C++ 将 std::unique_ptr 作为参数传递

Question

我尝试使用 std::unique_ptr 实现二叉树，但出现错误，我不明白输出错误。

代码如下：

#include <iostream>
#include <memory>
#include <functional>
#include <utility>

template <typename T>
class BinTreeNode {
public: 
    BinTreeNode(T key): data {key}, left {nullptr}, right {nullptr} {}
    ~BinTreeNode() {}
    T data;
    std::unique_ptr<BinTreeNode<T>> left;
    std::unique_ptr<BinTreeNode<T>> right;
};

template <typename T>
class BinTree {
public:
    BinTree() : root {nullptr} {} 
    ~BinTree() {}
    std::unique_ptr<BinTreeNode<T>> root;

    void insert(std::unique_ptr<BinTreeNode<T>> node, T key);
};

template <typename T>
void BinTree<T>::insert(
    std::unique_ptr<BinTreeNode<T>> node, 
    T key)
{
    if(node){ // != nullptr
        if(node->data < key) insert(node->right, key);
        else insert(node->left, key);
    } 
    else{
        std::unique_ptr<BinTreeNode<T>> u_ptr(new BinTreeNode<T>(key));
        node = std::move(u_ptr);
    }
}

int main(){
    BinTree<int> tree();
    tree.insert(tree.root, 10);
}

我假设错误出在插入函数中并且与参数初始化有关。

BinTree.cpp:65:27: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinTreeNode; _Dp = std::default_delete >]’ tree.insert(tree.root, 10); ^

In file included from /usr/include/c++/4.9/memory:81:0, from BinTree.cpp:2: /usr/include/c++/4.9/bits/unique_ptr.h:356:7: note: declared here unique_ptr(const unique_ptr&) = delete; ^

BinTree.cpp:35:6: error: initializing argument 1 of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’ void BinTree::insert( ^

BinTree.cpp: In instantiation of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’: BinTree.cpp:65:27: required from here BinTree.cpp:40:47: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinTreeNode; _Dp = std::default_delete >]’
if(node->data < key) insert(node->right, key); ^

In file included from /usr/include/c++/4.9/memory:81:0, from BinTree.cpp:2: /usr/include/c++/4.9/bits/unique_ptr.h:356:7: note: declared here unique_ptr(const unique_ptr&) = delete; ^

BinTree.cpp:35:6: error: initializing argument 1 of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’ void BinTree::insert( ^

BinTree.cpp:41:30: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinTreeNode; _Dp = std::default_delete >]’ else insert(node->left, key); ^

In file included from /usr/include/c++/4.9/memory:81:0, from BinTree.cpp:2: /usr/include/c++/4.9/bits/unique_ptr.h:356:7: note: declared here unique_ptr(const unique_ptr&) = delete; ^

BinTree.cpp:35:6: error: initializing argument 1 of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’ void BinTree::insert(

Answer 1

错误是由于您试图从 tree.root 复制构造 BinTree::insert 的参数造成的。 std::unique_ptr 只能移动。

我的猜测是 BinTree::insert 中的 node 应该通过引用传递。原因：

您必须 std::move 将 tree.root 放入其中（如果按值传递），这会窃取所有权
您在 BinTree::insert 中对其进行移动分配，这些更改不会通过 tree.root

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