class String
{
public:
  String(const char* = nullptr); //普通构造函数
  String(const String& other);  //拷贝构造函数
  ~String(void); //析构函数
  String& operator = (const String& other);  //赋值函数
  
private:
  char* m_data;
};
 
//普通构造函数
String::String(const char* str)
{
  if(str == nullptr){
    m_data = new char[1];  //对空字符自动申请存放结束标志'\0'的空
    *m_data = '\0';
  }else{
    m_data = new char[strlen(str) + 1];  //+1是为了多余一个字符存放'\0'
    strcpy(m_data, str);
  }
}
 
//拷贝构造函数
String::String(const String& other)
{
  if(other == nullptr){
    m_data = nullptr;
  }else{
    //注意下面括号里面都是other.m_data
    m_data = new char[strlen(other.m_data) + 1];
    strcpy(m_data, other.m_data);
  }
}
 
//析构函数
String::~String(void)
{
  if(m_data != nullptr){
    delete [] m_data;
    m_data = nullptr;
  }
}
 
//赋值运算符
String& String::operator=(const String& other)
{
  //判断是否是给自己赋值
  if(this != other){
    delete [] m_data;  //先释放掉原来的内存
    if(other == nullptr){
      m_data = nullptr;
    }else{ 
      m_data = new char[strlen(other.m_data) + 1];
      strcpy(m_data, other.m_data);
    }
  }
  return *this;
}

String& operator + (String& other)
{
  char* tmp = m_data;
  m_data = new char[strlen(m_data) + strlen(other.m_data) + 1];
  strcpy(m_data, tmp);  //复制第一个字符串
  strcpy(m_data, other.m_data);  //复制第二个字符串
  delete [] tmp; //记得删除这个内存
  return *this;
}
 
String& operator = (String& other)
{
  if(this = other){
    return *this;
  }
  if(m_data != nullptr){
    delete [] m_data;  //先释放之前的内存
  }
  m_data = new char [strlen(other.m_data) + 1];
  strcpy(m_data, other.m_data);
  return *this;
}