What is wrong with my ctor using linked list and templates?

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

score 3 · Accepted Answer · edited May 23 '17 at 10:34

3

You have a number of errors with your code:

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

edited May 23 '17 at 10:34

Community

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answered Feb 27 '12 at 18:50

GWW

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Also it's probably a bad design to use linked list classes to store pointers to objects on the stack. – Seth Carnegie Feb 27 '12 at 19:03
@SethCarnegie: I completely agree with you on that one – GWW Feb 27 '12 at 19:37

score 2 · Answer 2 · answered Feb 27 '12 at 18:46

2

There are a couple of problems here.

First you don't need ListNode<T>() simpley ListNode(...) for the constructor.
Second you can't have a defalut for a reference parameter to null if there is the potential for nex to be null it needs to be a pointer.

answered Feb 27 '12 at 18:46

rerun

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Yes you can have a default for a reference parameter, but you're right, he does need a pointer. – Seth Carnegie Feb 27 '12 at 18:47
Not of null for a refrence. and in this case it doesn't make sense. – rerun Feb 27 '12 at 18:50

score 0 · Answer 3 · answered Feb 27 '12 at 18:46

0

Default parameters are evil (IMO).

(just a thought) Create different constructors (one with no parameters, one with parameters, ... )

answered Feb 27 '12 at 18:46

Max

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What is wrong with my ctor using linked list and templates?

3 Answers3