[fpc-pascal] Re: fpc-pascal Digest, Vol 26, Issue 9

[Alexander Todorov]

Michael Van Canneyt wrote:

> > 1) If the Flag field is set to true and the programmer uses field from
> > the variant part which is declared in the false section, should there
> > be a run time error or not?
>
> No. No checking is performed on this.
>
> > Although it will be slowly should the compiler check V.Flag and
> > consider the other fields undefined? Something like "Inaccessible
> > record field used" error?
>
> No.
>
> The 'Flag' field is purely informational and has no restrictive meaning.
> It's a hint to the programmer, but nothing more.
> This is how pascal is defined.
>
> Michael.

Thanks for the explanation Michael.
Maybe it will be good these lines  to be added in the FPC docs for
reference to newbies.

Jonas Maebe wrote:

> > 2) In the test program the addresses of the two fields are the same
> > due to the default aligning used by the compiler. That's why the
> > output is the number 100.
> > In Delphi these addresses differ in 1 byte, so the ouput is :
> > V.Flag=FALSE, V.false_1=100,  V.true_1=200
> > V.Flag=TRUE,   V.false_1=100,  V.true_1=200
> > Is this by design or FPC has an inconsistency with Delphi?
>
> Both. All components of the variant part of a record always start at
> the same address by design (because it avoids changing the
> overlapping of the variant fields depending on the packrecords
> setting), and apparently this is different in Delphi.
>
>
> Jonas

Should this difference with Delphi considered a bug or not? IMO this
is not a bug.
Are there docs for the compiler desing and it's implementation of the
Pascal language?
I admit I haven't searched the site for that. Such differences should
be documented somewhere.