C++ Class Member Function converts String objects to upper case

#include <iostream>
#include <cstring>       //for strcpy(), etc
#include <cctype>        //for toupper()
using namespace std;
class String             //user-defined string type
{
   private:/*from   w ww  . j a va 2s .c o  m*/
   char* str;                    //pointer to string
   public:
   String(char* s)               //constructor, one arg
   {
      int length = strlen(s);    //length of string argument
      str = new char[length+1];  //get memory
      strcpy(str, s);            //copy argument to it
   }
   ~String()                     //destructor
   { delete str; }
   void display()                //display the String
   { cout << str; }
   void upit();                  //uppercase the String
};
void String::upit()                 //uppercase each character
{
   char* ptrch = str;               //pointer to this string
   while( *ptrch )                  //until null,
   {
      *ptrch = toupper(*ptrch);     //uppercase each character
      ptrch++;                      //move to next character
   }
}
int main()
{
   String s1 = "He who laughs last laughs best.";
   cout << "\ns1=";       //display string
   s1.display();
   s1.upit();             //uppercase string
   cout << "\ns1=";       //display string
   s1.display();
   cout << endl;
   return 0;
}