What is the main different of these 2 and why doesn't the second one work when like this?
template <class T> MyStack<T>::Node::Node(T& input, Node* head):next(head),value(input) {}
template <class T> MyStack<T>::Node::Node(T& input, Node* head) {next = head; value = input;}
My guess on why the second one doesn't work, is because it has nothing to assign variable when initializing.
MyStack<T>::value looks like it is of type T&. If that's the case, then remember that references must be initialized, and they cannot be rebound to a different object. Using the initializer list is the only way to correctly initialize references that are object data members; you cannot initialize them with assignment in the constructor. (In fact, if you omit the initializer list entry you should get some error similar to "uninitialized reference member.")
next and value might not have empty default constructors.
Besides that, const and reference members must be initialized in the initialization list, you can't assign to them.