赋值运算符的重载

#include <iostream>
#include <cstring>
using namespace std;

class String
{
private :
    char * str;
public:
    String() :str(NULL) 
    {
        cout << "the constructor is called " << endl;
    }
    const char * c_str()
    {
        return str;
    }
    char * operator =(const char *s);
    String & operator=(const String & s);
    ~String();
};
String::~String()
{
    if (str)
    {
        delete []str;
    }
    cout << "the destructor is called " << endl;
}
char * String::operator=(const char *s)
{
    if (str)
    {
        delete []str;
    }
    if (s)
    {
        str = new char[strlen(s) + 1];
        strcpy_s(str,strlen(s)+1, s);
    }
    else
    {
        str = NULL;
    }
    return (str);
}
String & String::operator=(const String &s)
{
    if (str == s.str) return (*this);
    if (str)
    delete[] str;
    str = new char[strlen(s.str) + 1];
    strcpy_s(str, strlen(s.str) + 1, s.str);
    return (*this);
}
int main()
{
    //String s2="Hello"//出错，因为这是初始化语句，不是赋值。
    String s1,s2,s3;
    s1 = "Hello world";
    s2 = "that";
    s3 = "this";
    //s1 = s2;//浅复制出错，两个指针指向同一空间，对象释放时会调用两次出错。
    s1 = s2;
    cout << s1.c_str() << endl;
    cout << s2.c_str() << endl;
    return 0;
}