C++ 中链表的读取访问冲突

Question

I have a very simple linked list that I want to perform functions on, but I keep getting the read access violation error on the 'root' node when I run my code.

Here is the error I get (I commented after the lines of code I get the errors in):

Exception thrown: read access violation. root was 0xCCCCCCCC. If there is a handler for this exception, the program may be safely continued.

This is the struct:

struct node {
int value;
node* link;

node(int val) {
    link = NULL;
    value = val;
}
};

First I initialize that linked list in main function like this:

int main() 
{
node *root;

addnode(root, 20);
addnode(root, 1);
addnode(root, 50);


node *curr;
for (curr = root; curr->link != NULL; curr = curr->link) { // I get error here
    cout << curr->value << " ";
}
cout << endl;

cout << "Number of elements " << countlist(root) << endl;

getchar();
return 0;
}

And the function that are called are (first one to add nodes, and the second to count the number of nodes in the list):

void addnode(node *&root, int val) {
if (root != NULL) { // I get error here
    node *temp=new node(val);
    temp->link = root;
    root = temp;
}
else
    root = new node(val);
}

int countlist(node *root) {
if (root != NULL) {
    int count = 0;

    do {
        count++;
        root = root->link;
    } while (root->link != NULL); // I get error here

    return count;
}
return 0;
}

The error I keep getting are in the lines that I mentioned in the comments in the code.

Answer 1

A good habit to avoid this kind of problem might be initializing all variables at declaration:

int main() 
{
   node *root = nullptr;
   // ...
}

Also, you do not want:

node *curr;
for (curr = root; curr->link != NULL; curr = curr->link) {
    cout << curr->value << " ";
}

but

for (node *curr = root; curr != nullptr; curr = curr->link) {
    cout << curr->value << " ";
}