wrap std::iterator in C++

Question

I have a need to wrap a vector iterator, but don't like the idea to rewrite it from scratch. And I can't subclass it as far as vector iterator doesn't seem to be cross-platform. At least gnu and ibm ones look different.

What I want to do is the following:

class MyContainer {
    vector<double> data;
    vector<int> indices;

    iterator
    begin()
    { return my_iterator(data, indices.begin()); }

    iterator
    end()
    { return my_iterator(data, indices.end()); }
}

MyContainer  cont;

Where indices vector contains integer positions within the data vector. Data is supposed to be much much bigger than the indices.

So I need an iterator that can go through the indices in any direction like a normal vector iterator does with the only exception: it must return a value of data vector when the value is going to be accessed. eg:

for(MyContainer::iterator it = cont.begin(); it != cont.end(); it++) {
    cout << *it << endl; // values of data should appear here
}

Basically it should look like a normal collection for the std world. You can iterate it in whatever direction you want, you can sort it, run unique, find_if, etc...

any simple solution?

Answer 1

There's a great Boost library for defining custom iterators. You need to provide a class with a few methods:

i.dereference()  Access the value referred to
i.equal(j)       Compare for equality with j
i.increment()    Advance by one position
i.decrement()    Retreat by one position
i.advance(n)     Advance by n positions
i.distance_to(j) Measure the distance to j

Then you get the rest from the iterator_facade .

Good luck!

Answer 2

This looks a lot like a permutation_iterator , one of the "built in" adapters from the Boost.Iterator Library

See this example (modified from the Boost docs) on codepad.

Answer 3

There's nothing in the standard C++ library, but you can probably get boost::iterator_adapter to do what you want. A preliminary examination suggests you'll need to override iterator_adapter::dereference and iterator_adapter::equal .

template <typename _Scalar=double,
          typename _Idx=int, 
          typename _Seq=std::vector<_Scalar>, 
          typename _IdxVector=std::vector<_Idx> >
class SelIter 
    : public boost::iterator_adaptor< SelIter<_Scalar, _Idx>, 
                                      typename _IdxVector::iterator, _Scalar > 
{
public:
    typedef boost::iterator_adaptor< SelIter, typename _IdxVector::iterator, _Scalar > Base;

    SelIter(_Seq& scalars, _IdxVector& idxs);
    SelIter(_Seq& scalars, typename _IdxVector::iterator pi);

    typename Base::reference dereference() const;
    bool equal(const SelIter& x) const;
private:
    // ...
}