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

From:

"Tom Serface" <tom@camaswood.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 11 May 2009 08:18:47 -0700

Message-ID:

<u8xjKvk0JHA.436@TK2MSFTNGP02.phx.gbl>

I agree with the others here. You will have to implement some sort of
versioning or typing in the structure. Perhaps you could add an enum to the
structure that says what kind of data is being used and/or create new
functions for each kind of data. I think you will have trouble with
backwards compatibility so you'll need to watch out for that of course.

I don't think you're going to be able to make this kind of radical change
without modifying the DLLs to at least understand this going forward. I
think Scott's idea of having your client/consumer application normalize the
structure first makes the most sense. Maybe you could create a front end
function in your application that calls the DLL and just enter that way.

Tom

"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