Trouble with templates - c++

Question

I have three files and I want to compile and run them, but I keep getting some errors and warnings. Redefinition of struct Node< T >. I don't know much about templates, but this looks right to me. And, I spent a lot of time trying to figure out whats wrong. Thanks.
//mystack.h
#ifndef MYSTACK_H
#define MYSTACK_H
template <class T>
struct Node
{
T info;
T *next;
};
template <class T>
class MyStack
{
private:
struct Node<T> *top;
public:
void Push(T item);
void Pop();
int Top();
void Print();
};
#endif
//mystack.cpp
#include <iostream>
#include "mystack.h"
template <class T>
struct Node
{
T info;
T* next;
};
template <class T>
class MyStack
{
private:
struct Node<T>* top;
public:
void Push(T item)
{
if(top == NULL)
{
top = new( struct Node<T> );
top->info = item;
top->next = NULL;
} else
{
Node<T>* temp;
temp = top;
top = new( struct Node<T> );
top->info = item;
top->next = temp;
}
}
void Pop()
{
if( top == NULL )
{
} else
{
Node<T>* temp;
temp = top->next;
delete top;
top = temp;
}
}
int Top()
{
return top;
}
void Print()
{
if(top != NULL)
{
Node<T>* temp;
temp = top;
while(temp != NULL)
{
std::cout << temp << std::endl;
temp = temp->next;
}
}
}
};

Answer 1

One mistake you did in the listings is that you redefine this structure as it says.
Thats the definition:
template <class T>
struct Node
{
T info;
T* next;
};
This definition is done in both listings.
Edit: The second thing is that your class method implementation does not look right. You will have the most success if you try not to split cpp and header files while using templates.

Answer 2

If you were designing a class instead of a template, what you're doing would be wrong because you're redefining types.
But since you're writing templates, you're wrong earlier than that: you can't separately compile templates.
Brief pointer on the C++ compilation model:
// Definition of Node
template<typename T>
struct Node {
T info;
T* next; // shouldn't that be a Node*?
};
// Definition of MyStack
template <typename T>
class MyStack
{
private:
Node<T> *top;
public:
// Declarations, but not definitions, of the Mystack function members.
void Push(T item);
void Pop();
int Top();
void Print();
};
// Example definition of MyStack::Push
template<typename T>
void
MyStack<T>::Push(T item)
{
// as before
}
The type definitions usually appear in headers (if they are to be reused in different TUs) with include guards as you are doing. The guards are here because the definitions must appear at most once per TU. Do not manually repeat a type definition (e.g. in a source file as you did). This is wrong as it should be: nobody wants copy-n-paste errors.
The function members definitions usually appear in source files, unless they are the members of a template. In the latter case it's simpler to put them in headers (they don't have to be inline either).
You can learn the details of the compilation model elsewhere on SO, or in books, or on the Internet. Searching for 'template definition' or 'one definition rule' (or ODR) can help.

Answer 3

First, In header, remove 'struct' from the line 'struct Node *top;'. In C++ structs are almost identical to classes, the only difference being that struct members are public by default and class members are private by default. You do not need to preface struct types with the struct keyword like in straight C.
Second, Your CPP is all wrong. Templates are instantiated by the compiler when needed, so they do not live in CPP files to be compiled into objects like normal (apart from template specializations). You can put your template definitions in the HPP itself or a better general solution is to use an IPP file, i.e
// mystack.ipp
#ifndef MYSTACK_IPP
#define MYSTACK_IPP
#include "mystack.h"
#include <iostream>
template <class T>
void MyStack<T>::Push(T item)
{
if(top == NULL)
{
top = new( struct Node<T> );
top->info = item;
top->next = NULL;
} else
{
Node<T>* temp;
temp = top;
top = new( struct Node<T> );
top->info = item;
top->next = temp;
}
}
template <class T>
void MyStack<T>::Pop()
{
if( top == NULL )
{
} else
{
Node<T>* temp;
temp = top->next;
delete top;
top = temp;
}
}
template <class T>
int MyStack<T>::Top()
{
return top;
}
template <class T>
void MyStack<T>::Print()
{
if(top != NULL)
{
Node<T>* temp;
temp = top;
while(temp != NULL)
{
std::cout << temp << std::endl;
temp = temp->next;
}
}
}
#endif
Then '#include "mystack.ipp"' in any file which makes use of the implementation of MyStack