Re: Changing a struct passed to a DLL without breaking everything?

From:

"Scot T Brennecke" <ScotB@MVPs.spamhater.org>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 11 May 2009 05:38:47 -0500

Message-ID:

<u8gwrSi0JHA.5288@TK2MSFTNGP03.phx.gbl>

You are planning to violate the "contract" of the DLL interfaces. Don't do that. If you want (and
are able) to redesign things "the right way", you can implement some versioning into the stucture
similar to what Giovanni suggested. But that will mean making all the DLLs aware of the structure
change to implement versioning.

If you want to continue without rebuilding the DLLs, the "next best right way" would be either make
completely separate new structures, and convert them back to the old struct layout before calling
the DLLs, or derive new structures from the old one and put your mods in the derived version. But,
of course, the DLL will be completely unaware of the new stuff. It will continue to see only the
old struct layout.

"Simon" <bad@example.com> wrote in message news:OcNYOhh0JHA.4716@TK2MSFTNGP02.phx.gbl...

Hi,

I know it might be a long shot, but hopefully someone has a 'neat' solution to my problem.

We have an exe that calls a function in a loaded DLL

// this is the function called on all the DLLs
void MyFunction( MY_OWN_STRUCTURE& sMyOwnStructure );

// the structure itself is defined
struct MY_OWN_STRUCTURE
{
  size_t numOfItems;
  int miscValues[ 255 ];
};

// I want to change the structure to be more dynamic, something like
struct MY_OWN_STRUCTURE
{
  size_t numOfItems;
  int* miscValues;

  // ... copy ~tor and so on...
};

Now, obviously the structure is totally different from the previous one, but I can easily convert
from the new one to the old one.

I do not want to change/recompile any of the older Dlls.
But, how can I safely pass the new structure to the old DLL? How can I tell what structure will be
used by the DLL?

Thanks

Simon