[fpc-pascal] High() and Low() for empty dynamic arrays

[Sven Barth]

On 09.02.2014 15:10, Fred van Stappen wrote:
>
>  > Date: Sun, 9 Feb 2014 13:08:16 +0100
>  > From: freepascal at ypa-software.de
>  > To: fpc-pascal at lists.freepascal.org
>  > Subject: Re: [fpc-pascal] High() and Low() for empty dynamic arrays
>  >
>  > Am 09.02.2014 13:05, schrieb Fred van Stappen:
>  > > if length(MyArray) > 0 then
>  > > for x := 0 to high(MyArray) do
>  > > MyArray[x].Free;
>  > >
>  > > But, if i use :
>  > >
>  > > setlength(MyArray, 0) ;
>  > >
>  > > would it do the same job ?
>  >
>  > No. Your array contains only references to the objects. The references
>  > where deleted, but the objects remain in memory without any
> destructor call.
>  >
>  > g
>  > Michael
>
> OK, many thanks Michael.
>
> Hum, i have a dynamic array of threads.
>
> mythread.create has :
> FreeOnTerminate := True;
>
> So, when the thread terminate, it frees the memory too ? (yes/no).

Yes, that frees the complete class instance of the thread.

> And it explain why i get a crash and error message if i try to do, for
> dynamic arrays of threads :
>
>   > if length(MyArray) > 0 then
>   > for x := 0 to high(MyArray) do
>   > MyArray[x].Free;
>
> Because the threads are already freed (yes/no) ?

Exactly.
>
> If so, how can i know if MyArray[x] was already freed ?

You can't. You must not access MyArray[x] anymore after the thread 
terminated.

>
> When i use:
> if assigned(MyArray[x]) then MyArray[x].Free;
>
> It does not work.

Yes, because MyArray[x] is not modified when the thread destroys the 
class at the end of its life. It doesn't even know that there is a 
variable that points to the class (class instance variables are 
basically pointers to the real class instance data). Thus the MyArray[x] 
still contains the pointer to the class instance data, but the later was 
already destroyed, so it's no longer valid memory. So: don't touch it! ;)

Regards,
Sven