HW02

main.cpp

@@ -2,118 +2,134 @@
 #include <cstdio>
 #include <memory>
 
-struct Node {
-    // 这两个指针会造成什么问题？请修复
-    std::shared_ptr<Node> next;
-    std::shared_ptr<Node> prev;
-    // 如果能改成 unique_ptr 就更好了!
-
-    int value;
-
-    // 这个构造函数有什么可以改进的？
-    Node(int val) {
-        value = val;
-    }
-
-    void insert(int val) {
-        auto node = std::make_shared<Node>(val);
-        node->next = next;
-        node->prev = prev;
-        if (prev)
-            prev->next = node;
-        if (next)
-            next->prev = node;
+struct Node : std::enable_shared_from_this<Node> {
+  // 这两个指针会造成什么问题？请修复
+  std::shared_ptr<Node> next;
+  std::weak_ptr<Node> prev; // 解决循环引用
+  // 如果能改成 unique_ptr 就更好了!
+
+  int value;
+
+  // 改进构造函数，初始化所有成员
+  explicit Node(int val) : next(nullptr), value(val) {}
+
+  void insert(int val) {
+    auto node = std::make_shared<Node>(val);
+    node->next = next;
+    node->prev = shared_from_this();
+    if (next)
+      next->prev = node;
+    next = node;
+  }
+
+  void erase() {
+    auto prev_node = prev.lock();
+    if (prev_node) {
+      prev_node->next = next;
     }
-
-    void erase() {
-        if (prev)
-            prev->next = next;
-        if (next)
-            next->prev = prev;
+    if (next) {
+      next->prev = prev;
     }
+  }
 
-    ~Node() {
-        printf("~Node()\n");   // 应输出多少次？为什么少了？
-    }
+  ~Node() {
+    printf("~Node()\n"); // 应输出多少次？为什么少了？
+  }
 };
 
 struct List {
-    std::shared_ptr<Node> head;
+  std::shared_ptr<Node> head;
 
-    List() = default;
+  List() = default;
 
-    List(List const &other) {
-        printf("List 被拷贝！\n");
-        head = other.head;  // 这是浅拷贝！
-        // 请实现拷贝构造函数为 **深拷贝**
-    }
+  List(List const &other) {
+    printf("List 被拷贝！\n");
+    // 这是浅拷贝！
+    // 请实现拷贝构造函数为 **深拷贝**
 
-    List &operator=(List const &) = delete;  // 为什么删除拷贝赋值函数也不出错？
+    if (!other.head)
+      return;
 
-    List(List &&) = default;
-    List &operator=(List &&) = default;
+    head = std::make_shared<Node>(other.head->value);
+    auto cur = head;
+    auto ne = other.head->next;
 
-    Node *front() const {
-        return head.get();
+    while (ne != nullptr) {
+      auto new_node = std::make_shared<Node>(ne->value);
+      cur->next = new_node;
+      new_node->prev = cur;
+      ne = ne->next;
+      cur = new_node;
     }
+  }
 
-    int pop_front() {
-        int ret = head->value;
-        head = head->next;
-        return ret;
-    }
+  List &operator=(List const &) = delete; // 为什么删除拷贝赋值函数也不出错？
 
-    void push_front(int value) {
-        auto node = std::make_shared<Node>(value);
-        node->next = head;
-        if (head)
-            head->prev = node;
-        head = node;
-    }
+  List(List &&) = default;
+  List &operator=(List &&) = default;
 
-    Node *at(size_t index) const {
-        auto curr = front();
-        for (size_t i = 0; i < index; i++) {
-            curr = curr->next.get();
-        }
-        return curr;
+  Node *front() const { return head.get(); }
+
+  int pop_front() {
+    int ret = head->value;
+    head = head->next;
+    if (head) {
+      head->prev.reset();
+    }
+    return ret;
+  }
+
+  void push_front(int value) {
+    auto node = std::make_shared<Node>(value);
+    node->next = head;
+    if (head)
+      head->prev = node;
+    head = node;
+  }
+
+  Node *at(size_t index) const {
+    auto curr = front();
+    for (size_t i = 0; i < index; i++) {
+      curr = curr->next.get();
     }
+    return curr;
+  }
 };
 
-void print(List lst) {  // 有什么值得改进的？
-    printf("[");
-    for (auto curr = lst.front(); curr; curr = curr->next.get()) {
-        printf(" %d", curr->value);
-    }
-    printf(" ]\n");
+void print(const List &lst) { // 有什么值得改进的？
+  printf("[");
+  for (auto curr = lst.front(); curr; curr = curr->next.get()) {
+    printf(" %d", curr->value);
+  }
+  printf(" ]\n");
 }
 
 int main() {
-    List a;
+  List a;
 
-    a.push_front(7);
-    a.push_front(5);
-    a.push_front(8);
-    a.push_front(2);
-    a.push_front(9);
-    a.push_front(4);
-    a.push_front(1);
+  a.push_front(7);
+  a.push_front(5);
+  a.push_front(8);
+  a.push_front(2);
+  a.push_front(9);
+  a.push_front(4);
+  a.push_front(1);
 
-    print(a);   // [ 1 4 9 2 8 5 7 ]
+  print(a); // [ 1 4 9 2 8 5 7 ]
 
-    a.at(2)->erase();
+  a.at(2)->erase();
 
-    print(a);   // [ 1 4 2 8 5 7 ]
+  print(a); // [ 1 4 2 8 5 7 ]
 
-    List b = a;
+  List b = a;
 
-    a.at(3)->erase();
+  a.at(3)->erase();
 
-    print(a);   // [ 1 4 2 5 7 ]
-    print(b);   // [ 1 4 2 8 5 7 ]
+  print(a); // [ 1 4 2 5 7 ]
+  print(b); // [ 1 4 2 8 5 7 ]
 
-    b = {};
-    a = {};
+  b = {};
+  a = {};
 
-    return 0;
+  return 0;
 }