[fpc-pascal] Why use pointers to arrays?

[Florian Klaempfl]

Graeme Geldenhuys schrieb:

> 
> 
> 1)  Why would you use a pointer to an array and not simply a variable
> what IS the array? In the program I am porting the following field
> variable is defined.
> 
>   FLines: ^TLinesArray;
> 
> Why couldn't I simply say:
> 
>   FLines: TLinesArray;

Because this array has a fixed size.

> 
> Is there some advantage for using a pointer to the array, when passing
> it around between procedures and functions?
> 
> 
> 2) The TLinesArray is a dynamic array, so shouldn't I maybe change the
> ported code to define the array as follows, instead of the [0..0]
> idea?
> 
>   TLinesArray = array of TLayoutLine;

A dynamic array does several magic behind your neck mainly taking care
of managed types. It depends on the code if it works or not.

> 
> 
> 
> 3) Maybe the strange usage (to me at least) of dynamic arrays could be
> explain in how the original program allocates memory for the array. In
> the class where FLines is defined, it initially reserves ten empty
> elements for the array in the class constructor.
> 
>   FAllocatedNumLines := 10;
>   GetMem( FLines, FAllocatedNumLines * sizeof( TLayoutLine ) );
> 
> And then later if it needs more space for elements, it redefines the
> array elements as follows:
> 
> Procedure TRichTextLayout.AddLineStart( Const Line: TLayoutLine );
> var
>   NewAllocation: longint;
> begin
>   if FNumLines >= FAllocatedNumLines then
>   begin
>     // reallocate the array twice the size
>     NewAllocation := FAllocatedNumLines * 2;
>     FLines := ReAllocMem(FLines, NewAllocation * sizeof(TLayoutLine));
>     FAllocatedNumLines := NewAllocation;
>   end;
>   FLines^[ FNumLines ] := Line;    //  ***** (1)
>   inc( FNumLines );
> end;
> 
> 
> Like I said, maybe this is related to different implementations of
> object pascal. By doesn't one allocate memory (array elements) via the
> SetLength() procedure?

SetLength is a Borland extension. The code as shown is standard pascal
code. It's relative complexity simply shows why dyn. arrays were invented.