[fpc-pascal] Arrays of objects

[Adrian Maier]

On 10/31/07, Joao Morais <post at joaomorais.com.br> wrote:
> Adrian Maier wrote:
> > On 10/31/07, Matt Emson <memsom at interalpha.co.uk> wrote:
> >> Adrian Maier wrote:
> >>> Hello,
> >>>
> >>> I'm seeking for advice about which is the best way to hold an array of
> >>> class instances
> >>> I need to access the elements using its position (like a regular
> >>> array) , and also i'd like
> >>> the structure to grow when I add more elements
> >> TObjectList
> >
> > I am aware that TObjectList is one of the possible ways.
> > But I was hoping to get a better understanding about the purpose of these
> > various classes or types of array ,  and more importantly what are the
> > drawbacks .
> >
> > Is there really no wiki page , tutorial, anything  that provides an overview of
> > the collections and types of arrays that FPC has ?
>
> You can use dynamic arrays, they work like an ansi string:
>
> var
>    VArray: array of TSomeClass;
> begin
>    SetLength(VArray, 10);
>    // now you have VArray[0] .. VArray[9];
>    SetLength(VArray, 20);
>    // now you have [0] .. [19];
>    // Length(VArray) = 20
>    // for I := 0 to Pred(Length(VArray)) is a valid statement
>
> They are reference counted, just like ansi strings, ie don't worry about
> memory leakages.

The detail that is not crystal clear to me is  :  after the first SetLength
and  i set the first 3  elements  ,   is it *guaranteed* that the
second SetLength
leaves those first 3 elements  untouched ?

That's what i meant by "resize" as opposed to  "reallocate" . Reallocation
could occur in some other place in memory , leading to loosing the
original contents.    I have actually tested it with a small program,  but since
i've been able to access indexes way beyond the allocated number of elements
before getting the program killed     i   want to be sure that i understand the
proper use of dynamic arrays.


Thanks  Joao  !


-- 
Adrian Maier