List using Templates

[Planet_EN]

Can anyone please tell me whats wrong with this code, why is it not executing???

My Compiler says:
Error: Unresolved external 'List<int>::List<int>(List<int>&)' referrenced from h:\data structures\myfile.obj

and if i remove comments from the commented part then i get like almost 6 more errors, can anyone please please help me out on this.............

[syntax="cpp"]
#include<iostream>
using namespace std;

template<class T>
class List{

protected:
struct Node
{
T _data;
Node *_next;
Node(T data = T(), Node* next = NULL):
_data(data),_next(next) { }
};

Node *Head, *Current;
List ( List & value );

public:
List():Head(new Node),Current(Head)
{ }

List( const List& );



void insert( const T& value )
{
Node* P = new Node( value , Current->_next );

if( P == NULL )
std::cerr << "Out of Space";
else
{
Current->_next = P;
Current = Current->_next;
}

}

void First( )
{
if( Head->_next != NULL )
Current = Head->_next;
}

int operator !() const
{
return Current != NULL;
}

List operator ++()
{
if( Current != NULL )
Current = Current->_next;
return *this;
}

T & operator ( ) ( ) const
{
if( Current != NULL )
return Current->_data;
else
return Head->_data;
}

/*
***********Problematic code**************

const List opeator = ( List & value )
{
for ( value.First( ); !value; value++ )
{
Current->_next = new Node( Value( ) , Curent->_next );
Current = Current->_next;
}

Current->_next = 0;
First();
value.First();
return *this;
}*/
int isEmpty() const { return Head->_next == NULL;}

};

template<class T>
void Print ( List<T> &L )
{
if( L.isEmpty() == NULL )
cout << "Empty List";
else
for( L.First(); !L; ++L )
cout << L( ) << ends;
}


int main()
{
List<int> b;
b.insert(5);
Print(b);
return 0;
}
[/syntax]

[Darryl]

1. you spelled operator wrong in: const List opeator = ( List & value )
2. you have 2 different copy constructor declarations(1 public, 1 protected) but no copy constructor definitions. Commententing those out allows compilation, however the default copy constructors are insufficient for copying pointers properly.

Implement your copy constructor(s) and all will be ok

[Planet_EN]

1. you spelled operator wrong in: const List opeator = ( List & value )
2. you have 2 different copy constructor declarations(1 public, 1 protected) but no copy constructor definitions. Commententing those out allows compilation, however the default copy constructors are insufficient for copying pointers properly.

Implement your copy constructor(s) and all will be ok

thank you its working now but somehow, it's displaying "Empty List" at its execution.

Here's the amended code:
[syntax="cpp"]
#include<iostream>
using namespace std;

template<class T>
class List{

protected:
struct Node
{
T _data;
Node *_next;
Node(T data = T(), Node* next = NULL):
_data(data),_next(next) { }
};

Node *Head, *Current;

public:
List():Head(new Node),Current(Head)
{ }

List( const List& );



void insert( const T& value )
{
Node* P = new Node( value , Current->_next );

if( P == NULL )
std::cerr << "Out of Space";
else
{
Current->_next = P;
Current = Current->_next;
}

}

void First( )
{
if( Head->_next != NULL )
Current = Head->_next;
}

int operator !() const
{
return Current != NULL;
}

List operator ++()
{
if( Current != NULL )
Current = Current->_next;
return *this;
}

T & operator ( ) ( ) const
{
if( Current != NULL )
return Current->_data;
else
return Head->_data;
}

void Print ( const List<T>& L )
{
if( L.Head->_next == NULL )
cout << "Empty List";
else
for( L.First(); !L; ++L )
cout << L( ) << ends;

}

const List operator = ( List & value )
{
for ( value.First( ); !value; value++ )
{
Current->_next = new Noe( Value( ) , Curent->_next );
Current = Current->_next;
}

Current->_next = 0;
First();
value.First();
return *this;
}
int isEmpty() const { return Head->_next == NULL;}

};

template<class T>
List<T>::List(const List<T>& L)
{
Head = L.Head;
Current = L.Current;
}

template<class T>
void Print ( List<T> &L )
{
if( L.isEmpty() == NULL )
cout << "Empty List";
else
for( L.First(); !L; ++L )
cout << L( ) << ends;
}


int main()
{
List<int> b;
b.insert(5);
b.insert(11);
Print(b);
return 0;
}
[/syntax]

[Darryl]

Code: Select all

int isEmpty() const {  return Head->_next == NULL;} //bad
if( L.isEmpty() == NULL ) // really bad:  (0 == 0) == true;


you really should use boolean types here;

Code: Select all

bool isEmpty() const {  return Head->_next == NULL;}//correct return
if( L.isEmpty() ) // or L.isEmpty() == true)// corect comparison

plus it reads clearer using booleans