c++ - Why am I getting this undefined symbols for architecture x86_64 error

I am getting an Undefined symbols for architecture x86_64 error but I'm not sure why.
I am making a stack data type using linked lists and templates.

StackLinkedList.h

#ifndef __StackLinkedList__StackLinkedList__
#define __StackLinkedList__StackLinkedList__

#include 
using namespace std;

#endif /* defined(__StackLinkedList__StackLinkedList__) */

template 
class StackLinkedList {

public:
    StackLinkedList();
    void push(Item p);

private:

    StackLinkedList* node;
    Item data;
};

StackLinkedList.cpp

#include "StackLinkedList.h"

template 

StackLinkedList::StackLinkedList() {
    node = NULL;
}

template 


void StackLinkedList::push(Item p) {
    if(node == NULL) {

        StackLinkedList* nextNode;
        nextNode->data = p;
        node = nextNode;
    }else {
        node->push(p);
    }
}

main.cpp

#include "StackLinkedList.h"

int main() {
    StackLinkedList* stack;

     stack->push(2);
}

Error details:

Undefined symbols for architecture x86_64:
  "StackLinkedList::push(int)", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I am using Xcode 6.1.

Answer

You have to declare/define your template function in the header file, as the compiler must use the information at compile time about the instantiation type. So put the definitions of the template functions inside the .h file, and not in the cpp.

See
Why can templates only be implemented in the header file?
for more details.