[Boost-users] [Iterators] How to implement proxy iterators?

Subject: [Boost-users] [Iterators] How to implement proxy iterators?
From: Oswin Krause (Oswin.Krause_at_[hidden])
Date: 2012-02-24 03:26:51

Hi Everybody,

I'm trying to implement a proxy_iterator for ublas::matrix which should
behave as an iterator over the matrix rows.

My "obvious" solution does not seem to work:

typedef boost::numeric::ublas::matrix<double> Matrix;
typedef boost::numeric::ublas::matrix_row<Matrix> MatrixRow;
typedef boost::numeric::ublas::vector<double> Vector;

struct MatrixIterator:
public boost::iterator_facade<
    MatrixIterator,
    Vector,//the semantic value type is a Vector
    boost::random_access_traversal_tag,
    MatrixRow//reference is a row of a matrix
>{
//snip constructors
private:
    friend class boost::iterator_core_access;

    //snip iterating stuff

    //fun part
    MatrixRow dereference() const {
        return row(*m_matrix,m_element);
    }
    std::size_t m_element;
    Matrix* m_matrix;
};

which reuslts in an error that MatrixRow can't be implicitely casted to
Vector. Which is correct. At the moment I'm using a 90% hacked workaround
which changes the ValueType of the Iterator to MatrixRow. This seems to
work as long as I don't use algorithms which rely on the actual value_type
of the iterator (for example iter_swap as implemented in the gnu libstdc++
will fail miserably even though a correct swap is implemented for the
MatrixRow...different story )

Since I need std::random_shuffle I am out of luck. I tried to find the
actual requirements of iterator_facade with respect to
ValueType/ReferenceType in the documentation, but was not able to find
them.
Does someone now a solution to this problem which works and results in an
iterator which can safely be used in random_shuffle? Or can point me to
the correct requirements?

Greetings from Germany,
Oswin Krause