How to catch current stream position before istream_iterator

Question

I am trying to create an extension of class istream_iterator that saves the input stream position before each read. How should I implement the constructor so that it can catch the stream position at the time the constructor was called? Because it is a subclass of istream_iterator, the constructor constructs the base class first; but after the base class has been constructed, the stream position will have changed (this is how istream_iterator works). Short of reimplementing istream_iterator (which is probably the best option because the istream_iterator does not let me ask its underlying stream for its current position, meaning that I need to keep another reference to the same input stream), what are my options here?

Answer 1

Check the next approach too:

class MyIterator : istream_iterator<char>
{
public:

    MyIterator( istream& stream )
        : MyIterator( stream, stream.tellg() )
    {
    }

private:

    MyIterator( istream& stream, istream::pos_type pos )
        : istream_iterator( stream ), startPos( pos )
    {
    }

    istream::pos_type const startPos;
};

Answer 2

Something like this should work:

MyIterator::MyIterator(istream& stream)
    : istream_iterator((cur_pos = stream.tellg(), stream)) {}