Is it possible to subclass a C struct in C++ and use pointers to the struct in C code?

Question

Is there a side effect in doing this:

C code:

struct foo {
      int k;
};

int ret_foo(const struct foo* f){ 
    return f.k; 
}

C++ c

Answer 1

It probably will work but I do not believe it is guaranteed to. The following is a quote from ISO C++ 10/5:

A base class subobject might have a layout (3.7) different from the layout of a most derived object of the same type.

It's hard to see how in the "real world" this could actually be the case.

EDIT:

The bottom line is that the standard has not limited the number of places where a base class subobject layout can be different from a concrete object with that same Base type. The result is that any assumptions you may have, such as POD-ness etc. are not necessarily true for the base class subobject.

EDIT:

An alternative approach, and one whose behaviour is well defined is to make 'foo' a member of 'bar' and to provide a conversion operator where it's necessary.

class bar {
public:    
   int my_bar() { 
       return ret_foo( foo_ ); 
   }

   // 
   // This allows a 'bar' to be used where a 'foo' is expected
   inline operator foo& () {
     return foo_;
   }

private:    
  foo foo_;
};