c++ 初始化二叉树节点

Question

I get these errors: e0289 no instance of constructor...matches argument list and c2440: 'initializing' cannot convert from initializer list to BinaryTreeNod

I am trying to point head to left and right subtree.

---

#include <memory>

using namespace std;

template <typename T>
struct BinaryTreeNode{
  T data;
  unique_ptr<BinaryTreeNode<T>> left, right;

  explicit BinaryTreeNode(const T& data) : data(data) {}
  BinaryTreeNode(T data, unique_ptr<BinaryTreeNode<T>> left,
  unique_ptr<BinaryTreeNode<T>> right) : data(data), left(move(left)),
  right(move(right)) {}
};

int main()
{

  BinaryTreeNode<int> subtree_0{ 5 };
  BinaryTreeNode<int> subtree_1{ 7 };
  BinaryTreeNode<int> head{3, subtree_0, subtree_1 };
}

---

the error comes from the line BinaryTreeNode head{3, subtree_0, subtree_1 }; . Is something wrong with the constructor? Or am I doing smth wrong trying to initialize like that? Just trying to make a simple binary tree (the code of the BinaryTree Node is from "Elements of Programming Interviews"

Answer 1

std::unique_ptr does not support implicit pointer to unique pointer conversion, but it has an explicit constructor to do so. But assigning it a stack allocated pointer is not advised as it might try to delete it later on.

To resolve this, we need to make the two nodes out of unique pointers.

std::unique_ptr<BinaryTreeNode<int>> subtree_0 = std::make_unique<BinaryTreeNode<int>>(BinaryTreeNode<int>(5));
std::unique_ptr<BinaryTreeNode<int>> subtree_1 = std::make_unique<BinaryTreeNode<int>>(BinaryTreeNode<int>(7));

But as the spec states, you cannot copy any unique pointers. Which means that we need to move these two pointers to the tree node object. For this we need to accept them as rvalues in the constructor. Then we can move the pointers to the object without an issue.

template <typename T>
struct BinaryTreeNode {
    ...
    BinaryTreeNode(T data, std::unique_ptr<BinaryTreeNode<T>>&& left,
        std::unique_ptr<BinaryTreeNode<T>>&& right) : data(data), left(std::move(left)), right(std::move(right)) {}
};

int main()
{
    std::unique_ptr<BinaryTreeNode<int>> subtree_0 = std::make_unique<BinaryTreeNode<int>>(BinaryTreeNode<int>(5));
    std::unique_ptr<BinaryTreeNode<int>> subtree_1 = std::make_unique<BinaryTreeNode<int>>(BinaryTreeNode<int>(7));
    BinaryTreeNode<int> head{ 3, std::move(subtree_0), std::move(subtree_1) };
}