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:
[code=cpp]
template<class T> class List_Iterator : public Iterator<T>
{
public:
T* first();
T* next();
T* previous();
List_Iterator(n List<typename T>& l): list_(l){};
private:
nList& list_;
nList::Link current_;
};
[/code]

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