Error on template functions returning pointers

[edyam]

I'm trying to template a pointer-based list class and several functions (the ones in which I try to return a pointer) are producing error messages.

The class:

template <typename T>
class List : public BasicADT
{
    public:
 
        List();     // constructor
        List(const List<T>& aList);    // copy constructor
        virtual ~List();    // destructor
 
        // overloaded assignment operator
        List<T> & operator=(const List<T>& rhs);
 
        // member functions
        virtual bool isEmpty() const;
        virtual int getLength() const;
        virtual void insert(int index, const T& newItem);
        virtual void remove(int index);
        virtual void retrieve(int index, T& dataItem) const;
        virtual void removeAll();
 
    private: 
 
        struct ListNode
        {
            T item;
            ListNode *next;
        };    // end ListNode
 
        int size;
        ListNode *head;
        ListNode *find(int index) const;
        void copyListNodes(List<T> origList);
 
    protected:
 
        void setSize(int newSize);
        ListNode *getHead() const;
        void setHead(ListNode *ptr) const;
        T getNodeItem(ListNode *ptr) const;
        ListNode *getNextNode(ListNode *ptr) const;
 
} ;    // end List

The functions:

template <typename T>
ListNode *List<T>::find(int index) const
{
    if ( (index < 1) || (index > getLength()) )
    {
        return null;
    }
 
    else
    {
        ListNode *cur = head;
 
        for (int skip = 1; skip < index; ++skip)
        {
            cur = cur->next;
        }    // end for
 
        return cur;
    }    // end if    
}    // end find
 
template <typename T>
ListNode *List<T>::getHead() const
{
    return head;
 
}    // end getHead
 
template <typename T>
ListNode *List<T>::getNextNode(ListNode *ptr) const
{
    return ptr->next;
}    // end getNextNode

I'm getting the same error on the first line of each of the above functions:

error: expected constructor, destructor, or type conversion before '*' token

I've written constructors and destructors for the templated List class, so I'm not sure why only these three functions are giving me problems. The majority of my code for this project was copied out of the textbook, as per the assignment, so I'm not sure what the problem is. All of the other member functions seem to compile just fine.

Any help would be greatly appreciated!

[Laharl]

Try moving the * to right after ListNode. So, ListNode* List<T>::getHead(), rather than ListNode *List<T>::getHead(). The second one implies that the class is called *List<T> and the function returns a ListNode. The first tells the compiler that the class is called List<T> and the function returns a ListNode pointer.

[edyam]

Hmm... Thanks for the suggestion, but it didn't seem to make a difference. Any other ideas?

Thanks again!

[Ganon11]

Try moving the * to right after ListNode. So, ListNode* List<T>::getHead(), rather than ListNode *List<T>::getHead(). The second one implies that the class is called *List<T> and the function returns a ListNode. The first tells the compiler that the class is called List<T> and the function returns a ListNode pointer.

Laharl, actually, the whitespace here is a matter of preference - especially since classnames cannot start with the * character (else every time you said int *x, you would get an error).

I expect that, since ListNode is itself a template struct, you need to treat it as a template in your return type:

ListNode<T>* List<T>::blahblahblah()

[edyam]

I expect that, since ListNode is itself a template struct, you need to treat it as a template in your return type:

ListNode<T>* List<T>::blahblahblah()

Thanks, Ganon, but all that does is produce the following error...

error: expected constructor, destructor, or type conversion before '<' token

Thanks again!

[tavianator]

Actually, ListNode is a nontemplate, nested struct. Your function should return a List<T>::ListNode*. So

template <typename T>
List<T>::ListNode* List<T>::foo();
 

when declaring member functions inside classes, however, the List<T>:: is unnecessary.

[edyam]

Actually, ListNode is a nontemplate, nested struct. Your function should return a List<T>::ListNode*. So

template <typename T>
List<T>::ListNode* List<T>::foo();
 

when declaring member functions inside classes, however, the List<T>:: is unnecessary.

That makes a lot of sense, tavianator, but I'm still getting the same error. Thanks anyway.

[edyam]

I tried commenting out the three functions that were giving me errors, but that just caused me to get the following error on the first line of every member function:

error: expected initializer before '<' token

Any ideas/suggestions? Thanks in advance!

[Ganon11]

Wait, I just remembered;

Are you defining these functions in the same file as your class? Template functions have to be defined in the same place they are declared (i.e. in the .h file).

[tavianator]

Actually, List<T>::ListNode is a dependent name, so typename is required. Declare the functions like this:

template <typename T>
typename List<T>::ListNode* List<T>::find()
 

etc.

[edyam]

Re: Ganon

Adding 'typename' before the function results in the following error:
error: expected nested-name-specifier before 'List'

Re: tavianator

They're in two seperate files, but I've included

#include "ListTemplate.cpp"

at the bottom of ListTemplate.h, as per my professor's instructions for the project. This should be equivalent to having them in the same file, correct?

[tavianator]

I'm pretty sure you switched "re: Ganon" and "re: tavianator".

Anyway, the following code compiles fine with g++ 4.2.2. Some old compilers handle typename incorrectly, and that may be why the error persisted when you put typename List<T>::ListNode *List<T>::find(). Make sure you write it that way in all cases.

template <typename T>
class List
{
  public:
 
    List();  // constructor
    List(const List<T>& aList);// copy constructor
    virtual ~List();  // destructor
 
    // overloaded assignment operator
    List<T> & operator=(const List<T>& rhs);
 
    // member functions
    virtual bool isEmpty() const;
    virtual int getLength() const;
    virtual void insert(int index, const T& newItem);
    virtual void remove(int index);
    virtual void retrieve(int index, T& dataItem) const;
    virtual void removeAll();
 
  private: 
 
    struct ListNode
    {
      T item;
      ListNode *next;
    }; // end ListNode
 
    int size;
    ListNode *head;
    ListNode *find(int index) const;
    void copyListNodes(List<T> origList);
 
  protected:
 
    void setSize(int newSize);
    ListNode *getHead() const;
    void setHead(ListNode *ptr) const;
    T getNodeItem(ListNode *ptr) const;
    ListNode *getNextNode(ListNode *ptr) const;
 
} ;// end List
 
template <typename T>
typename List<T>::ListNode *List<T>::find(int index) const
{
  if ( (index < 1) || (index > getLength()) )
  {
    return 0;
  }
 
  else
  {
    ListNode *cur = head;
 
    for (int skip = 1; skip < index; ++skip)
    {
      cur = cur->next;
    } // end for
 
    return cur;
  } // end if 
} // end find
 
template <typename T>
typename List<T>::ListNode *List<T>::getHead() const
{
  return head;
 
} // end getHead
 
template <typename T>
typename List<T>::ListNode *List<T>::getNextNode(ListNode *ptr) const
{
  return ptr->next;
} // end getNextNode
 

[Ganon11]

Re: tavianator

They're in two seperate files, but I've included

#include "ListTemplate.cpp"

at the bottom of ListTemplate.h, as per my professor's instructions for the project. This should be equivalent to having them in the same file, correct?

Yeah, this should have been Re:Ganon, but I figured it out XD

No, this is not the same. #include "ListTemplate.cpp" just means your program knows about the existence of the .cpp file and will look there for the function definitions. You need to define template functions in the same file i.e. in ListTemplate.h. You can still have the other functions in ListTemplate.cpp.

[edyam]

Sorry about the mixup! lol. Both of you have been extremely helpful. Using both of your suggestions, I'm no longer getting the original error messages, but the whole project still wont compile correctly. I intend to work on it some more later this evening, and I'll be sure to let you know about any new error messages I may get.

Thanks again for all your help.