partial specialization for iterator type of a specified container type

Question

I have a template struct, which accepts a Iterator type for the template argument. now I need to specialize that class for iterators of different containers. I have tried with std::vector

template<typename Iterator>
struct AC {

};

template<typename T, typename Alloc>
struct AC<typename std::vector<T, Alloc>::iterator> { //this doesn't work

};

but I got this compiler error(VS11): 'T' : template parameter not used or deducible in partial specialization

Can someone please tell me why this doesn't work? And how to make it work?

Answer 1

You can't deduce types left of a nesting ::. Indeed, your question makes no sense. Consider this simpler counter-example:

template <typename> struct Foo;
template <> struct Foo<bool> { typedef float type; };
template <> struct Foo<char> { typedef float type; };

template <typename> struct DoesntWork;

template <typename T> struct DoesntWork<typename Foo<T>::type> { };

Now if I say DoesntWork<float>, what should T be?

The point is that there is no reason that any T should exist for which Foo<T>::type is a thing you want to match, and even if there were one, there's no reason why it would be unique.