自己的迭代器不起作用

Question

I work on own iterator , that should iterate double linked list. If I'm iterating, for cycle is immediately skipped. I haven't errors, but from debugging I don't read much.

for cycle:

for(SemestralWork::DoubleList<Student>::iterator it = link->begin(); it != link->end(); ++it){
         //something...
    }

iterator + begin() + end() :

class iterator {
private:
    Node<T> * node;
public:
    iterator(){}
    iterator(const iterator& cit){}
    iterator(T t) {

    }
    ~iterator(){}

    iterator& operator=(const iterator& second){
        node = second.node;
        return(*this);
    }
    iterator& operator++(){     
        if (node != NULL){
            node = node->GetNext();
        }
        return(*this);
    }
    iterator operator++(int){
        iterator tmp = *this;   //iterator tmp(*this)
        operator++();
        return tmp;
    }
    bool operator==(const iterator& second) {
        return node == second.node;
    }
    bool operator!=(const iterator& second) {
        return node != second.node;
    }
    T& operator*() {return node->GetData();}
    T* operator->(){return((DoubleList<T>::iterator)*this);} 
};

iterator begin(){
    return iterator(first->GetData());
}

iterator end(){
    return iterator(last->GetData());
}

Node :

template <class U>
    class Node{
        Node<U> * next;
        Node<U> * previous;
        U data;
    public:
        Node(const U &data){
            next = NULL;
            previous = NULL;
            this->data = data;
        }
        void SetNext(Node<U> *next) { this->next = next; }
        Node<U> *GetNext(){ return next; }
        void SetPrevious(Node<U> *previous) { this->previous = previous; }
        Node<U> *GetPrevious(){ return previous; }
        U &GetData() { return data; }
    };

Answer 1

Here are a few things I noticed. Whether any of these will actually resolve your problem I don't know because the posted code is incomplete:

1. Your iterator should not be constructed from a T object and the implementation of the constructor should actually do something (I would guesss the fact that iterator::iterator(T) doesn't do anything at all is your actual problem). Instead, the iterator should be constructed from an Node<T>* .
2. The preincrement operator should not check if there is actually a next element! It is a precondition for the operator++() that the iterator can be incremented. If anything, the operator should report a misuse with debug settings enabled.
3. I'm suspicious of your use of last : note that the end iterator is a position one past the last element.

4. Your comparision oerators should be const members and typically operator!=() just delegates to operator==() , ie:

    bool iterator::operator!=(iterator const& other) const { return !(*this == other); } 

   The advantage of this implementation is that the operator!=() is consistent with the operator==() even if the implementation of operator==() is changed.

Answer 2

You didn't post the list implementation showing first and last , but I'm assuming last points at the last element. With iterators, end() should point beyond the last element, not to the last element. For example, if the list contains exactly 1 element, your for-loop won't run at all since first == last and therefore begin() == end() .