从导入的queue.front（）获取奇怪的段错误

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

我不知道这是否会按您期望的那样工作，但是一个快速的解决方法可能是使用操作数的地址并实际分配指针而不是取消对它的引用：

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();