下面代码中operator=中str.m_data m_data不是私有的吗，为何跑起来没有问题，真是非常奇怪。
string.h

#ifndef __MYSTRING__
#define __MYSTRING__

#include <iostream>
using namespace std;

class String
{
public:                                 
   String(const char* cstr=0);                     
   String(const String& str);                    
   String& operator=(const String& str);         
   ~String();                                    
   char* get_c_str() const { return m_data; }
private:
   char* m_data;
};

#include <cstring>

inline
String::String(const char* cstr)
{
   if (cstr) {
      m_data = new char[strlen(cstr)+1];
      strcpy(m_data, cstr);
   }
   else {   
      m_data = new char[1];
      *m_data = '\0';
   }
}

inline
String::~String()
{
   delete[] m_data;
}

inline
String& String::operator=(const String& str)
{
   if (this == &str)
      return *this;

   delete[] m_data;
   m_data = new char[ strlen(str.m_data) + 1 ];
   strcpy(m_data, str.m_data);
   return *this;
}

inline
String::String(const String& str)
{
   m_data = new char[ strlen(str.m_data) + 1 ];
   strcpy(m_data, str.m_data);
}



ostream& operator<<(ostream& os, const String& str)
{
   os << str.get_c_str();
   return os;
}

#endif

main.cpp

#include "string.h"
#include <iostream>

using namespace std;
class str{
public:
private:
    char * m_data;
};

int main()
{
  String s1("hello"); 
  String s2("world");

  String s3(s2);
  cout << s3 << endl;
  
  s3 = s1;
  cout << s3 << endl;     
  cout << s2 << endl;  
  cout << s1 << endl;      
}