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

itemprop="text">

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 


using
            namespace std;


int main () {
 int x =
            10;


 ListNode node4();

            ListNode node3(10
            ,node4);
}

while
this is "list.h" :

#ifndef
            LIST_NODE_H
#define LIST_NODE_H


#include
            
using namespace std;

template             T>
class ListNode {

 ListNode* next;
 T
            data;


 public:

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

};

#endif

I
can't understand why this line: ListNode 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

class="post-text" itemprop="text">

class="normal">Answer

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 href="https://stackoverflow.com/questions/4364536/c-null-reference">here).




2. For
   a constructor with no arguments you need to omit the () (you
   can read about why href="https://stackoverflow.com/questions/180172/why-is-it-an-error-to-use-an-empty-set-of-brackets-to-call-a-constructor-with-no">here)

Something
like this should work:

using
            namespace std;


template 
class
            ListNode {
 ListNode* next;
 T data;


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



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