C++ 复制构造函数 - 指针分段错误的深度复制

问：

我最近开始通过高级计算机科学基础MOOC学习C++。我们有一个在隐藏的 .h 文件中声明的挑战（无法更改），我们需要对 .h 文件中初始化的所有成员函数/构造函数/析构函数进行编码。

为了避免作弊或任何人为我做这项工作，如果有人能帮助解释我做错了什么以及我做了什么来获得分段错误，我将不胜感激。我已经搜索并搜索了所有深度与浅层的复制指南，但似乎无法理解我做错了什么。

下面是 .h 文件：

  class Pair {
    public:
      int *pa,*pb;
      Pair(int, int);
      Pair(const Pair &);
      ~Pair();
   };

从中需要指出的两件事是，pa/pb 是指向 int 的指针，而不仅仅是 ints，并且没有赋值运算符的位置，即使我读到了“三巨头规则”，它解释了如果我有复制构造函数或析构函数，我也应该有赋值运算符。

继续前进，我尝试了很多事情来让它工作，并包含大量的诊断信息，可以看到我在哪里搞砸了，但不明白为什么或我应该做什么。

自定义构造函数我认为这没关系：

 Pair::Pair (int a,int b) {
      int *pa = new int(a);
      int *pb = new int(b);
      std::cout << "pa points to value : " << *pa <<std::endl;
      std::cout << "pb points to value : " << *pb <<std::endl;
      std::cout << "custom constructor resolved "<<std::endl;
    }

Copy 构造函数请原谅我刚刚试图排除故障的所有评论。

    Pair::Pair(const Pair &obj) {

    int *const * a = &obj.pa;
    int *const * b = &obj.pb;
    int *pa = new int;
    int *pb = new int;
    pa = *a;
    pb = *b;

    std::cout << "obj.pa address is : " << &obj.pa <<std::endl;
    std::cout << "obj.pb address is : " << &obj.pb <<std::endl;
    std::cout << "obj.pa points at : " << obj.pa <<std::endl;
    std::cout << "obj.pb points at : " << obj.pb <<std::endl;

    std::cout << "pa address is : " << &pa <<std::endl;
    std::cout << "pb address is : " << &pb <<std::endl;
    std::cout << "pa is pointing at : " << pa <<std::endl;
    std::cout << "pb is pointing at : " << pb <<std::endl;
    std::cout << "copy constructor called "<<std::endl;
     }

破坏者我想我在这方面做得很好：

    Pair::~Pair() {
      delete pa; pa = nullptr;
      std::cout << "pa deleted " << std::endl;

      delete pb; pb = nullptr;
      std::cout << "pb deleted " << std::endl;
    }

主要系列测试

int main() {
  Pair p(15,16);
  Pair q(p);
  Pair *hp = new Pair(23,42);
  delete hp;

  std::cout << "If this message is printed,"
    << " at least the program hasn't crashed yet!\n"
    << "But you may want to print other diagnostic messages too." << std::endl;
  return 0;
}

从这个角度来看，程序将创建一个对 p;然后创建一个深拷贝对 Q;然后创建一个指向堆上一对 23,42 的配对 HP 的指针;然后删除指向配对 hp 的指针。除了复制构造函数之外，一切似乎都可以正常编译和运行。主要问题是 &obj.pa 似乎只是拉取对象 obj 的指针 *pa 指向的地址，而不是检索实际取消引用的值。

有关其他信息，以下是执行 .main 时的终端输出（添加附加注释）：

pa points to value : 15            // pair p.pa correct
pb points to value : 16            // pair p.pb correct
custom constructor resolved        // pair p made
obj.pa address is : 0x7fff1887c780 // address original object pointer pa stored 
obj.pb address is : 0x7fff1887c788 // address original object pointer pb stored
obj.pa points at : 0x2             // address pointer obj pa is directed to (I want the value not this)
obj.pb points at : 0x40102d        // address pointer obj pb is directed to (I wand the value not this)
pa address is : 0x7fff1887c728     // pa address on stack
pb address is : 0x7fff1887c730     // pb address on stack
pa is pointing at : 0x2            // address value copied not address itself
pb is pointing at : 0x40102d       // address value copied not address itself
copy constructor called            // copy constructor runs through albeit incorrect
pa points to value : 23            // hp.pa
pb points to value : 42            // hp.pb
custom constructor resolved        // constructor made on heap from pointer hp
pa deleted deleted original pa     // (made from pointer to pair hp)
pb deleted deleted original pa     // (made from pointer to pair hp)
If this message is printed, at least the program hasn't crashed yet!
But you may want to print other diagnostic messages too.
pa deleted                         // deleted original pa (made from pair p)
pb deleted                         // deleted original pb (made from pair p)

我跑了一遍，在执行之后得到了我不太理解的“分段错误”。而且在C++的早期，我甚至对我的术语没有信心来帮助我的搜索能力。