string的部分实现

#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;}

还是有问题的，待改动！