[fpc-pascal] Calling C++ object

[Jonas Maebe]

On 15 May 2009, at 05:06, <Josh.Bowden at csiro.au>  
<Josh.Bowden at csiro.au> wrote:

> I am trying to call a C++ object which is in a dll from a Pascal  
> interface unit as described in Rudy's Delphi corner: (http://rvelthuis.de/articles/articles-cppobjs.html 
> ). I use the inheritance method, making my C++ object a descendent  
> from a Virtual ancestor.
>
> I end up with run time errors in both fpc and the cross 64 bit  
> compiler (my desired target is Win64):
>
> error 210, (=R6025 - pure virtual function call)

Run time error 211 is for abstract function calls (= "pure virtual"  
function calls). 210 = "Object not initialized". You have range  
checking (or -CR) turned on. In this case, FPC inserts a run time  
check before calling any method, which verifies that the object/class  
is actually of the type that you declared in the source code.

If you make a class instance point to a C++ class instance, that check  
will obvious fail.

> However, the code works using Delphi.

It relies on the VMT of the Pascal and C++ classes having an identical  
layout. This may work with certain versions of certain C++ compilers  
on certain platforms with Delphi and might also work in FPC (with the  
same caveats), but it can break at any time and is simply a hack. I  
would strongly recommend against using this at all, let alone in  
production software.


Jonas