[fpc-devel] Variants and strings

[Joost van der Sluis]

> > I've found a problem in the string-implementation of the variants, but I
> > thought I would better discus it first, before I start doing all sort of
> > work for nothing.
> >
> > In variants string-values are stored as PWideChar's. (called vOleStr)
> > But if you set a string-value to a variant, it's stored and initialized
> > as a WideString. Widestrings are ref-counted, and since it's getting
> > casted into a PWideChar, things are going wrong. Example:
> >
> > var v : variant;
> >     s : string;
> >
> > begin
> >   v := 'Hello World';
> >   s := v;
> >   writeln(s);
> >   s := v;
> >   writeln(s);
> > end;
> >
> > returns only one 'ello World' or in some cases '   d'. That's because in
> > the assignment (s:=v) the memory of the Widestring is already freed
> > before it's value is assigned to the string. (A new pwidechar is
> > initialised at the same memory-location, wich sets the first character
> > to #0, the termination of the newly-initialised pwidechar)
> >
> > I've fixed this in changing the assignment-part to use a PWideChar
> > instead of the Widestring. The problem of that solution is that you have
> > to allocate memory for the PWideChar. If you change the value of the
> > variant, memory should be re-allocated. And I'm curious if the memory
> > will be freed when the variant is freed. Writing all those things would
> > mean that you're writing all sorts of code, which is already there for
> > Widestrings!
> 
> You must be careful with this. The implementation on Linux will be
> different than on Windows: On Windows the windows variant routines
> are used to manage the variants (because they can be passed to other
> processes via COM) and on Linux, FPC specific routines are used.

That explains the location of there implementation. I have no experience
with COM, windows-variant internals etc. Can someone try the above
example under Windows?

> We should take care that the two remain compatible.

Are you sure they are now?

> > So I want to suggest to change the string-support in variants form
> > PWideChar into Widestrings. This is also more Delphi-compatible.
> > According to the Delphi help strings in variants are stored in
> > Widestrings...
> 
> This is not quite true. (or it may depend on the Delphi version)
> 
> "if a variant containing an AnsiString is assigned to an OleVariant,
> the AnsiString becomes a WideString."

Yep, but in FPC (Linux, haven't looked into win32 stuff), variants can't
contain any ansistrings. There are only two 'string-types' available in
FPC's variants: PWideChar (which they call OleVariant, partially
implemented as widestrings as I stated above) and VarString, which
aren't implemented or used any further.

On Delphi there are shortstrings, widestrings, ansistrings, pwidechars,
widechars and pchars. (see the help on TVarRec) This is all very
different from the implementation in FPC.

> So this needs some thinking, and some input from the compiler guys.

Probably, yes. (compiler guys, anyone?)
Basically I think that we should look how win32 handles this, and
implement it that way into linux/other OS's.

-- 
Met vriendelijke groeten,

  Joost van der Sluis
  CNOC Informatiesystemen en Netwerken
  http://www.cnoc.nl