Adding new members to C++ structure and backwards compatibility

I have a struct which is used by others somewhere, they rely on sizeof(struct S) as well. Below is my struct,

struct S{
   int  aVar1;
   void *ptr1;
}

In future, I might add new members to it(Not sure right now) like..

struct S{
   int  aVar1;
   void *ptr1;
   int aVar2;
   void *ptr2;
}

I know this breaks the backward compatibility with the users who rely on sizeof.

To avoid this I plan to add some dummy pointers to struct which can be used in future.

struct S{
   int  aVar1;
   void *ptr1;
   void *dummyptr1;
   void *dummyptr2;
   void *dummyptr3;
   void *dummyptr4;
   void *dummyptr5;
   void *dummyptr6;
 }

Any better solution please to maintain backward compatibility ?