从派生 class 调用的复制构造函数

Question

有人可以解释这里的输出吗？ createTreap() 工作正常，并且 r 不是 nullptr，但是 createTreapPair() 在 treapPair.first.display(); treapPair.second.display(); 之后留下 r == nullptr。为什么？两者有什么区别？这是怎么回事？

#include <iostream>
#include <memory>

class BinaryTree {
public:
    class Node {
        int value;
        std::shared_ptr<Node> left = nullptr, right = nullptr, parent = nullptr;
        friend class BinaryTree;  friend class Treap;
    public:
        Node (int v) : value(v) {}
        virtual ~Node() = default;
        Node (const Node&);
    };
    BinaryTree() : root(nullptr) {}
    BinaryTree (const BinaryTree& other) : root(std::shared_ptr<Node>(new Node(*other.root))) {std::cout << "BinaryTree copy constructor called.\n";}
    void setRoot (const std::shared_ptr<Node>& node) {root = node;}
protected:
    std::shared_ptr<Node> root;
};

BinaryTree::Node::Node (const Node& other) : value(other.value) {
    std::cout << "Node copy constructor called, value = " << value << '\n';
    if (other.left) left = std::shared_ptr<Node>(new Node(*other.left));
    if (other.right) right = std::shared_ptr<Node>(new Node(*other.right));
}

class Treap : public BinaryTree {
public:
    class Node : public BinaryTree::Node {
        int priority;
        friend class Treap;
    public:
        Node (int value) : BinaryTree::Node(value), priority(std::rand() % 100) {}
        Node (const Node& other) : BinaryTree::Node(other), priority(other.priority) {}  // Treap::Node copy constructor.
    };
    void display() const {
        std::shared_ptr<Node> r = std::dynamic_pointer_cast<Node>(root);  // Casting from BinaryTree::Node to Treap::Node.
        std::cout << "r = " <<  r << '\n';
        if (root) std::cout << "Root exists and has value " << root->value << ".\n";
    }
};

Treap createTreap() {
    std::cout << "\n\ncreateTreap() called.\n";
    Treap treap;
    std::shared_ptr<Treap::Node> r = std::make_shared<Treap::Node>(4);
    treap.setRoot(r);
    return treap;
}

std::pair<Treap, Treap> createTreapPair() {
    std::cout << "\n\ncreateTreapPair() called.\n";
    Treap treap1, treap2;
    std::shared_ptr<Treap::Node> r = std::make_shared<Treap::Node>(11);
    treap1.setRoot(r);
    treap2.setRoot(r);
    return std::make_pair(treap1, treap2);
}

int main() {
    const Treap treap = createTreap();
    treap.display();  // Works fine, r != nullptr.

    const std::pair<Treap, Treap> treapPair = createTreapPair();
    treapPair.first.display();  // r is nullptr!
    treapPair.second.display();  // r is nullptr!

    std::cin.get();
}

如何修复上面的代码，使 r 在 treapPair.first.display(); treapPair.second.display(); 之后不转为 nullptr？ r在createTreap()和createTreapPair()中都是std::make_shared<Treap::Node>类型，为什么r = std::dynamic_pointer_cast<Node>(root);在Treap::display()中把r变成nullptr呢？在 createTreapPair()?

之后

Answer 1

@IgorTandetnik 已经指出了你代码中的问题。

BinaryTree copy constructor explicitly slices away Treap::Node, and creates an instance of BinaryTree::Node that root holds on to. Then of course dynamic_cast<Treap::Node>(root) returns nullptr.

解决这个问题的一种方法是创建一个virtual成员函数来克隆一个BinaryTree::Node并在Node和BinaryTree的复制构造函数中使用它。

这是您的代码的更新版本：

class BinaryTree {
   public:
      class Node {
         int value;
         std::shared_ptr<Node> left = nullptr, right = nullptr, parent = nullptr;
         friend class BinaryTree;  friend class Treap;
         public:
         Node (int v) : value(v) {}
         virtual ~Node() = default;
         Node (const Node&);

         //////////////////////////////
         // New code
         //////////////////////////////
         virtual Node* clone() const = 0;

      };
      BinaryTree() : root(nullptr) {}
      BinaryTree (const BinaryTree& other) : root(nullptr)
      {
         //////////////////////////////
         // Updated code
         //////////////////////////////
         if ( other.root )
         {
            root = std::shared_ptr<Node>(other.root->clone());
         }

         std::cout << "BinaryTree copy constructor called.\n";
      }
      void setRoot (const std::shared_ptr<Node>& node) {root = node;}
   protected:
      std::shared_ptr<Node> root;
};

BinaryTree::Node::Node (const Node& other) : value(other.value) {
   std::cout << "Node copy constructor called, value = " << value << '\n';

   //////////////////////////////
   // Updated code
   //////////////////////////////
   if (other.left) left = std::shared_ptr<Node>(other.left->clone());
   if (other.right) right = std::shared_ptr<Node>(other.right->clone());
}

class Treap : public BinaryTree {
   public:
      class Node : public BinaryTree::Node {
         int priority;
         friend class Treap;
         public:
         Node (int value) : BinaryTree::Node(value), priority(std::rand() % 100) {}
         Node (const Node& other) : BinaryTree::Node(other), priority(other.priority) {}  // Treap::Node copy constructor.

         //////////////////////////////
         // New code
         //////////////////////////////
         virtual Node* clone() const
         {
            return new Node(*this);
         }
      };
      void display() const {
         std::shared_ptr<Node> r = std::dynamic_pointer_cast<Node>(root);  // Casting from BinaryTree::Node to Treap::Node.
         std::cout << "r = " <<  r.get() << '\n';
         if (root) std::cout << "Root exists and has value " << root->value << ".\n";
      }
};

从派生 class 调用的复制构造函数

Copy constructor called from derived class

c++

copy-constructor

c++11