Hi All,

I have two simple questions:

1) As far as I know class variables in FPC are automatic initialize to the NULL value (e.g. 0 or "" or whatever), does that include dynamic arrays? e.g.

TMyClass = class
    myarray: array[0..100] of Integer;
end;

will myarray be initialized to nil?

2) After SetLength(myarray, 0), is myarray nil or pointing to some invalid address?

Thanks

Sorry the array should be dynamic, without [0..100].

2013/3/12 Xiangrong Fang

Xiangrong Fang wrote

TMyClass = class myarray: array[0..100] of Integer; end;

You could answer such questions yourself:

program Project1;
{$mode objfpc}
type
TMyClass = class
   myarray: array of Integer;
end;
var my:TMyClass;
begin
   my:=TMyClass.Create;
   try
    writeln('After creation the value of my.myarray is
',PtrUInt(my.myarray));
    SetLength(my.myarray, 0);
    writeln('After SetLength(), value of my.myarray is
',PtrUInt(my.myarray));
   finally
     my.Free;
   end;
   ReadLn;
end.

"Xiangrong Fang" wrote

Sorry the array should be dynamic, without [0..100].

Yes, in that case it will be Nil (Note: in the static case the elements of
the array would be 0).

Regards,
Sven

"Xiangrong Fang" wrote

2) After SetLength(myarray, 0), is myarray nil or pointing to some

invalid address?

It will be Nil. An equivalent alternative would be:

MyArray := Nil;

Regards,
Sven

Yes, in that case it will be Nil (Note: in the static case the elements of the array would be 0).

So, it is even OK to use SetLength() on static arrays? interesting.

Not with this simple test.

Mattias

2013/3/13 Sven Barth

Sorry, I didn't see this reply just now...   You mean, by MyArray := nil;
but NOT calling SetLength(MyArray, 0); there will NOT be memory leaks?

Could you show a more complex test that would give a satisfactory answer to the OP's questions?

Howard

"Xiangrong Fang" wrote

> > Yes, in that case it will be Nil (Note: in the static case the elements

of the array would be 0).

So, it is even OK to use SetLength() on static arrays? interesting.

No, I meant that the content of a static array will be initialized to zero inside a class instance.

Regards,
Sven

invalid address?

but NOT calling SetLength(MyArray, 0); there will NOT be memory leaks?

As dynamic arrays are reference counted they are equal.

Regards,
Sven

Xiangrong wanted to know if the above array is initialized. If you want to test that then you have to either single step via the debugger or to compare the allocated memory before and after. The problem is that you do not know the memory position before the allocation.
You can try to create the class, change it, free it and allocate it again. If the two instances are at the same position, then you can check for 0 or nil.

Mattias

Thanks for the clear explanation. I now see that the question is about the data stored in memory, not about the memory address.

Howard