help please with list<string> iterator problem

[cakeathon]

I'm working my way through the accelerated C++ book, and exercise 10-5, & 10-6 have me stuck

I have the follwing class in a header file:

class String_list
{
public:
    String_list();
    void addString (const std::string s);
    void print_strings () const;
    std::string* biIterator(int dx);
private:
    std::string _dummy;
    std::list<std::string> _strings;
    std::list<std::string>::const_iterator _iter;
};
 
and its functions in a cpp file:
 
#include "String_list.h"
 
String_list::String_list()
{
    _iter = _strings.begin();
}
 
void String_list::addString(const string s)
{
    _strings.push_back(s);    
}
 
void String_list::print_strings () const
{
    list<string>::const_iterator it;
    for(it = _strings.begin();it!=_strings.end();++it)
        cout << *it << endl;
}
 
 
string* String_list::biIterator(int dx)
{
    //assigning _iter here to begin works, yet initializing in constructor doesnt
    //_iter = _strings.begin();
    if (dx>0){
        for(int count = 0;count!=dx;++count)
            ++_iter;
    }else{
        for(int count = dx;count!=0;--count)
            --_iter;
    }
    cout << *_iter << endl;
    _dummy = *_iter;
    string* pS = &_dummy;
    return pS;
}
 

All compiles OK, but I get a runtime error "list iterator not incrementable"

The bug goes away if instead of assigning _iter in the constructor I assign it in the biIterator function. Assigning _iter in the addString function gives the same error. I understand why I cant initialize it in the constructor, but why not in the addString func ? what is going on, can someone please help ?

[cakeathon]

I think I have found the bug after several hours of head scratching,

I created my String_list using a function that called addString and pass a copy back to main. In passing the copy back to main the iterator to begin is no longer valid.