member iterator - request for comment

Hi,

Recently I've met the need to perform operations like this quite
often:

struct A {
   int val;
}

....
std::vector<A> as;
....
std::set<int> vals;
std::vector<A>::const_iterator it, end = as.end();
for (it = as.begin(); it != end; ++it) {
   vals.insert(it->val);
}

that is, to perform some action on one of the members of each of the
collection's elements. I couldn't find any solutions within the STL,
so I've created my own MemberIterator class. The code is:

template<class Iterator, class PointerToMember, class Member>
class MemberIterator : public Iterator {
public:

     MemberIterator(Iterator it, PointerToMember ptr) : Iterator(it),
ptr_(ptr) {
     }

     Member& operator*() {
         return Iterator::operator*().*ptr_;
     }

private:

     PointerToMember ptr_;

};

template<class Member, class Iterator, class PointerToMember>
MemberIterator<Iterator, PointerToMember, Member> makeMemberIterator(
         Iterator it, PointerToMember ptr) {
     return MemberIterator<Iterator, PointerToMember, Member>(it, ptr);
}

Given this solution, I can write:

std::copy(makeMemberIterator<int>(as.begin(), &A::val),
makeMemberIterator<int>(as.end(), &A::val), std::back_inserter(vals));

and it works just fine.

I would like to know your opinion on
1. whether this class is implemented correctly or how I could make it
better
2. whether I should attempt to write stuff like this or use some STL
provided solution that I'm not aware of

Thanks,
Mikolaj

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