Alternative schemes for implementing vptr?

Question

This question is not about the C++ language itself(ie not about the Standard) but about how to call a compiler to implement alternative schemes for virtual function.

Answer 1

The short answer is that no, I don't know of any switch to do this with any common C++ compiler.

The longer answer is that to do this, you'd just about have to build most of the intelligence into the linker, so it could coordinate distributing the IDs across all the object files getting linked together.

I'd also point out that it wouldn't generally do a whole lot of good. At least in a typical case, you want each element in a struct/class at a "natural" boundary, meaning its starting address is a multiple of its size. Using your example of a class containing a single int, the compiler would allocate one byte for the vtable index, followed immediately by three byes of padding so the next int would land at an address that was a multiple of four. The end result would be that objects of the class would occupy precisely the same amount of storage as if we used a pointer.

I'd add that this is not a far-fetched exception either. For years, standard advice to minimize padding inserted into structs/classes has been to put the items expected to be largest at the beginning, and progress toward the smallest. That means in most code, you'd end up with those same three bytes of padding before the first explicitly defined member of the struct.

To get any good from this, you'd have to be aware of it, and have a struct with (for example) three bytes of data you could move where you wanted. Then you'd move those to be the first items explicitly defined in the struct. Unfortunately, that would also mean that if you turned this switch off so you have a vtable pointer, you'd end up with the compiler inserting padding that might otherwise be unnecessary.

To summarize: it's not implemented, and if it was wouldn't usually accomplish much.