Getting a strange seg fault from imported queue.front() - c++

Question

So I am currently working on a graph abstract data type for my CS163 class. Everything else for this program works awesome BUT the depth-first traversal of the graph. When testing, I add all the vertices I want, then connect them as I need to. I check and everything is indeed linked as it should be so we are good to go. I jump into this function below, given the name of the friend(vertex) to start from. The line I commented "*current_buddy = myQueue.front();" is the line I found in GDB to be the offending piece of code that causes a seg fault. Also using GDB, I was able to successfully print the proper vertex using "p myQueue.front()"
So perhaps a sample test to give a bit more context. say I have the points a,b,c,d, and e. a is connected to c,d, and e. b is connected to c. I want to start at vertex "a" for this test. Passing in a to the function, it finds the proper index with the find_location function which simply returns said index integer. It will then push that vertex onto the queue and mark that vertex as visited so that upon traversal, I don't return to that point to push it onto the queue again. I create a node "current" that is attached to the first node in the edge list for the "a" vertex. When it goes into the first loop, I use the previously created vertex "current_buddy" to point to the first object in the queue, which is the vertex "a." This is the point that the program hits a seg fault and I cannot for the life of me figure out what is going on with that line of code that would cause this.
Both "vertex" and "node" are structs I have created and the queue I am using is from the standard library #include queue more on the queue bit here if needed Any and all information would be greatly appreciated! Obviously since this is a school assignment, I don't expect anyone to just give me the answer but I am lost at the moment.
bool graph::breadth_first(char * start)
{
if(adjacency_list[0].buddy_name == NULL)
return false;
int location = find_location(start);
queue<vertex> myQueue;
myQueue.push(adjacency_list[location]);
adjacency_list[location].visited = true;
node * current = adjacency_list[location].head;
vertex * current_buddy = NULL;
while(myQueue.empty() == false)
{
*current_buddy = myQueue.front();//THIS LINE SEG FAULTS
cout << "THIS IS A FRIEND IN THE BREADTH-FIRST TRAVERSAL" << current_buddy->buddy_name << endl;
current = current_buddy->head;
myQueue.pop();
while(current != NULL)
{
if(current->connected_buddy->visited == false)
{
current_buddy = current->connected_buddy;
location = find_location(current_buddy->buddy_name);
myQueue.push(adjacency_list[location]);
adjacency_list[location].visited = true;
current = current->next;
}
}
}
for(int i = 0; adjacency_list[i].buddy_name != NULL; ++i)
{
adjacency_list[i].visited = false;
}
return true;
}

Answer 1

I don't know if this will work as you expect, but a quick fix might be to use the address-of operand and actually assign the pointer instead of dereferencing it:
current_buddy = &myQueue.front();

Answer 2

(EDIT: For the first part of this question, the solution/answer provided by Joachim may perform better. I just like avoid pointer when possible.)
You could try:
vertex * current_buddy = NULL;
while(myQueue.empty() == false)
{
vertex front_buddy = myQueue.front();
cout << "THIS IS A FRIEND IN THE BREADTH-FIRST TRAVERSAL" << front_buddy.buddy_name << endl;
current = front_buddy.head;
BTW - I think you need to check for null here:
while(current != NULL)
{
if (current->connected_buddy == NULL)
{
// add error handling
}
else
{
// normal code
}
}
So that you don't get another crash
This part
while(current != NULL)
{
if(current->connected_buddy->visited == false)
{
// your code
current = current->next;
}
}
looks like an endless loop in case current->connected_buddy->visited is true.
Maybe you want
while(current != NULL)
{
if(current->connected_buddy->visited == false)
{
// your code
}
// Moved out of the if-statement
current = current->next;
}

Answer 3

It's segfault because you are attempting to deference a NULL pointer.
vertex * current_buddy = NULL;
while(myQueue.empty() == false)
{
// V de-referencing a NULL pointer
*current_buddy = myQueue.front();//THIS LINE SEG FAULTS
You should change it to
// vertex * current_buddy = NULL; // < Remove this line
while(myQueue.empty() == false)
{
vertex *current_buddy = &myQueue.front();