The behavior of istream.

I was trying to understand the concept of Iterator from the book "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
I was trying to understand the program in Chapter 20.6.2
But the exact copy of the code was crashing and was not able to read the newline also from the console.
So I did little bit of experiment and change the code as follows

#include <iostream>
#include<list>
#include <vector>
using namespace std;
class Text_iterator;

typedef vector<char> Line;

class Text_iterator {
    list<Line>::iterator ln;
    Line::iterator pos;
public:
    Text_iterator( list<Line>::iterator ll, Line::iterator pp ):ln(ll),pos(pp) {

    }
    char& operator *() {
        return *pos;
    }
    Text_iterator & operator++() {
        if ( pos == ln->end() ){
            ++ln;
            pos = ln->begin();
        }
        else {
        ++pos;
        }
        return *this;
    }
    bool operator ==( const Text_iterator &rhs ) const{
        return ln == rhs.ln && pos ==rhs.pos;
    }

    bool operator != (const Text_iterator &rhs ) const{
        return !(*this == rhs);

    }

};

struct Document {
    list<Line> line;
    Document() {
        line.push_back(Line());
    }
    Text_iterator begin() {
        return Text_iterator(line.begin(),(line.begin())->begin() );

    }
    Text_iterator end() { //This is the main modification to

        list<Line>::iterator last = line.end();
        --last;
        return Text_iterator(last, last->end());
        //return Text_iterator(line.end(),(line.end())->end() );

    }

};

void print(Document &d) {
    for (Text_iterator p = d.begin(); p != d.end(); ++p) {
        cout << *p;
    }
}

//Insert into Document
istream & operator>>(istream &is, Document &d) {

    char ch;
    while (is.get(ch)) { //This is also modified to read '\n' and space
        d.line.back().push_back(ch);
        if (ch == '\n') {
            d.line.push_back(Line());
        }
    }
    return is;
}

int main() {
    Document dd;
    cin>>dd;
    print(dd);
    return 0;
}

But while I ran the code with the following input I get extra character in output. That's the place I am perplexed.

Input
+++++++++
hello
how are you
I am fine
 I
++++++++++
Output
hello
how are you
I am fine
 I
a
++++++
So I am getting extra character "a" in output.
Could you please help me to understand the reason for this.

--
Somenath