How to implement an iterator for a custom class?

March 3, 2022

For an assignment, we had to create custom list class. One of the functions is to insert an element into the middle of the list. So far I have,

 template<class T>
 class MyList : public list<T> {
 public:
   void addInMiddle(const T&);
   void insertInSortedOrder(const T&);
   void printList();
 };


template<class T>
void MyList<T>::addInMiddle(const T& x) {

 list<T>::iterator it = this->begin();

 int location = (this->size()) / 2;     //where we want to insert the new element
 for (int i = 0; i < location; i++) {
     it++;
 }
 this->insert(it,x);
}


 int main()
{
MyList<int> list1;
list1.push_back(1);
list1.push_back(2);
list1.push_back(3);
list1.push_back(4);
list1.addInMiddle(5);
list1.printList();
return 0;
}

The program compiles and shows 1,2,3,4. It should actually show 1,2,5,3,4. I don’t think the iterator is working. Any thoughts?

>Solution :

Your code works fine for me once I add typename in front of list<T>::iterator, since the type of the iterator is dependent on the type of the T template parameter, eg:

typename list<T>::iterator it = this->begin();

Online Demo

See Where and why do I have to put the "template" and "typename" keywords?

Alternatively, you can (and should) just use auto instead:

auto it = this->begin();

Online Demo

That being said, standard C++ containers are not intended to be inherited from (no virtual destructor, etc). You should use encapsulation instead of inheritance, eg:

template<class T>
class MyList {
private:
    list<T> m_list;
public:
    void addToBack(const T&);
    void addInMiddle(const T&);
    void insertInSortedOrder(const T&);
    void printList();
};

template<class T>
void MyList<T>::addToBack(const T& x) {
    m_list.push_back(x);
}

template<class T>
void MyList<T>::addInMiddle(const T& x) {

    list<T>::iterator it = m_list.begin();
    // or: auto it = m_list.begin();

    int location = m_list.size() / 2;     //where we want to insert the new element
    for (int i = 0; i < location; i++) {
        it++;
    }

    m_list.insert(it, x);
}

int main()
{
    MyList<int> list1;
    list1.addToBack(1);
    list1.addToBack(2);
    list1.addToBack(3);
    list1.addToBack(4);
    list1.addInMiddle(5);
    list1.printList();
    return 0;
}

Online Demo