Question

I have a code of doubly Linked list and there are few lines of that code which idea is not clear . I request for experts comments on the lines which have comment in the following .Its long since I last time used C++ . There are just two line which i indicated are not understandable for me.

template <typename T>
class double_linked
{
    struct node
    {
        T data;
        node* prev;
        node* next;
        node(T t, node* p, node* n) : data(t), prev(p), next(n) {}
    };
    node* head;
    node* tail;
public:
    double_linked() : head( NULL ), tail ( NULL ) {}
    template<int N>
    double_linked( T (&arr) [N]) : head( NULL ), tail ( NULL ) 
    {
        for( int i(0); i != N; ++i)
            push_back(arr[i]);
    }

    bool empty() const { return ( !head || !tail ); } // this doing? 
    operator bool() const { return !empty(); } // this doing? i know operators need in C++ but dont know the use of it here 
    void push_back(T);
    void push_front(T);
    T pop_back();
    T pop_front();

    ~double_linked()
    {
        while(head)
        {
            node* temp(head);
            head=head->next;
            delete temp;
        }
    }
};

Answer 1

operator bool() const is a conversion operator. If an instance of double_linked is used in a context where a bool is required, this function will be called to do the conversion. (And will evaluate to true if the list is not empty in your case.)

The other function is a plain old function, which will return true if either head or tail is null.

For more about conversion operators, see: How do conversion operators work in C++?