Tuesday, March 6, 2012

virtual function adding complexity (letter to friend

Hi LA

I guess why many C programming teams avoid virtual keyword is because this innocent-looking keyword can cause complexity explosion when mixed with other elements --

- dynamic cast for pointer or reference variables
- pure virtual functions
- slicing -- without virtual, slicing is more consistent and easier to debug
- passing a RTTI object by reference -- note an object is an RTTI object IFF it has a virtual pointer.
- throwing an RTTI exception object by reference, by pointer or by value -- again, things are simpler without virtual
- member function hiding (by accident) -- without virtual, the hiding rule is simpler
- non-trivial destructors
- pointer delete -- for example, deleting a derived object via a base pointer is undefined behavior if the destructor is non-virtual. If you avoid virtual completely, we would less likely write this kind of buggy code.
- double pointers -- pointer to pointer to an RTTI object
- double dispatch -- usually involves virtual functions. Double-dispatch may not be very meaningful to non-RTTI objects.
- container of RTTI objects, such as vector of pointer-to-base, where each pointee can be a Derived1 object or Derived2 object... -- again, non-RTTI cases are simpler
- templates -- remember STL internally uses no virtual function, and perhaps very little inheritance
- smart pointers holding RTTI objects
- private inheritance
- multiple and virtual inheritance

Some experts advocate all base classes should have virtual destructor. In that case, avoiding virtual means avoiding inheritance. That would definitely reduce complexity.

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