c++中的C风格字符串和操作符重载

我试图在我自己的C styled版本的string类上重载+操作符。

一切都很好，除了display()调用第二个字符串s2显示垃圾值。

class string_ {
   char *str;
   int len;
   public :
         ...
         void display()
         {
              cout << str;
         }
};
string_ :: string_()
{
   str = 0;
   len = 0;
}
string_ :: ~string_()
{
   delete []str;
   len = 0;
}

string_ :: string_(char *s,int l)
{
   len = l + 1; // +1 for 
   str = new char[len];
   int i;
   for(i=0;i<len;i++)
          str[i] = s[i];
   str[i] = '';
}
string_ string_ :: operator +(string_ c)
{      
   int j = 0, i = 0;
   string_ s;
   s.len = len + c.len - 1;
   s.str = new char[s.len];
   while (str[i] != '')
   {s.str[i] = str[i]; i++;}
   while (c.str[j] != '')
   {s.str[i] = c.str[j]; i++; j++; }
   s.str[i] = '';
   //The below statements gives the desired output
   cout <<"nIN operator +" << str;
   cout <<"nIN operator +" << c.str;
   cout <<"nIN operator +" << s.str;
   return s;
 }
int main()
{
   char *str = "Hello";
   char *str1 = " World";
   string_ s1(str,5);
   string_ s2(str1,6);
   string_ s3 = s1 + s2;
   cout << "nstring s1 : ";
   s1.display();
   cout << "nstring s2 : ";
   s2.display(); //PROBLEM
   cout << "nConcatenated string : ";
   s3.display();
   return 0;
}