Are data structures an appropriate place for shared_ptr?

Question

I'm in the process of implementing a binary tree in C++. Traditionally, I'd have a pointer to left and a pointer to right, but manual memory management typically ends in tears. Which leads me to my question...

Are data structures an appropriate place to use shared_ptr?

Answer 1

I think it depends on where you'd be using them. I'm assuming that what you're thinking of doing is something like this:

template <class T>
class BinaryTreeNode 
{
    //public interface ignored for this example
    private:
        shared_ptr<BinaryTreeNode<T> > left;
        shared_ptr<BinaryTreeNode<T> > right;
        T data;
}

This would make perfect sense if you're expecting your data structure to handle dynamically created nodes. However, since that's not the normal design, I think it's inappropriate.

My answer would be that no, it's not an appropriate place to use shared_ptr, as the use of shared_ptr implies that the object is actually shared - however, a node in a binary tree is not ever shared. However, as Martin York pointed out, why reinvent the wheel - there's already a smart pointer type that does what we're trying to do - auto_ptr. So go with something like this:

template <class T>
class BinaryTreeNode 
{
    //public interface ignored for this example
    private:
        auto_ptr<BinaryTreeNode<T> > left;
        auto_ptr<BinaryTreeNode<T> > right;
        T data;
}

If anyone asks why data isn't a shared_ptr, the answer is simple - if copies of the data are good for the client of the library, they pass in the data item, and the tree node makes a copy. If the client decides that copies are a bad idea, then the client code can pass in a shared_ptr, which the tree node can safely copy.