This is just a kernel of an idea. It addresses a problem on which I have "subbed my toe" many, many times, namely that forward declarations do not express the inheritance hierarchy. This issue is significant enough that the Google C++ style guide advices against using forward declarations:
Pros:- Forward declarations can save compile time, as #includes force the compiler to open more files and process more input.
- Forward declarations can save on unnecessary recompilation. #includes can force your code to be recompiled more often, due to unrelated changes in the header.
Cons:- It can be difficult to determine whether a forward declaration or a full #include is needed. Replacing an #include with a forward declaration can silently change the meaning of code:
// b.h:
struct B {};
struct D : B {};
// good_user.cc:
#include "b.h"
void f(B*);
void f(void*);
void test(D* x) { f(x); } // calls f(B*)
If the #include was replaced with forward decls for B and D, test() would call f(void*).
The cited problem would disappear if struct D's forward declaration were allowed to include its base class:
struct D : B;
I am not enough of a language lawyer to know why this would not work and whether it might have problems in the presence of multiple base classes.
/john