Question

I am a beginner in c++ so please excuse me if I my mistakes below turn out to be silly. Still, I am stuck with my code and would appreciate any help.

I get the following error when trying to compile through make via g++:

In file included from A.cpp:2:
List.h:20: error: ‘List’ is not a template type
A.cpp: In member function ‘void A::NowyObiekt(int)’:
A.cpp:6: error: ‘list_a’ was not declared in this scope
make: *** [A.o] Error 1

My code is separated into the following tiny files:

• A.h : http://pastebin.com/QQ04xx2j (header)

• A.cpp : below

  #include "A.h"
  #include "List.h"
  
  void A::NewObject(int i)
  {
          list_a.Add(i);
  }
  
  int A::Compare(int a, int b)
  {
          if ( a>b ) return 1;
          if ( a<b ) return -1;
          else return 0;
  }
  

• List.h : below (header)

  #ifndef LIST_H
  #define LIST_H
  template<typename T>
  class Node
  {
      Node()
      {
          nxt = pre = 0;
      }
      Node(const T& el, Node *n = 0, Node *p = 0 )
      {
          dana = el; nxt = n; pre = p;
      }
      T dana;
      Node *nxt, *pre;
  };
  
  template<typename T>
  class List
  {
  public:
      List()
      {
          head = tail = 0;
      }
      void Add(const T&);
  protected:
      Node<T> *head,*tail;
  };
  
  #endif
  

• List.cpp : http://pastebin.com/a3HQ9yZ4

• prog.cpp : below (main)

  #include "List.h"
  #include "A.h"
  
  int main()
  {
      int i = 5;
      class List list_a;
      class A obj;
  
      obj.Add(i);
  }
  

• and the makefile is : http://pastebin.com/GTR5jW54

As noted, I am still a beginner, so please be understanding. I would be thankful for any help and clear explanations. Thanks in advance.

Answer 1

There are a couple of problems with your code: The first is that you don’t declare any variable named list_a anywhere. That error should be pretty obvious. The other is that you use the List class without giving it template parameters.

And last a small note about your question: As your files are indeed very small, you could put them in the question and not link to them.

Edit: About the List template problem.

You already use Node properly in List, i.e. declare the nodes as Node<T>. When you use List you simply has to do the same. For example, to declare a list of integers:

List<int> my_int_list;

Also, as you only use public functions in List from the class A, you don’t need the friend declaration. If you do need to use protected or private members (which IMO is a sign of bad design) you need to make that friend-declaration templated as well:

friend class List<sometype>;

And finally, your code will not compile anyway… The reason being that when you are using a template-class, the whole class has to be fully defined (i.e. complete with its function implementations). You can solve this by putting the functions in the header file. And when defining the functions, you need the template parameter there as well:

template<typename T>
void List<T>::Add(const T& el)
{
    Node<T>* head = new Node<T>(el);
    if ( Compare(el,i) > i )
        std::cout << "Ok" << std::endl;
}

Note that I added the template parameter in a couple of places.