Using unique_ptr instead of shared_ptr in BST

Question

I am trying to implement BST with unique_ptr. I got a working program for shared_ptr. How do I go about using unique_ptr instead to enforce the single ownership semantics of the BinarySearchTree?

When I replace shared_ptr with unique_ptr, I get compilation errors beyond my understanding.

#include <iostream>
#include <memory>

template<class T>
class BinarySearchTree{
    struct TreeNode;
    typedef std::shared_ptr<TreeNode> spTreeNode;
    struct TreeNode{
        T data;
        spTreeNode  left;
        spTreeNode  right;
        TreeNode(const T & value):data(value),left(nullptr),right(nullptr){}
    };



    spTreeNode root;
    bool insert(spTreeNode node);
    void print(const spTreeNode) const ;
public:
    BinarySearchTree();
    void insert( const T & node);
    void print()const;
};

template<class T>
BinarySearchTree<T>::BinarySearchTree():root(nullptr){}

template<class T>
void BinarySearchTree<T>::insert(const T & ref)
{
    TreeNode *node = new TreeNode(ref);
    if (root==nullptr)
    {
        root.reset(node);
    }
    else
    {
        spTreeNode temp = root;
        spTreeNode prev = root;
        while (temp)
        {
            prev = temp;
            if (temp->data < ref)
                temp = temp->right;
            else
                temp = temp->left;
        }
        if (prev->data  < ref)
            prev->right.reset(node);
        else
            prev->left.reset(node);
    }
}

template<class T>
void BinarySearchTree<T>::print()const
{
    print(root);
}

template<class T>
void BinarySearchTree<T>::print(const spTreeNode node)const
{
    if (node==nullptr)
        return;
    print(node->left);
    std::cout << node->data<< std::endl;
    print(node->right);
}

int main()
{
    BinarySearchTree<int> bst;
    bst.insert(13);
    bst.insert(3);
    bst.insert(5);
    bst.insert(31);
    bst.print();
    return 0;
}

EDIT: Compilation errors in case anyone is interested. Warning: Wall of text.

    prog.cpp: In instantiation of ‘void BinarySearchTree<T>::insert(const T&) [with T = int]’:
prog.cpp:75:18:   required from here
prog.cpp:39:27: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinarySearchTree<int>::TreeNode; _Dp = std::default_delete<BinarySearchTree<int>::TreeNode>]’
         spTreeNode temp = root;
                           ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:273:7: error: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^
prog.cpp:40:27: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinarySearchTree<int>::TreeNode; _Dp = std::default_delete<BinarySearchTree<int>::TreeNode>]’
         spTreeNode prev = root;
                           ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:273:7: error: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^
prog.cpp:43:18: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinarySearchTree<int>::TreeNode; _Dp = std::default_delete<BinarySearchTree<int>::TreeNode>]’
             prev = temp;
                  ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:274:19: error: declared here
       unique_ptr& operator=(const unique_ptr&) = delete;
                   ^
prog.cpp:45:22: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinarySearchTree<int>::TreeNode; _Dp = std::default_delete<BinarySearchTree<int>::TreeNode>]’
                 temp = temp->right;
                      ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:274:19: error: declared here
       unique_ptr& operator=(const unique_ptr&) = delete;
                   ^
prog.cpp:47:22: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinarySearchTree<int>::TreeNode; _Dp = std::default_delete<BinarySearchTree<int>::TreeNode>]’
                 temp = temp->left;
                      ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:274:19: error: declared here
       unique_ptr& operator=(const unique_ptr&) = delete;
                   ^
prog.cpp: In instantiation of ‘void BinarySearchTree<T>::print() const [with T = int]’:
prog.cpp:79:15:   required from here
prog.cpp:59:15: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinarySearchTree<int>::TreeNode; _Dp = std::default_delete<BinarySearchTree<int>::TreeNode>]’
     print(root);
               ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from prog.cpp:2:
/usr/include/c++/4.8/bits/unique_ptr.h:273:7: error: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^
prog.cpp:63:6: error:   initializing argument 1 of ‘void BinarySearchTree<T>::print(BinarySearchTree<T>::spTreeNode) const [with T = int; BinarySearchTree<T>::spTreeNode = std::unique_ptr<BinarySearchTree<int>::TreeNode, std::default_delete<BinarySearchTree<int>::TreeNode> >]’
 void BinarySearchTree<T>::print(const spTreeNode node)const
          ^

Answer 1

unique_ptrs are not assignable but moveable. I reworked your example and now works with unique_ptrs. Notice, that I use std::move in order to move contents from one unique_ptr to another. Also due to the fact that unique_ptr isn't copyable, I pass unique_ptrs in member functions by reference and not by value:

#include <iostream>
#include <memory>

template<class T>
class BinarySearchTree{
    struct TreeNode;
    typedef std::unique_ptr<TreeNode> spTreeNode;
    struct TreeNode{
        T data;
        spTreeNode  left;
        spTreeNode  right;
        TreeNode(const T & value):data(value),left(nullptr),right(nullptr){}
    };



    spTreeNode root;
    bool insert(spTreeNode &node);
    void print(const spTreeNode&) const ;
public:
    BinarySearchTree();
    void insert( const T & node);
    void print()const;
};

template<class T>
BinarySearchTree<T>::BinarySearchTree():root(nullptr){}

template<class T>
void BinarySearchTree<T>::insert(const T & ref)
{
    std::unique_ptr<TreeNode> node(new TreeNode(ref));
    if (root == nullptr) {
        root = std::move(node);
    } else {
        TreeNode* temp = root.get();
        TreeNode* prev = root.get();
        while (temp != nullptr) {
            prev = temp;
            if (temp->data < ref)
                temp = temp->right.get();
            else
                temp = temp->left.get();
        }
        if (prev->data < ref)
            prev->right = std::move(node);
        else
            prev->left = std::move(node);
    }
}

template<class T>
void BinarySearchTree<T>::print()const
{
    print(root);
}

template<class T>
void BinarySearchTree<T>::print(const std::unique_ptr<TreeNode> &node) const
{
    if(node == nullptr) return;
    print(node->left);
    std::cout << node->data<< std::endl;
    print(node->right);
}

int main()
{
    BinarySearchTree<int> bst;
    bst.insert(13);
    bst.insert(3);
    bst.insert(5);
    bst.insert(31);
    bst.print();
    return 0;
}

LIVE DEMO