte; _BufSize: integer);
var
  i: Integer;
begin
  for i := 0 to _BufSize - 1 do
    HandleByte(_Buffer[i]);
end;