On 10/22/07, Mathias Gaunard <mathias.gaunard@etu.u-bordeaux1.fr> wrote:
Corrado Zoccolo wrote:
BTW, the idea of the hand made vtable born in the discussion with gpd. My first implementation used just one function ptr to encapsulate all the behaviour (with a signature to fit all the methods), and was substantially less efficient than the current solution.
Instead of a hand make vtable, why not this?
struct impl_base { virtual void something(void*) = 0; };
template<typename T> struct impl { void something(void* p) { // do something useful } };
template<typename T> class poly { template<typename U> poly(const U& u) { new(&type_info) impl<U>; ... }
typename boost::aligned_storage< sizeof(impl<T>), boost::alignment_of< impl<T> > >::type type_info; };
This simply assumes that all instances of impl have the same size and alignment.
Because the vtables in C++ are not smart enough. If I make an hand made vtable, I can put not only function pointers, but also values inside it. This means that for example, I have to make a call through a function pointer to query if the object was allocated on heap or on stack, or if it has a no-throw copy constructor (all things that I can put directly in my hand made vtable). I think a valuable addition to c++0x would be "virtual constexpr" methods, i.e. virtual methods that are guaranteed to return compile time constants, but different classes in the hierarchy can return different values. This allows for a big optimization, i.e. the compiler can store the value of the constant into the vtable, and save an unpredictable call through a pointer. Corrado _______________________________________________
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-- __________________________________________________________________________ dott. Corrado Zoccolo mailto:zoccolo@di.unipi.it PhD - Department of Computer Science - University of Pisa, Italy -------------------------------------------------------------------------- The self-confidence of a warrior is not the self-confidence of the average man. The average man seeks certainty in the eyes of the onlooker and calls that self-confidence. The warrior seeks impeccability in his own eyes and calls that humbleness. Tales of Power - C. Castaneda