在BST中使用unique_ptr而不是shared_ptr

Question

我正在尝试用unique_ptr实现BST。我有一个shared_ptr的工作程序。如何使用unique_ptr来强制BinarySearchTree的单一所有权语义？

当我用unique_ptr替换shared_ptr时，我会出现无法理解的编译错误。

#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;
}

编辑：编译错误，以防有人感兴趣。警告：文字墙。

    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_ptr s不可分配，但可移动。我重新编写了您的示例，现在使用unique_ptr。请注意，我使用std::move是为了将内容从一个unique_ptr移动到另一个。同样由于unique_ptr是不可复制的，我在成员函数中通过引用而不是值传递unique_ptrs：

#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;
}

现场演示

Answer 2

模板风格的错误总是很可怕，但不要惊慌！所有这些"删除函数的使用"都是关于你试图复制一个unique_ptr，它的超能力来自于只能移动。跳到每一行，分析情况：

您是否希望转让指定对象的所有权？然后通过右值引用传递唯一指针，并将其移动到新的保持器。

// Take unique_ptr by rvalue reference
void TreeNode::setLeft(std::unique_ptr<TreeNode> &&node) {
    // Move it in a member variable
    left = std::move(node);
    // Now it's ours !
}

你只是想介绍一下对方吗？使用对unique_ptr的常量左值引用，或从unique_ptr.get((.传递常量原始指针

// Take raw const pointer
bool isNodeLeft(TreeNode const *node) const {
    // Look at it but don't disturb it
    return node->value <= value;
}

一旦所有这些都解决了，您要么会有一个编译代码，要么会解决一些其他错误，要么我会更新这个答案。

注意：TreeNode *node = new TreeNode(ref);在为异常安全大声疾呼。您应该使用make_unique(来自C++1y，或者自己制作(。

这是：

template <class T, class... Args>
std::unique_ptr<T> make_unique(Args&&... args) {
    return unique_ptr<T>(new T(std::forward<Args>(args)...));
}