🚀 go-pugleaf

Exporting an object out of a DLL is possible but not generally
recommended. The problem is that the export is language and often
compiler specific. This means that by exporting objects you are
probably forcing clients to generate their code using the same compiler
that you used. This defeats one of the main purposes of DLLs.

The generally accepted way to 'export an object' is to provide a handle
based interface to it. You can either export an 'int' that the DLL uses
to index into an array:

// DLL
TObject * objectArray[ MaxObjects ];

int method_TObjectWibble( int Handle,
                          char const * info )
{
  return objectArray[ Handle ]->Wibble( info );
}

int method_TObjectWibbleCTOR()
{
  int freeHandle=getFreeSlot();

  if( freeHandle!=-1 )
    objectArray[ freeHandle ]=new TObject();

  return freeHandle;
}

Or export a pointer to the object but declare it as 'void *':

// DLL
int method_TObjectWibble( void * Handle,
                          char const * info )
{
  return reinterpret_cast<TObject *>( Handle )->Wibble( info );
}

void * method_TObjectWibbleCTOR()
{
  return new TObject();
}

The first solution is a bit slower and requires more 'infrastructure'
in the DLL but is easier to police.

The advantage of both methods is that you are exporting things that
every language understands and thus anyone can use your DLL. The
downside is that you lose the encapsulation to an extent but you can
always provide a wrapping object that the EXE writer can add to their
project to hide the API.
-- 
Andrue Cope [TeamB]
[Bicester, Uk]
http://info.borland.com/newsgroups/guide.html