11.2节中，见识到了c++内存管理的险恶，如何解决

Hard Copy

出现原因：多个对象同时指向一块内存空间。
解决方案：所有的对象都拥有自己的内存数据。

11.2中的例子中，错误的罪魁祸首就是C++默认提供的Copy Constructor，它把指针直接赋值了。

  • Provide a user-defined copy constructor. ```cpp MyString::MyString(const MyString & ms) { this->buf_len = 0; this->characters = NULL; create(ms.buf_len, ms.characters); }
  2. - `create()` release the current memory and **allocate a new one.**
  3.    - **会显示放当然内存，然后重新申请一个新的内存空间。**
  4. - `**t**his->characters` will not point to `ms.characters`.
  5.    - 保证了当前对象的指针不跟参数的指针相同
  6. - **It's a hard copy!**
  8. 第二个错误产生的原因：赋值运算符出现了问题<br />默认的运算符重载做了两个变量的复制，我们可以自定义运算符重载函数，
  10. - Provide a user-defined copy assignment
  11. - 使用create去申请内存，使得每个对象都有自己的内存，不在共享
  12. ```cpp
  13. #pragma once
  14. #include <iostream>
  15. #include <cstring>
  17. class MyString
  18. {
  19. private:
  20.     int buf_len;
  21.     char *characters;
  23. public:
  24.     MyString(int buf_len = 64, const char *data = NULL)
  25.     {
  26.         std::cout << "Constructor(int, char*)" << std::endl;
  27.         this->buf_len = 0;
  28.         this->characters = NULL;
  29.         create(buf_len, data);
  30.     }
  31.     MyString(const MyString &ms)
  32.     {
  33.         std::cout << "Constructor(MyString&)" << std::endl;
  34.         this->buf_len = 0;
  35.         this->characters = NULL;
  36.         create(ms.buf_len, ms.characters);
  37.     }
  38.     ~MyString()
  39.     {
  40.         release();
  41.     }
  42.     MyString &operator=(const MyString &ms)
  43.     {
  44.         create(ms.buf_len, ms.characters);
  45.         return *this;
  46.     }
  47.     bool create(int buf_len, const char *data)
  48.     {
  49.         release();
  51.         this->buf_len = buf_len;
  53.         if (this->buf_len != 0)
  54.         {
  55.             this->characters = new char[this->buf_len]{};
  56.         }
  57.         if (data)
  58.             strncpy(this->characters, data, this->buf_len);
  60.         return true;
  61.     }
  62.     bool release()
  63.     {
  64.         this->buf_len = 0;
  65.         if (this->characters != NULL)
  66.         {
  67.             delete[] this->characters;
  68.             this->characters = NULL;
  69.         }
  70.         return 0;
  71.     }
  72.     friend std::ostream &operator<<(std::ostream &os, const MyString &ms)
  73.     {
  74.         os << "buf_len = " << ms.buf_len;
  75.         os << ", characters = " << static_cast<void *>(ms.characters);
  76.         os << " [" << ms.characters << "]";
  77.         return os;
  78.     }
  79. };