[fpc-devel] Type Compatibility: dynarray <=> Pointer

[Thorsten Engler]

A zero length dynamic array is represented by a nil pointer instead of a pointer to the heap with the information that the length of the array is 0.

arr := nil;

is equivalent to:

SetLength(arr, 0);

And the nil assignment variant is pretty much ubiquitous in any code involving dynamic arrays that I'm aware of. 

> -----Original Message-----
> From: fpc-devel <fpc-devel-bounces at lists.freepascal.org> On Behalf
> Of Martok
> Sent: Tuesday, 17 April 2018 01:21
> To: fpc-devel at lists.freepascal.org
> Subject: [fpc-devel] Type Compatibility: dynarray <=> Pointer
> 
> Hi all,
> 
> I have started debugging 0031215, and discovered something slightly
> unrelated, but odd. I hope someone can clear that up.
> While testing, I found out that the following compiles, and becomes
> a call to
> fpc_dynarray_assign:
> 
> var
>   arr: array of byte;
> begin
>   arr:= Pointer(42);
> end.
> 
> This eventually happens because of the conversion case
> arraydef<=pointerdef in defcmp.pas:1151, where a comment says: "nil
> and voidpointers are compatible with dyn. arrays". This comment was
> there since the very first SVN import.
> 
> My question: why *are* voidpointers assignment compatible to
> dynarrays? It does seem to be Delphi compatible, but I couldn't
> find any mention in either documentation that this is possible -
> only the reserved/constant "nil" is compatible, and handled
> elsewhere.
> 
> --
> Regards,
> Martok
> 
> 
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