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.

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.

Thorsten Engler wrote

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

Yes. I know ;-)

> only the reserved/constant "nil" is compatible, and handled elsewhere

I asked specifically about assigning arbitrary untyped pointers.

FWIW this has been changed in Delphi 10.2 and does not compile anymore but gives: E2010 Incompatible types: 'Dynamic array' and 'Pointer'

Stefan Glienke wrote

FWIW this has been changed in Delphi 10.2 and does not compile anymore but gives: E2010 Incompatible types: 'Dynamic array' and 'Pointer'

That is what I would have expected.
Could you please test what the following does in D10.2?

var arr: array of byte;
begin
  arr:= Pointer(0);
end.

What I'd like to know: do they check the "nil" literal, or pointers of value nil?

That compiles as well because Pointer(0) is converted by the compiler to nil.

There is even a dedicated warning for this when used in a const:

http://docwiki.embarcadero.com/RADStudio/Tokyo/en/W1013_Constant_0_converted_to_NIL_(Delphi)

---
Diese E-Mail wurde von Avast Antivirus-Software auf Viren geprüft. https://www.avast.com/antivirus