Iterator Implementation Compile error

[pnayak]

Hi,

I am trying to implement an iterator to a List class. The code is included below. I get the following compiler error. At the point where I use a class that is defined inside a template (nList<T>::Link). The Link class is defined inside of a template. (I assume that is possible?). Any one know what I doing wrong?

Many Thanks,
Purush

Error from Compiler (gcc 3.4.6)
nList.cc:52: error: expected `;' before "current_"
*** Error code 1

Stop in /usr/home/annu/work/c++/Containers/dlists.

#include <iostream>
 
template<class T> class Iterator
{
    public:
        virtual T* first() = 0;
        virtual T* next() = 0;
        virtual T* previous() = 0;
};
 
template<class T> class List_Iterator;
 
template<class T> class nList
{
    public:
        nList(): head_(0), tail_(0), size_(0) {}
        void push_front(const T& r);
        void push_back(const T& r);
        void insert(int position, const T r);
        int size() const;
        void print() const;
        class Bad_Range{};
        //friend class List_Iterator<T>;
 
        class Link
        {
            public:
            Link* next;
            Link* previous;
            T val;
            Link(T v):next(0), previous(0),val(v) {}
            Link():next(0), previous(0) {}
        };
 
 
    private:
        Link* head_;
        Link* tail_;
        int size_;
};
 
 
template<class T> class List_Iterator : public Iterator<T>
{
    public:
        T* first();
        T* next();
        T* previous();
        List_Iterator(nList<T>& l): list_(l){};
    private:
        nList<T>& list_;
        nList<T>::Link current_;
};
 
//template<class T> T* List_Iterator<T>::first()
//{
//    return &list_.front();
//}
//
//template<class T> T* List_Iterator<T>::next()
//{
//    if (current_)
//        current_ = current_->next;
//
//    return &current_->val;
//}
 
//template<class T> T* List_Iterator<T>::previous()
//{
//    if (current_)
//        current_ = current_->previous;
//
//    return &current_->val;
//}
 
template<class T> int nList<T>::size() const
{
    return size_;
}
 
template<class T> void nList<T>::print() const
{
    Link* l;
 
    for (l = head_; l; l = l->next)
        std::cout<<l->val<<'\n';
 
}
 
template<class T> void nList<T>::push_front(const T& r)
{
    Link* l = new Link(r);
 
    l->next = head_;
    l->previous = 0;
 
    if (head_)
        head_->previous = l;
 
    head_ = l;
 
    if (!tail_)
        tail_ = l;
 
    size_++;
}
 
template<class T> void nList<T>::push_back(const T& r)
{
    Link* l = new Link(r);
 
    l->previous = tail_;
    l->next = 0;
 
    if (tail_)
        tail_->next = l;
 
    tail_ = l;
 
    if (!head_)
        head_ = l;
 
    size_++;
}
 
template<class T> void nList<T>::insert(int position, const T r)
{
 
    if (position > size_)
        throw Bad_Range();
 
    Link* l = new Link(r);
 
    Link* iterator = head_;
    for (int i = 0; i < position; i++)
        iterator = iterator->next ;
 
    if (iterator)
    {
        iterator->previous->next = l;
        l->next = iterator;
        iterator->previous = l;
    }
    else
    {
        push_back(r);
    }
    size_++;
}
 
int main()
{
    nList<int> il;
}
 

[gpraghuram]

I havent gone through the code.
But i tried compiling in my G++(cygwin) version 2.95.3-4 and it compiled successfully


raghuram

[pnayak]

I tried on another box with a 4.2 version of gcc and same problem. However the Error message in this case is,

nList.cc:50: error: type 'nList<T>' is not derived from type 'List_Iterator<T>'
nList.cc:50: error: expected ';' before 'current_'

[weaknessforcats]

nList<T>::Link current_;

is a thing called a dependent type. Probably the class should look like:

template<class T> class List_Iterator : public Iterator<T>
{
    public:
        T* first();
        T* next();
        T* previous();
        List_Iterator(nList<typename T>& l): list_(l){};
    private:
        nList& list_;
        nList::Link current_;
};
 

Visual Studio.NET 2008 gets the same error you get but that error goes away when the class is changed as above.