Accessing members of a class within a template

[chadwickstein]

Hi everybody -
I'm trying to develop simple list template that can create lists of various types of objects. My code is partially based on an example given in SAMS Teach Yourself C++ in 21 days.

The following code includes my header for the template and also a short driver program. The header alone compiles just fine, but when I compile it with main(), I get errors (see end of post):

#include <iostream>

using namespace std;

//begin template header
template <class Type>
class List
{
      public:
             List():head(0),tail(0), theCount(0) {std::cout<<"Constructing list...";}
             virtual ~List();
             void insert(Type value);
             void append(Type value);
             int is_present(Type value);
             int is_empty() const {return head == 0;}
             int count() const {return theCount;}
             void displayList() const;
      
      private:
             class ListCell
             {
             public:
                    ListCell( Type value, ListCell * cell = 0): val(value), next(cell){}
                    Type val;
                    ListCell * next;
             };
             ListCell * head;
             ListCell * tail;
             int theCount;
};


//destructor
template<class Type>
List<Type>::~List()
{
   ListCell * pCurrent = head;
   while (pCurrent != 0)
   {
      ListCell * pTemp = pCurrent->next;
      delete pCurrent;
      pCurrent = pTemp;
   }
}

//insert implementation - inserts a new value at the head of the list.
template<class Type>
void List<Type>::insert(Type value)
{
   ListCell * pTemp = new ListCell(value, head);
   if (head == 0) pTemp = tail;
   pTemp = head;
   theCount++;
}

//append implementation - inserts a new value at the tail of the list.
template<class Type>
void List<Type>::append(Type value)
{
   ListCell * pTemp = new ListCell(value);
   if (head == 0) pTemp = head;
   else tail->next = pTemp;
   tail = pTemp;
   theCount++;
}

//is_present implementation - checks to see if a given value is already present in the list
template<class Type>
int List<Type>::is_present(Type value)
{
   if (head == 0)
   {
      return 0;
   }
   
   ListCell * pCurrent = head;
   while (pCurrent != 0)
   {
      if (*pCurrent.val == value)
         return 1;
      else
          pCurrent = *pCurrent.next;
   }
   
   return 0;
}

//displayList implementation - displays all current members of the list
//assumes that any class/type passed to the template List has overloaded the << operator.
template<class Type>
void List<Type>::displayList() const
{
   ListCell * pCurrent = head;
   for (int i = 1; pCurrent != 0; i++)
   {
      cout<<i <<":" <<*pCurrent.val;  //causes compiler error when instantiated in main()
      pCurrent = *pCurrent.next;  //causes compiler error when instantiated in main()
   }
}
//end header


//driver program    
int main()
{
    int value;
    List<int> theList;
    
    cout<<"Enter a value to insert:";
    cin>> value;
    cin.ignore();
    theList.insert(value);
    cout<<"You have " <<theList.count() <<" members in your list."; //this line causes error
    theList.displayList();

    cin.get();
}

I'm using Dev-C++ 4.9.9.2. When I try to compile, I get the following errors:

Line 96: error: request for member `val' in `pCurrent', which is of non-class type `List<int>::ListCell*'
Line 97: error: request for member `next' in `pCurrent', which is of non-class type `List<int>::ListCell*'

Somehow I can't access the members of the ListCell class... any help would be appreciated.

Also, this is only my second thread on these forums - have I posted too much code?

[Desolation]

You haven't specified ListCell as a templated class like you did for 'List'.

[chadwickstein]

Thanks for responding, desolation. I tried making ListCell a templated class also, but I still receive the same error message. Here is the revised code:

#include <iostream>

using namespace std;

//begin template header
template <class Type>
class List
{
      public:
             List():head(0),tail(0), theCount(0) {std::cout<<"Constructing list...";}
             virtual ~List();
             void insert(Type value);
             void append(Type value);
             int is_present(Type value);
             int is_empty() const {return head == 0;}
             int count() const {return theCount;}
             void displayList() const;
      
     private:
             template <class T>
             class ListCell
             {
             public:
                    ListCell( T value, ListCell * cell = 0): val(value), next(cell){}
                    Type val;
                    ListCell * next;
             };
             ListCell<Type> * head;
             ListCell<Type> * tail;
             int theCount;
};


//destructor
template<class Type>
List<Type>::~List()
{
   ListCell<Type> * pCurrent = head;
   while (pCurrent != 0)
   {
      ListCell<Type> * pTemp = pCurrent->next;
      delete pCurrent;
      pCurrent = pTemp;
   }
}

//insert implementation - inserts a new value at the head of the list.
template<class Type>
void List<Type>::insert(Type value)
{
   ListCell<Type> * pTemp = new ListCell<Type>(value, head);
   if (head == 0) pTemp = tail;
   pTemp = head;
   theCount++;
}

//append implementation - inserts a new value at the tail of the list.
template<class Type>
void List<Type>::append(Type value)
{
   ListCell<Type> * pTemp = new ListCell<Type>(value);
   if (head == 0) pTemp = head;
   else tail->next = pTemp;
   tail = pTemp;
   theCount++;
}

//is_present implementation - checks to see if a given value is already present in the list
template<class Type>
int List<Type>::is_present(Type value)
{
   if (head == 0)
   {
      return 0;
   }
   
   ListCell<Type> * pCurrent = head;
   while (pCurrent != 0)
   {
      if (*pCurrent.val == value)
         return 1;
      else
          pCurrent = *pCurrent.next;
   }
   
   return 0;
}

//displayList implementation - displays all current members of the list
//assumes that any class/type passed to the template List has overloaded the << operator.
template<class Type>
void List<Type>::displayList() const
{
   ListCell<Type> * pCurrent = head;
   for (int i = 1; pCurrent != 0; i++)
   {
      cout<<i <<":" <<*pCurrent.val;
      pCurrent = *pCurrent.next;
   }
}
//end header


//driver program    
int main()
{
    int value;
    List<int> theList;
    
    cout<<"Enter a value to insert:";
    cin>> value;
    cin.ignore();
    theList.insert(value);
    cout<<"You have " <<theList.count() <<" members in your list.";
    theList.displayList();

    std::cin.get();
}

Still the same error as before. Is this what you meant, desolation?

