Return pointer to template's nested class problem

Kimmo · #1 19-Sep-2007, 14:14

(Sorry for the subject..)

I had hoped to reveal my linked list implementation to the world this evening, but alas, I ran into a problem I can't overcome.

The problem is, as best described, this.

I have a class LinkedList, in which I have a nested class Node. Some of my methods in LinkedList return pointers to class Node objects. Everything seemed to work (as best I could test) before templating the LinkedList class. After templating I get the following warnings and errors on my method definitions that try to return pointer to Node objects

Code:

Warning	1	warning C4346: 'LinkedList<dataType>::Node' : dependent name is not a type
Error	2	error C2143: syntax error : missing ';' before '*'	
Error	3	error C4430: missing type specifier - int assumed.

Here are the class declarations

CPP / C++ / C Code:

template <typename dataType>
class LinkedList
{

	class Node
	{
	private:
		
		Node(const Node&);
		Node& operator=(const Node&);
		
	public:

		dataType m_data;
		Node* m_next;
		Node* m_previous;

		Node(dataType data);
		Node();
		~Node() {}

	};

	

private:

	Node* m_firstNode;
	Node* m_lastNode;
	unsigned int m_numberOfNodes;

	LinkedList(const LinkedList&);
	LinkedList& operator=(const LinkedList&);

public:

	LinkedList();
	~LinkedList();

        // methods...

        // The culprits
	// getFirstNode() returns a pointer to first Node in list
	Node* getFirstNode();

	// getLastNode() returns a pointer to last Node in list
	Node* getLastNode();
};

And here are the method definitions

CPP / C++ / C Code:

template <typename dataType>
LinkedList<dataType>::Node* LinkedList<dataType>::getFirstNode()
{

	// if list is empty, return 0
	if (m_numberOfNodes == 0)		
	{
		return 0;
	}
	else
	{
		return m_firstNode;
	}
}

// similar one for getLastNode()

So there's probably something amiss in the scoping of Node class? But I don't know what, since other methods like

CPP / C++ / C Code:

template <typename dataType>
bool LinkedList<dataType>::isEmpty() const

compile fine. And the same pattern is used for the nested class.

Yes, I tried looking for explanations, but didn't come up with any. Or I just missed them as irrelevant.

Kimmo · #2 20-Sep-2007, 00:37

Okay, in the morning I'm back in business and found this poor solution. Making the methods inline works. As in the following

CPP / C++ / C Code:

template <typename dataType>
class LinkedList
{
private:
// stuff
// Node class declaration

public:

// stuff

    Node* getFirstNode()
    {

        // if list is empty, return 0
        if (m_numberOfNodes == 0)		
        {
            return 0;
        }
        else
        {
            return m_firstNode;
        }
    }
};

Now apparently I don't want to use this solution for more complex methods, so I would be grateful on tips how to get this to work non-inline.

Kimmo · #3 20-Sep-2007, 23:39

Yet another morning and more business.

So I thought "I wonder how this compiles on GCC" and went off to test it. And, believe it or not, GCC told me in plain English what was wrong.

Here's how it's supposed to be for those interested.

CPP / C++ / C Code:

template <typename dataType>
typename LinkedList<dataType>::Node* returnNode()
{}

However, before getting this plain-Englished answer handed to me, I realized that returning a Node is just plain wrong. It's the data I want to return, not Node. So I changed the design already and this is not that relevant in my linked list anymore, but might come in handy sometime.

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Return pointer to template's nested class problem

Re: Return pointer to template's nested class problem

Re: Return pointer to template's nested class problem