Re: double dispatch, half compile-time

Ethan Eade <>
Fri, 2 Mar 2007 18:45:44 CST

One solution would probably be to use External Polymorphism, like:


int main()
    Variant v;
    Value<int> val;
    Adapter<Variant, long> av(v);
    I<long> *iV = &av;
    Adapter<Value<int>, long> aval(val);
    I<long> *iVal = &aval;

Right here you have assumed simultaneous explicit knowledge at compile
time of both int (Value<int>) and long (A == long). Once you have this,
the solution is easy (as you have shown). The problem is that I don't
have that. I know A at compile time, but I only know T up to abstract
base class:

template <class A> void single_bound(A& a, const Variant& v)
    // what can I do here to turn v into Adapter<A,Value<T> > ?

So unfortunately, your suggestion might be useful when you know A at the
time of the creation of the Variant, but not when A is supplied elsewhere.


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