C++ 赋值运算符=重载

#include <iostream>using namespace std;class String{public:    String():str(NULL) {}~String(){if(str)delete[] str;}const char * c_str() {return str;}char * operator= (const char *s);String& operator= (const String &s);private:    char *str;};char * String::operator=(const char *s){if(str)delete[] str;if(s){str = new char[strlen(s)+1];strcpy(str,s);}else{str = NULL;}return str;}String& String::operator= (const String &s){// if it is self-copyif(str == s.str)return *this;if(str)    delete[] str;if(s.str){str = new char[strlen(s.str)+1];    strcpy(str,s.str);}else{str = NULL;}return *this;}int main(){    String s;s = "Good luck,";cout << s.c_str() << endl;s = "Shenzhou 8";cout << s.c_str() << endl;return 0;}