[fpc-pascal] PWideChar vs. ^

Marc Weustink marc at dommelstein.net
Sat Mar 14 00:17:40 CET 2009

Marc Santhoff wrote:
> Hi,
> maybe this is a dumb question, but:
> If I have a variable declared as
>   var
>     s: PWideString;
> why is an exception (AV) thrown when using
>     something := s^;
> but not when using
>     something := PWideChar(s);
> ?

You're mixing things up.

A Widestring is a dynamically allocated type which holds wide chars.

So when declaring

   W: Widestring;

you are under the hood in fact declaring a pointer to a dynamically 
allocated (referencecounted on non windows) struct holding the string.

What you declare is a pointer to this

   S: PWideString;

Like a variable of a PChar type this pointer doesn't contain anything 
but random noise, unless you initialized it.

When you assign
   something := s^;

you are dereferencing random data, which in most cases cause an AV. 
(even worse in this case since you assume this random data is a pointer 
to a widestring)

When you cast and assign
   something := PWideChar(s);

you treat this random pointer as a pointer to widechars. If you are 
lucky you are allowed to read this data and it is somewhere #0#0 
termintated, so that is can be intrepreted and converted to a something.

> Maybe this is special behaviour of the routine getting the variable, it
> is
>   TTreeView.AddChild(ParentNode: TTreeNode; const s: string): TTreeNode

I guess this is not the same S


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