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

A couple of observations:

1. Yes, a smaller value could be used to represent the class, but some processors require data to be aligned so that saving in space may be lost by the requirement to align data values to e.g. 4 byte boundaries. Further, the class-id must be in a well defined place for all members of a polymorphic inheritance tree, so it is likely to be ahead of other date, so alignment problems can't be avoided.

2. The cost of storing the pointer has been moved to the code, where every use of a polymorphic function requires code to translate the class-id to either a vtable pointer, or some equivalent data structure. So it isn't for free. Clearly the cost trade-off depends on the volume of code vs numer of objects.

3. If objects are allocated from the heap, there is usually space wasted in orer to ensure objects are alogned to the worst boundary, so even if there is a small amount of code, and a large number of polymorphic objects, the memory management overhead migh be significantly bigger than the difference between a pointer and a char.

4. In order to allow programs to be independently compiled, the number of classes in the whole program, and hence the size of the class-id must be known at compile time, otherwise code can't be compiled to access it. This would be a significant overhead. It is simpler to fix it for the worst case, and simplify compilation and linking.

Please don't let me stop you trying, but there are quite a lot more issues to resolve using any technique which may use a variable size id to derive the function address.

I would strongly encourage you to look at Ian Piumarta's Cola also at Wikipedia Cola

It actually takes a different approach, and uses the pointer in a much more flexible way, to to build inheritance, or prototype-based, or any other mechanism the developer requires.