详解C++句柄类

class MyString
{
public:
 // 默认构造函数
 MyString()
 {
  std::cout << "MyString()" << std::endl;

  buf_ = new char[1];
  buf_[0] = '\0';
  len_ = 0;
 }

 // const char*参数的构造函数
 MyString(const char* str)
 {
  std::cout << "MyString(const char* str)" << std::endl;

  if (str == nullptr)
  {
   len_ = 0;
   buf_ = new char[1];
   buf_[0] = '\0';
  }
  else
  {
   len_ = strlen(str);
   buf_ = new char[len_ + 1];
   strcpy_s(buf_, len_ + 1, str);
  }
 }

 // 拷贝构造函数
 MyString(const MyString& other)
 {
  std::cout << "MyString(const MyString& other)" << std::endl;

  len_ = strlen(other.buf_);
  buf_ = new char[len_ + 1];
  strcpy_s(buf_, len_ + 1, other.buf_);
 }

 // str1 = str2;
 const MyString& operator=(const MyString& other)
 {
  std::cout << "MyString::operator=(const MyString& other)" << std::endl;

  // 判断是否为自我赋值
  if (this != &other)
  {
   if (other.len_ > this->len_)
   {
    delete[]buf_;
    buf_ = new char[other.len_ + 1];
   }

   len_ = other.len_;
   strcpy_s(buf_, len_ + 1, other.buf_);
  }

  return *this;
 }

 // str = "hello!";
 const MyString& operator=(const char* str)
 {
  assert(str != nullptr);

  std::cout << "operator=(const char* str)" << std::endl;

  size_t strLen = strlen(str);
  if (strLen > len_)
  {
   delete[]buf_;
   buf_ = new char[strLen + 1];
  }

  len_ = strLen;
  strcpy_s(buf_, len_ + 1, str);
  
  return *this;
 }
 
 // str += "hello"
 void operator+=(const char* str)
 {
  assert(str != nullptr);

  std::cout << "operator+=(const char* str)" << std::endl;

  if (strlen(str) == 0)
  {
   return;
  }

  size_t newBufLen = strlen(str) + len_ + 1;
  char* newBuf = new char[newBufLen];
  strcpy_s(newBuf, newBufLen, buf_);
  strcat_s(newBuf, newBufLen, str);

  delete[]buf_;
  buf_ = newBuf;

  len_ = strlen(buf_);
 }

 // 重载 ostream的 <<操作符 ，支持 std::cout << MyString 的输出
 friend std::ostream& operator<<(std::ostream &out, MyString& obj)
 {
  out << obj.c_str();
  return out;
 }

 // 返回 C 风格字符串
 const char* c_str()
 {
  return buf_;
 }

 // 返回字符串长度
 size_t length()
 {
  return len_;
 }

 ~MyString()
 {
  delete[]buf_;
  buf_ = nullptr;
 }

private:
 char* buf_;
 size_t len_;
};

#include "MyString.h"

int _tmain(int argc, _TCHAR* argv[])
{
 MyString str1("hello~~");
 MyString str2 = str1;
 MyString str3 = str1;

 std::cout << "str1=" << str1 << ", str2=" << str2 << ", str3=" << str3;

 return 0;
}

class RefCount
{
public:

 RefCount() : count_(new int(1)){};

 RefCount(const RefCount& other) : count_(other.count_)
 {
  ++*count_;
 }

 ~RefCount()
 {
  if (--*count_ == 0)
  {
   delete count_;
   count_ = nullptr;
  }
 }

 bool Only()
 {
  return *count_ == 1;
 }

 void ReAttach(const RefCount& other)
 {
  // 更新原引用计数的信息
  if (Only())
  {
   delete count_;
  }
  else
  {
   --*count_;
  }

  // 更新新的引用计数的信息
  ++*other.count_;
  
  // 绑定到新的引用计数
  count_ = other.count_;
 }

 void MakeNewRef()
 {
  if (*count_ > 1)
  {
   --*count_;
   count_ = new int(1);
  }
 }

private:
 int* count_;
};

class MyStringHandle
{
public:
 MyStringHandle() : pstr_(new MyString){}

 // 这两种参数的构造函数必须构造一个新的MyString对象出来
 MyStringHandle(const char* str) : pstr_(new MyString(str)) {}
 MyStringHandle(const MyString& other) : pstr_(new MyString(other)) {}

 // 拷贝构造函数，将指针绑定到参数绑定的对象上，引用计数直接拷贝构造，在拷贝构造函数内更新引用计数的相关信息
 MyStringHandle(const MyStringHandle& ohter) : ref_count_(ohter.ref_count_), pstr_(ohter.pstr_) {}

 ~MyStringHandle()
 {
  if (ref_count_.Only())
  {
   delete pstr_;
   pstr_ = nullptr;
  }
 }

 MyStringHandle& operator=(const MyStringHandle& other)
 {
  // 绑定在同一个对象上的句柄相互赋值，不作处理
  if (other.pstr_ == pstr_)
  {
   return *this;
  }

  // 若当前引用唯一，则销毁当前引用的MyString
  if (ref_count_.Only())
  {
   delete pstr_;
  }

  // 分别将引用计数和对象指针重定向
  ref_count_.ReAttach(other.ref_count_);
  pstr_ = other.pstr_;

  return *this;
 }

 // str = "abc" 这里涉及到对字符串内容的修改，
 MyStringHandle& operator=(const char* str)
 {
  if (ref_count_.Only())
  {
   // 如果当前句柄对MyString对象为唯一的引用，则直接操作改对象进行赋值操作
   *pstr_ = str;
  }
  else
  {
   // 如果不是唯一引用，则将原引用数量-1，创建一个新的引用，并且构造一个新的MyString对象
   ref_count_.MakeNewRef();
   pstr_ = new MyString(str);
  }

  return *this;
 }

private:
 MyString* pstr_;
 RefCount ref_count_;
};

int _tmain(int argc, _TCHAR* argv[])
{
 // 构造MyString
 MyStringHandle str1("hello~~");

 // 不会构造新的MyString
 MyStringHandle str2 = str1;
 MyStringHandle str3 = str1;
 MyStringHandle str4 = str1;

 // 构造一个空的MyString
 MyStringHandle str5;

 // 将str1赋值到str5，不会有内存拷贝
 str5 = str1;

 // 修改str5的值
 str5 = "123";
 str5 = "456";

 return 0;
}