有人可以在這裏解釋輸出嗎? createTreap()
工作正常,並且r不是nullptr,但是createTreapPair()
在treapPair.first.display(); treapPair.second.display();
後離開r == nullptr
。爲什麼?兩者有什麼區別?這裏發生了什麼?從派生類中調用複製構造函數
#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();
}
如何解決上面的代碼,這樣r
不轉treapPair.first.display(); treapPair.second.display();
後nullptr? r
在createTreap()
和createTreapPair()
中都是std::make_shared<Treap::Node>
,那麼爲什麼r = std::dynamic_pointer_cast<Node>(root);
在Treap::display()
中將r轉換爲nullptr?之後createTreapPair()
?
變量'treap1'和'treap2'走出去的範圍,不是嗎? – Chiel
'BinaryTree'拷貝構造函數明確地切掉'Treap :: Node',並且創建'root'保存的'BinaryTree :: Node'的實例。當然''dynamic_cast(root)'返回'nullptr'。 'createTreap'能夠避開拷貝構造函數,但'createTreapPair'不是。 –
@Chiel這就是爲什麼需要調用複製構造函數的原因。 @Igor所以'Treap :: Node'的拷貝構造函數是不夠的?我需要爲'Treap'定義一個拷貝構造函數?那麼'BinaryTree'拷貝構造函數還沒有做什麼?那個拷貝構造函數是什麼樣的? – prestokeys