Any further thoughts out there? Again, all help is greatly appreciated.

[CornedBee]

*pCurrent.val

The member access operator has a higher precedence than the dereference operator, so this code tries to access the val member of pCurrent and dereference that. Of course, a pointer has no members, so it complains.

Use the indirect member access operator instead:
[cod]pCurrent->val[/code]

The other error is for the same reason.

[bejiz]

I don't know if it can be usefull but I post what I did with the same exercise of the same book. If I remember, there was a problem with Display and for returning an array, so I replaced it by a vector.

#include <iostream>
using namespace std;
#include <string>
#include <vector>
template<class T>
class List
{
public:
List():head(0),tail(0),theCount(0){}
virtual~List();
void insert( T value );
void append( T value );
int is_present( T value) const;
int is_empty( ) const{ return head == 0; }
int count() const { return theCount; }
void List_Display();
T List_getValues()const;
vector<T> List_getValues_array()const;
void List_Count() { cout<<" Count is : "<<theCount<<endl;}
int List_Count1()const { return theCount;}
template<class T>
friend int operator== (const T& lhs,const T& rhs );
private:
class ListCell
{
public:
ListCell( T value, ListCell * cell = 0): val(value),
next(cell) {}
T val;
ListCell * next;
T getValue() const { return val; }
void display(ListCell * pt)
{cout<<" Value is : "<<pt->getValue()<<endl; }
};
ListCell * head;
ListCell * tail;
int theCount;
};
template<class T>
List<T> :: ~List()
{
ListCell * pt = head;
while(pt)
{
ListCell * tmp = pt;
pt =pt->next;
delete tmp;
}
head = 0;
tail = 0;
}
template<class T>
void List<T> :: insert(T value ) 
{
ListCell * pt = new ListCell( value, head );
if( head==0) tail = pt;
head = pt;
theCount++;
}
template<class T>
void List<T> :: append(T value ) 
{
ListCell * pt = new ListCell( value);
if( head==0) head = pt;
else {tail->next = pt;}
tail = pt;
pt->next = 0;
theCount++;
}
template<class T>
int List<T> :: is_present( T value ) const
{
if(head==0) return 0;
if(head->val==value) return 1; ListCell * pt = head->next;
for(;pt != tail; pt = pt->next )
if(pt->val==value) return 1;

return 0;
} 
template<class T>
void List<T> :: List_Display()
{
ListCell * pt = head;
while(pt!=tail->next)
{
cout<<" Value is : "<<pt->getValue()<<endl;
pt = pt->next;
}
}
template<class T>
T List<T> :: List_getValues()const
{
T value;
T value_array[100];
ListCell * pt = head;
while(pt!=tail->next)
{
value += pt->getValue();
pt = pt->next;
}
for(int i=0;i<=theCount;i++){if(pt!=tail->next){value_array[i]=pt->getValue();pt=pt->next;}}
return value;
//return value_array;
}
template<class T>
vector<T> List<T> :: List_getValues_array()const
{
vector<T>val_vect;
T value_array[100];
ListCell * pt = head;

for( int i=0; i<=theCount; i++ )
{
if(pt!=tail->next) 
{ value_array[i] = pt->getValue();
val_vect.push_back(value_array[i]);
cout<<" "<<value_array[i];
pt = pt->next; }
}
//return value_array[0];
return val_vect;
}




template<class T>
int operator== (const T& lhs,const T& rhs )
{//compare lengths first
T value1; T value2;
if( lhs.theCount != rhs.theCount ) 
return 0;//lengths differ 
if( lhs.List_getValues()!= rhs.List_getValues() )
return 0;
return 1;//if they dont differ , they must match
}

class cat
{ };
int main()
{
List<string> test;
List<cat> test1;
List<int> test2;
test.List_Count();
test.append("qw");
test.List_Count();
test.insert("yu");test.List_Count();
test.append("az");test.List_Count();
test.insert("kim");test.List_Count();
test.List_Display();

cout<<'\n';
test2.List_Count();
test2.append(786);test2.List_Count();
test2.append(9000);test2.List_Count();
test2.List_Display();

cout<<'\n';
List<string> test3; 
test3 = test;
test3.List_Display();
if( test3 == test ) cout<< " \n1111 ";
else cout<<" \n0000 ";

cout<<test.List_getValues();

vector<string>t = test.List_getValues_array();
vector<string>t3 = test3.List_getValues_array();

cout<<'\n';
for( int i = 0; 
i < (test.List_Count1());
i++ ) 
cout<<t[i]<<" ";
if( t == t3 ) cout<< " test = test3 ...\n";
getchar(); return 0;
}

[CornedBee]

You're using code tags incorrectly. Place [code] before the code and [/code] after it.

[CornedBee]

You can edit old posts, and the slash needs to go before the word code in the end tag.

[CornedBee]

The formatting is still completely broken. Preserving indentation is kind of the point of code tags.

Annoying, ain't I?

[bejiz]

I have deleted my last message since the first has been corrected but frankly I have not the sense that the formatting is especially broken? thanks again.

[CornedBee]

You have absolutely no indentation. It is impossible, without counting braces, to tell the structure of the code. So yes, it is broken.

[bejiz]

I see what you mean now, that's a mistake I did when working with Dreamweaver, but I didn't write my code like this. And I made the same mistake with the other pages of code I put on my site!