为什么列表上的迭代失败？

Question

I'm currently working my way through the Stanford open CS106B, and I'm running into a problem on Assignment 3, Part B. I am given a struct Node as follows:

struct Node {
string name;   // my person's name
string killer; // who eliminated me
Node* next;    // ptr to next node
Node(string name, Node* next) {...}
};

I have to implement a class that makes a list of Nodes. I have the constructor working properly, but when I try to iterate through the list, my program crashes. My iteration code:

void AssassinsList::printGameRing() {
    Node* current;
    for(current = ring; current->next != NULL; current = current->next) {
        cout << endl << "  " << current->name << " is targeting " << current->next->name;
    }
    cout << endl << "  " << current->name << " is targeting " << ring->name << endl;
}

However, if I use a for-loop to loop the number of times I know I need to for a certain list length, it works. Help? The link to the assignment pdf: http://www.stanford.edu/class/cs106b/homework/3-tiles-assassins/spec.pdf

Thanks!

Answer 1

I am guessing that you don't initialize * next to nullptr . So for all the links you setup between nodes it is fine, but the last object in the list points to garbage.

Sorry, nullptr is c++11. If your compiler older then its just NULL .

Answer 2

There's a chance that if cur is NULL or doesn't point to anything, you could be dereferencing a bad pointer and therefore crashing your program. The other option is that as woolstar pointed out, you don't have a terminating node in your list (that points to NULL.) Observe the following code:

Node* head = new Node{0};
Node* cur = head;
for (int i = 1; i <= 10; i++)
{
    cur->next = new Node{i};
    cur = cur->next;
}

// Set terminating node
cur->next = nullptr;

// We'll iterate until cur is null
// So if we access cur->next
// It won't result in undefined behavior
for (cur = head; cur != nullptr; cur = cur->next)
{
    std::cout << cur->value;
}

// cur should be nullptr now
if (!cur)
    std::cout << "end of list";

Answer 3

You can also use 0. Yeah, it's not as cool as nullptr , but it's supported. Fixed constructor:

Node(string name_, Node* next_=0): name(name_), next(next_) {}

Answer 4

The fact a fixed length loop works, but a NULL terminated loop doesnt work shows that its likely you have an invalid address in the last Node's next field.

I would expect your problem comes from either your Node constructor or your list code or the interplay between them.

Try setting next to 0 / nullptr in the Node constructor, that should help.

Alternatively, have your list set the next field to 0 when you add the first element to the list, or add any element to the end of the list.