segmentation fault C++ - c++

Question

I don't know why my code is seg faulting, im assuming im looking at a point in the queue where theres nothing there, but I thought that I do push elements onto the queue.
heres my code:
template <typename T>
class btree {
public:
btree(size_t maxNodeElems);
~btree() {}
struct node {
list <T> elements;
node *lvl;
};
private:
size_t maxNodeElems;
node* root;
};
template <typename T>
btree<T>::btree(size_t maxNodeElems) {
if (maxNodeElems > 0) max = maxNodeElems;
root = new node;
root->lvl = new node*[max+1];
for (int i = 0; i < (int) max+1; i++) root->lvl[i] = new node;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, const btree<T>& tree) {
queue <typename btree<T>::node*> q;
q.push(tree.root);
int loop = 0;
while (!q.empty()) {
loop++;
typename btree<T>::node* temp = q.front();
int i = 0;
class list <T>::iterator itr = temp->elements.begin();
for (; itr != temp->elements.end(); ++itr) {
os << *itr << " ";
if (!temp->lvl[i]->elements.empty()) {
q.push(temp->lvl[i]);
}
i++;
}
q.pop();
}
return os;
}
could someone help me out, im lost because when I check if the if statement is working inside the for loop, it does go into it, but im not sure if its pushing
EDIT: more code
template <typename T>
pair <typename btree<T>::iterator, bool> btree <T>::insert (const T& elem) {
pair <typename btree<T>::node, bool> start;
start = addElement (elem, root);
pair <typename btree<T>::iterator, bool> final;
return final;
}
template <typename T>
pair <typename btree<T>::node, bool> btree<T>::addElement (const T& e, typename btree<T>::node*& n) {
pair <typename btree<T>::node, bool> elemPair;
if (n->elements.size() == max) {
int count = 0;
class list <T>::iterator itr = n->elements.begin();
for (; itr != n->elements.end(); ++itr) {
count++;
if (e < *itr) {
count--;
elemPair = addElement (e, n->lvl[count]);
} else if (e == *itr) return make_pair (*n, false);
}
} else {
n->elements.push_back(e);
n->elements.sort();
}
return make_pair(*n, true);
}
in my test.cpp:
int main (void) {
btree<char> b(2);
b.insert('Z'); b.insert('J'); b.insert('Y');
cout << b;
return 0;
}

Answer 1

The code you post doesn't compile and so I can't verify your error.
The following does compile and so might provide some help. Its similar to the code you provided.
However, the following is not good code - for example, you need to be vary careful about using the new keyword in a constructor, because exception handing in a constructor is more difficult (and there is the chance of an allocation exception being thrown).
template <typename T>
class btree {
public:
btree(size_t maxNodeElems);
//if your using new then you need to delete - do that in the destructor.
~btree();
struct node {
list <T> elements;
//you use this as a pointer to an array,
// it can be demoted to a double pointer -
// but not a single pointer as you had.
node ** lvl;
};
private:
size_t maxNodeElems; //you don't actually use this variable in your code
node* root;
//You are doing memory allocations in the constructor,
// therefore need to overload operator= and copy constructor.
// Since we are not overloading these, we make them private.
void operator=(const btree&);
btree(const btree&);
};
//In your code you had a undeclared variable max.
// No idea what it was or why you had it - removed.
template <typename T>
btree<T>::btree(size_t v_maxNodeElems)
: maxNodeElems(v_maxNodeElems) // construct the variable you define
{
root = new node;
root->lvl = new node*[maxNodeElems+1]; //root->lvl is a double pointer
for (int i = 0; i < (int) maxNodeElems+1; i++)
root->lvl[i] = new node;
}
//delete in reverse order to you constructing
template <typename T>
btree<T>::~btree() {
for (int i = 0; i < (int) maxNodeElems+1; i++)
delete root->lvl[i]; //first the elements of the array
delete[] root->lvl; //then the array - note the delete[]
delete root; //then the root.
}
int
main (int ac, char **av)
{
btree<char> b(2);
}