Why is my linked list deleting after a function call? - c++

Question

In the code below I attempt to create a linked list of strings. I then use the linked list to store output generated by a function (named myFunction) which calls itself recursively. When testing/debugging the code, I noticed that if I print the contents of the linked list after executing the function (which should add items to the linked list) nothing prints out. However, if I attempt to print the linked list after adding items from inside the function it works fine.
It appears that the entire linked list is deleted after the call to myFunction. On the other hand, I'm using dynamic memory allocation when I add elements to the linked list so I don't see the issue.
Please help!
#include <cstdlib>
#include <iostream>
template <class T>
class node{
public:
node *next;
T data;
node(){next=0;};
void print();
};
template <class T>
void node<T>::print(){
std::cout << data;
}
template <class T>
class List{
public:
node<T> *head;
List(){head=0;};
void add(T data);
void print();
int len();
};
template <class T>
int List<T>::len(){
int i=0;
node<T> *current=head;
while(current!= 0){
i++;
current=current->next;
}
return i;
};
template <class T>
void List<T>::add(T myData){
node<T> *current=head;
if(head==0){
head= new node<T>;
head->data=myData;
}
else{
while(current->next!=0){
current=current->next;
}
current->next = new node<T>;
current->next->data=myData;
}
}
template <class T>
void List<T>::print(void){
node<T> *current=head;
if(head==0){
return;
}
else{
do{
std::cout << current->data << " ";
current=current->next;
}while(current!=0);
}
}
void myFunction(List<std::string> myList, int n, std::string starter, int leftParens, int rightParens){
int remainingLength = leftParens+rightParens;
if(remainingLength==0){
myList.add(starter);
std::cout <<myList.len() << std::endl;
}
if(leftParens >0){
myFunction(myList, n, starter+"(", leftParens-1, rightParens);
}
if(leftParens==0 and rightParens >0){
myFunction(myList, n, starter+")", leftParens, rightParens-1);
}
}
int main(int argc, char** argv) {
List<std::string> myList;
myFunction(myList, 5, "", 5, 5);
std::cout <<myList.len();
}

Answer 1

You are passing myList to myFunction by value. Any changes made to myList in the function are changes to the copy, not the original myList in main.
Change myFunction so that it accepts its argument by reference. Then, any changes made to it in myFunction will also be visible in main.
void myFunction(List<std::string>& myList, int n,
// ^^
std::string starter, int leftParens, int rightParens){

Answer 2

You need to use reference if you want to update the variable in the caller context (in other words, if you want to change the variable in main).
Whenever a class allocates memory, you probably need to follow the "rule of three" (constructor, copy-constructor, copy-assignment operator). If you don't, you'll get into trouble if you ever make a copy of the original class [like your call to myFunction as it currently stands]