[fpc-devel] TBytes

[Mattias Gaertner]

On Tue, 28 Feb 2012 09:23:34 +0100
Sven Barth <pascaldragon at googlemail.com> wrote:

> Am 28.02.2012 05:04, schrieb Martin Schreiber:
> > Hi,
> > I read that we should use TBytes instead of AnsiString in order to
> > implement combined binary/character buffers with automatic memory
> > management. With AnsiString we used setlength() in order to allocate not
> > initialized memory.
> > TBytes is defined as
> > "
> > TBytes = array of Byte;
> > "
> > where setlength() fills the buffer with zeros. What should we use instead?
> 
> For strings SetLength also fills the string with zeros as the following 
> example shows:
> 
> === example begin ===
> 
> program setlengthtest;
> 
> var
>    barr: array of Byte;
>    b: Byte;
>    s: AnsiString;
>    c: Char;
> begin
>    SetLength(barr, 20);
>    for b in barr do
>      Write(b, ' ');
>    Writeln;
>    SetLength(s, 20);
>    for c in s do
>      Write(Ord(c), ' ');
>    Writeln;
> end.
> 
> === example end ===
> 
> === output begin ===
> 
> PS P:\tests\oneshots> .\setlengthtest.exe
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

That's because the OS gives you on first time a lot of zeroed mem.
SetLength does not zero memory. Try this:

var
  i: Integer;
  s: string;
begin
  for i:=1 to 100000 do begin
    SetLength(s,1000000); // size does not matter for speed
    SetLength(s,1);
  end;
end.

Mattias