String class using linked list - c++

Question

I have to write this String class which is represented using linked list. I seem to have problems with my copy constructor and I have no idea how to write my assignment operator. Any idea where the mistake is and how to write them? The shown code is for the copy constructor with the following operation String s="word". I'm not sure how to write one for String s1=s2. Here is my code so far:
struct Element
{
char data;
Element* next;
};
class String
{
Element* top;
public:
String();
bool empty() const;
char last() const;
char pop();
void push(char);
friend std::ostream& operator<<(std::ostream&, const String&);
friend std::istream& operator>>(std::istream&, String&);
String(const char*);
};
String::String(const char *p)
{
int l = strlen(p);
for(int i=0; i < l+1 ; i++)
{
Element *newElement;
newElement = new Element;
newElement->data = p[i];
newElement->next = NULL;
if(top == NULL)
{
top = newElement;
}
else
{
Element *tmp = top;
while(tmp->next != NULL)
{
tmp = tmp->next;
tmp->next = newElement;
}
}
}
}
int main()
{
String s="Hello";
std::cout<<s;//operator<< works tested it
}

Answer 1

If you defined single-linked list when it is much better to add new elements at the head of the list instead of the tail. Nevertheless your function could look the following way
String::String(const char*p)
{
for ( ; *p; ++p )
{
Element *newElement = new Element;
newElement->data = *p;
newElement->next = NULL;
if ( top == NULL )
{
top = newElement;
}
else
{
Element *tmp = top;
while ( tmp->next != NULL ) temp = tmp->next;
tmp->next = newElement;
}
}
}
By the way it is not a copy constructor. As for the copy constructor then it could be defined as
String::String( const String &s ) : top( NULL )
{
if ( s.top )
{
top = new Element;
top->data = s.top->data;
top->next = NULL;
for ( Element *tmp1 = top, tmp2 = s.top; tmp2->next; tmp1 = tmp1->next, tmp2 = tmp2->next )
{
tmp1->next = new Element;
tmp1->next->data = tmp2->next->data;
tmp1->next->next = NULL;
}
}
}

Answer 2

In the following code, your while loop will never execute since the top->next will be NULL.
Element *tmp = top;
while(tmp->next != NULL)
{
...
}
Also, as #Claptrap commented, you assign newElement on every loop iteration.
Given all of this, I'm not seeing why it would fail, but it may be in code that you have not posted. Please try to reduce your code to the minimum necessary that reproduces the problem and then include that code in your post.