Question

I am trying to practice a little bit with cpp and want to make a class of linked list and than to manipulate it reverse it, finding circle and etc. but I can’t understand what is the problem with my ctor/

I have this main:

#include "list.h"
#include <iostream>


using namespace std;


int main () {
  int x = 10;

  ListNode<int> node4();
  ListNode<int> node3(10 ,node4);
}

while this is “list.h” :

#ifndef LIST_NODE_H
#define LIST_NODE_H

#include <iostream>
using namespace std;

template <class T>
class ListNode {

  ListNode* next;
  T data;

 public:

  ListNode<T>( T dat = 0 ,const ListNode<T> &nex = NULL):data(dat),next(nex){};

};

#endif

I can’t understand why this line: ListNode<int> node3(10 ,node4);
makes this errors, what is the problem?

list.cpp: In function ‘int main()’: list.cpp:12:33: error: invalid
conversion from ‘ListNode ()()’ to ‘int’ [-fpermissive]
list.h:15:3: error: initializing argument 1 of
‘ListNode::ListNode(T, const ListNode&) [with T = int]’
[-fpermissive] list.cpp:12:33: warning: passing NULL to non-pointer
argument 1 of ‘ListNode::ListNode(T, const ListNode&) [with T =
int]’ [-Wconversion-null] list.cpp:12:33: error: recursive evaluation
of default argument for ‘ListNode::ListNode(T, const ListNode&)
[with T = int]’ list.h:15:3: warning: in passing argument 2 of
‘ListNode::ListNode(T, const ListNode&) [with T = int]’ [enabled
by default] list.h: In constructor ‘ListNode::ListNode(T, const
ListNode&) [with T = int]’: list.cpp:12:33: instantiated from
here list.h:15:76: error: cannot convert ‘const ListNode’ to
‘ListNode’ in initialization

Answer 1

You have a number of errors with your code:

1. In your constructor you have a null reference as your default value, which is invalid (see here).

2. For a constructor with no arguments you need to omit the () (you can read about why here)

Something like this should work:

using namespace std;

template <class T>
class ListNode {
    ListNode* next;
    T data;

    public:
          ListNode<T>( T dat = 0 ,ListNode<T> * nex = NULL):data(dat),next(nex){};
};


int main(){
    ListNode<int> node4;
    ListNode<int> node3(10, &node4);
}