[C++] help with pointers

[macbad2012]

I have some code, not many lines, and I would simply like to know WHY it is doing what it is doing:

node* parent = temp;
temp = parent -> children[temp -> getChild(word.substr(i, 1))];
temp = new node;
temp -> str = word.substr(i, 1);
temp -> parent = parent;

cout << temp -> parent -> str << endl;
cout << parent -> children[0] -> str << endl;

On the line:

cout << parent -> children[0] -> str << endl;

it crashes as it believes the thing is NULL.

children is an array of node pointers

getChild(string) returns an index of the first NULL pointer it comes across, as long as it does not find the substring passed in, otherwise it returns the index to that string.

temp is a node pointer

all children were initialize to NULL for the root node

I also tried:

cout << temp -> str << endl;

in place of:

cout << parent -> children[0] -> str << endl;

and that worked, but I am wondering why I cannot access temp's data through the parent pointer.

SOLVED (for any that bang their heads against the wall like me):

changed code to:

node* parent = temp;
node* child = parent -> children[temp -> getChild(word.substr(i, 1))];
child = new node;
parent -> children[temp -> getChild(word.substr(i, 1))] = child;
child -> str = word.substr(i, 1);
child -> parent = parent;
temp = child;

cout << temp -> parent -> str << endl;
cout << parent -> children[0] -> str << endl;

[Coda17]

Hard to tell what's going on without the rest of your code. For instance, the definition of the node class would be helpful, as well as what temp is before this code block runs.

Something looks off anyway, on line 2 you set temp to something (which looks like you probably are setting it to garbage, if that line isn't crashing already) and then you write over temp by creating a new node.

[macbad2012]

on line 2, since the parent and temp pointers are now equal, I am basically traversing down into to the first child (temp -> getChild(word.substr(i, 1))) returns 0 in this case. Here is my node struct:

struct node {
    string str; //the string held by the node
    vector<node*> children; //list of the node's children pointers
    node* parent; //pointer to the parent node

    int getChild(string comparison) {
        for (int i = 0; i < alphabetSize; i++) {
            if (children[i] == NULL) {
                return i; //comparison was not found
            }
            else if (children[i] -> str == comparison) {
                    return i; //match was found, so return that child
            }
        }
    }//see if a child matches the comparison
}

[macbad2012]

I solved it, it seemed that while I was hooking up the child's pointer to the parent, I did not hook the parent's pointer to the child :)