Doubly Linked Objects

From:
Timie Milie <tim.milstead@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 27 Jun 2009 22:33:13 CST
Message-ID:
<3d678741-dcaf-4ac3-b2ca-31e1f48c1493@y34g2000yqd.googlegroups.com>
I am trying to design an object with connections to parent and child
objects of the same class. I can't seem to produce an implementatin
without draw backs. It's code follows (along with some questions and
problems in comments):

#include <iostream>

class Node {
protected:
    Node* parent;

    Node* child;

    std::string name;
private:
    void init() {
        parent = 0;
        child = 0;
        name = "";
    }
public:

    Node() {
        init();
    }

    Node(const std::string name) {
        init();
        this->setName(name);
    }

    Node(const Node& node) {
        init();
        set(node);
    }

    virtual ~Node() {
        // Q: Should I be concerned that the children may have been created
on the stack and passed in using the & operator?
        if (getChild() != 0) {
            delete getChild();
        }
    }

    Node& operator=(const Node& node) {
        set(node);
        return *this;
    }

    void set(const Node &node) {
        setName(node.getName());
        if (node.getChild() != 0) {
            Node* newChild = new Node(*node.getChild()); // Q: Is this
dangerous?
            setChild(newChild);
        }
    }

    void setName(const std::string& name) {
        this->name = name;
    }

    std::string getName() const {
        return name;
    }

    Node* getParent() const {
        return parent;
    }

    void setParent(Node* node) {
        parent = node;
    }

    Node* getChild() const {
        return child;
    }

    void setChild(Node* node) {
        child = node;
        node->setParent(this);
    }

    // Using boost::shared_ptr<Node> would not work here because n=0 can
not return boost::shared_ptr<Node>(this);
    Node* getDescendant(const int& n) {
        if (n==0) {
            return this;
        } else if (getChild() != 0) {
            return getChild()->getDescendant(n-1);
        } else {
            return 0;
        }
    }

    void display(std::ostream& out) const {
        out << "Node@" << this << "(name = " << getName().c_str() << ")";
        if (getChild() != 0) {
            out << std::endl << "|" << std::endl;
            getChild()->display(out);
        }
    }
};

std::ostream& operator<<(std::ostream& out,const Node& node) {
    node.display(out);
    return out;
}

void test() {
    Node a("A");

    Node* b = new Node("B");
    a.setChild(b);

    // Uncommenting this creates an error because the code attemtps to
delete b through a's destructor and when it goes out of scope.
    // Q: Should I be concerned by this.
     //Node b("B");
    //a.setChild(&b);

    Node* c = new Node("C");
    b->setChild(c); // We can do this because b was not copied into a

    std::cout << a << std::endl << std::endl;

    Node d(a);
    std::cout << d << std::endl << std::endl;

    std::cout << *d.getDescendant(2) << std::endl << std::endl;

    // Q: Is it right that we rely on a's destructor to destroy b and c?
    // delete(c); // Uncommenting this produces an error because the app
tries delete c twice.
}

int main(int argc, char* argv[]) {
    test();
    std::cout << "Finished" << std::endl;
    return 0;
}

/* Output:

Node@0012FE54(name = A)
|
Node@003662F0(name = B)
|
Node@00366358(name = C)

Node@0012FE08(name = A)
|
Node@00366458(name = B)
|
Node@003664C0(name = C)

Node@003664C0(name = C)

Finished
*/

Does anyone have any responses to my anxious questions in the code
above? Am I worrying too much?

If I use references instead of pointers in the setChild and setParent
I then have to copy which would stop me creating the chain without
doing:

a.getChild().getChild().getChild().setParent(blah);

If I use smart pointers than I can't do the getDescendants function.

Any ideas on a nice implementation.

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