Question

I hope this hasn’t been covered in some question before. I looked as best I could, but I think part of the problem in the first place is that I don’t understand what is up, and that may have prevented me from finding a previous answer. My apologies if so, but otherwise…

For practice with templates and generally understanding C++ and code design better, I’ve set to writing a (currently quite simple) implementation of a linked list, mostly seeking to mimic std::list. I’ve been working at implementing iterators properly, and the other components logically, but I’ve run into a snag. I’d guess it is with template syntax somewhere, but I’m not sure. It could be just some silly mistake.

Here’s the general structure of the class:

template <class T>
class LinkedList {
public:
    LinkedList();
    class Iterator;
    void push_front(const T&);
    void push_back(const T&);
    void pop_front();
    void pop_back();
    T& front();
    T& back();
    unsigned int size() const;
    bool empty() const;
    Iterator begin();
    Iterator end();
private:
    struct ListNode;
    ListNode* m_front;
    ListNode* m_back;
    unsigned int m_size;
};

template <class T>
class LinkedList<T>::Iterator {
public:
    Iterator();
    Iterator(const Iterator& rhs);
    Iterator(ListNode* const& node);
    Iterator operator=(const Iterator& rhs);
    T& operator*();
    bool operator==(const Iterator& rhs) const;
    bool operator!=(const Iterator& rhs) const;
    Iterator operator++();
private:
    ListNode* m_node;
};

template <class T>
struct LinkedList<T>::ListNode {
    T* m_data;
    ListNode* m_next;
};

And here is the offending function:

template <class T>
void LinkedList<T>::push_front(const T&) {
    if (m_front == NULL) {
        m_front = new ListNode;
        *(m_front->m_data) = T;
        m_front->m_next = NULL;
        m_back = m_front;
    } else if (m_front == m_back) {
        m_front = new ListNode;
        *(m_front->m_data) = T;
        m_front->m_next = m_back;
    } else {
        ListNode* former_front(m_front);
        m_front = new ListNode;
        *(m_front->m_data) = T;
        m_front->m_next = former_front;
    }
}

And the error given by GCC 4.6.3:

linkedlist.hpp: In member function ‘void pract::LinkedList<T>::push_front(const T&)’:
linkedlist.hpp:75:31: error: expected primary-expression before ‘;’ token
linkedlist.hpp:80:31: error: expected primary-expression before ‘;’ token
linkedlist.hpp:85:31: error: expected primary-expression before ‘;’ token

I hope all that helps, but if anything else would be desirable please do ask.
Thanks all.

Answer 1

The problems is on these lines:

*(m_front->m_data) = T;

This is attempting to assign a type to a variable, which is clearly not possible. Probably you want a named argument and to use said argument for this assignment:

template <class T>
void LinkedList<T>::push_front(const T& t) {
    if (m_front == NULL) {
        m_front = new ListNode;
        *(m_front->m_data) = t;
        m_front->m_next = NULL;
        m_back = m_front;
    } else if (m_front == m_back) {
        m_front = new ListNode;
        *(m_front->m_data) = t;
        m_front->m_next = m_back;
    } else {
        ListNode* former_front(m_front);
        m_front = new ListNode;
        *(m_front->m_data) = t;
        m_front->m_next = former_front;
    }
}