String实现

#include <iostream>
using namespace std;
class String
{
public:
	String(const char *str = NULL); // 通用构造函数
	String(const String &another); // 拷贝构造函数
	~ String(); // 析构函数
	String& operator =(const String&);// 赋值函数
	friend ostream& operator << (ostream&, const String& s);
	friend istream& operator>>(istream& in, String& s);
	friend String operator+(const String&,const String&);
	bool operator==(const String &);
private:
	char *m_data; // 用于保存字符串
};
String::String(const char *str)
{
   if (str == NULL)
      m_data = NULL;
   else
   {
	   m_data = new char[strlen(str)+1];
       strcpy(m_data,str);
   }

}
String::String(const String &another)
{
   m_data = new char[strlen(another.m_data)+1];
   strcpy(m_data,another.m_data);
}
String::~String()
{
	if(m_data != NULL)
	{
		delete []m_data;
		m_data = NULL;
	}
}
bool String::operator==(const String &s)
{
	if (strcmp(this->m_data,s.m_data))
	    return true;
	return false;
}
String & String::operator=(const String &rhs)
{
  if (*this == rhs)
      return *this;
  delete []m_data;
  m_data = NULL;
  m_data = new char[strlen(rhs.m_data)+1];
  strcpy(m_data,rhs.m_data); 
  return *this;
}
ostream& operator << (ostream& out, const String& s)
{
  if(s.m_data)
   out << s.m_data << endl;
  return out;
}
istream& operator>>(istream& in, String& s)
{
	char q[1000];
	in >> q;
   s.m_data = new char[strlen(q)+1];
   strcpy(s.m_data,q); 
   return in;
}
String operator+(const String& lhs,const String&rhs)
{
	String *ret = new String(lhs);
	strcat(ret->m_data,rhs.m_data);
	return *ret;
}
int main()
{
   String s ;
   cin >> s;
   String s1;
   cin >> s1;
   String s2 = s+s1;//this sentence has question , but i don't know why
   cout << s2;
}