Operator<< in C++ with custom Data Structures

Question

I'm trying to make a custom set of header files to use a generic List and use operator<< to make it print into a ostream object There are a couple of errors which I cannot resolve.

I'm trying to make a generic List class using a generic Node class. The generic List should be printed using the operator<< . However I'm getting a lot of errors.

#include <iostream>
using std::cout;
using std::ostream;

class List;

template<class T>
class Node
{
    friend class List<Node>;
    private:
    T data_;
    Node *next_;

    public:
    Node(T data);
    T get_data();
};

template<typename T>
Node<T>::Node(T data)
{
    data_ = data;
    next_ = 0;
}

template<typename T>
T Node<T>::get_data()
{
     return data_;
}

template<typename T>
class Node;

template<typename T>
class List
{
    template<typename T>
    friend ostream& operator<<(ostream& o , const List<T> head);

    private:
    Node<T> *start_;
    bool is_empty();

    public:
    List();
    ~List();
    void insert(T data);
    void remove();  
};

template<typename T>
bool List<T>::is_empty()
{
    if(start_ == 0)
        return true;        
    return false;
}

template<typename T>
List<T>::List()
{
    start_ = 0;
}

template<typename T>
List<T>::~List()
{
    if( !is_empty())
    {
        Node<T> *current = start_;
        Node<T> *temp;
        while(current != 0)
        {
            temp = current;
            current = current->next_;
            delete temp;
        }
    }
}

template<typename T>
void List<T>::insert(T data)
{
    if(is_empty())
    {
        Node<T> *temp = new Node<T>(data);
        start_ = temp;
    }
    else
    {
        Node<T> *temp = start_;
        while(temp->next_ != 0)
        {
            temp = temp->next_;
        }
        temp->next_ = new Node<T>(data);
    }
}

template<typename T>
void List<T>::remove()
{
    if(start_->next_ == 0)
    {
        Node<T> *temp = start_->next_;
        start_->next_ = 0;
        delete temp; 
    }
    else if(!is_empty())
    {
        Node<T> *stay = start_;
        Node<T> *remove = stay->next_;
        while(remove->next_ != 0)
        {
            stay = remove;
            remove = remove->next_;
        }
        stay->next_ = 0;
        delete remove;
    }
}


 // Experimental Stuff out here
template<typename T>
ostream& operator<<(ostream& o , const List<T> *head)
{
    Node<T> *temp = head->start_;
    if(!is_empty<T>())
    {
        while(temp->next_ != 0)
        {
            o << temp.get_data() << "\t";
            o << "\n";
            temp = temp->next_;
        }
    }
    return o;
}
//  End of Experimental Stuff

What are the errors you're getting? You might read through this: http://stackoverflow.com/questions/4421706/operator-overloading — Retired Ninja, Apr 05 '15 at 10:00
temp.get_data() This should be temp->get_data(), since temp is a pointer — codingEnthusiast, Apr 05 '15 at 10:01
There are no compiler errors related to operator overloading here. — n. m. could be an AI, Apr 05 '15 at 10:17
If you want help concerning the overloaded operator, strip all non-relevant code (see http://sscce.org/) and share the error message! — Ferdinand Beyer, Apr 05 '15 at 10:57

rwols · Accepted Answer · 2015-04-05T10:25:21.023

Assuming clients of your class instantiate it like this:

List<int> my_list;

You need to define your operator << overload like this:

template <typename T>
std::ostream& operator << (std::ostream& os, const List<T>& list)
{
    // I'm gonna assume you have C++11
    auto node = list.start_;
    while (node != nullptr)
    {
        os << '\t' << node->get_data() << '\n';
        node = node->next_;
    }
    return os;
}

Additionally, there are loads of errors in your code regarding templates, so please fix them first.

Operator<< in C++ with custom Data Structures

1 Answers1