🚀 go-pugleaf

I've written a class for string datatype.  The program runs fine but crashes
in the end . However if I initialize the variable po in the main function
 int i, po= 8;) instead of getting it from the user (cin >>po) the program
exits nicely and doesn't crash.
I'll be very grateful for any assistance. I'm using turbo c++ 3.0.

Thanks


#include <iostream.h>
#include <conio.h>
#include <stdio.h>
#include <string.h>

class string
{
 private:
      char *head;
      int len;
 public:
      string()
   {
   }

  string(int l)
  {
   len=l;
   head=new char[len];
  }

  string(char a[])
  {
   strcpy(head,a);
  }

  ~string()
  {
     delete head;
  }

  int length();
  char atindex(int);
  int substring(char []);
  void print();
  void concatenate(char []);
  void upper();
  void lower();
  void proper();
  int compare(char []);
};

int string::length()
{
 int index=0;
 while (*(head+index))
  index++;
 len=++index;
 return index;
}

void string::print()
{
 int index=0;
 while (*(head+index))
 {
  cout<<*(head+index);
  index++;
 }
}

char string::atindex(int index)
{
    return *(head+index);
}

void string::upper()
{
 int index=0;
 while (*(head+index))
 {
  if ((*(head+index)>=97) && (*(head+index)<=122))
   *(head+index)-=32;
  index++;
 }
}

void string::lower()
{
 int index=0;
 while (*(head+index))
 {
  if ((*(head+index)>=65) && (*(head+index)<=90))
   *(head+index)+=32;
  index++;
 }
}

void string::proper()
{
 int index=0;
 while (*(head+index))
 {
  if (((index==0)||(*(head+index-1)==32))&&((*(head+index)>=97) &&
(*(head+index)<=122)))
   *(head+index)-=32;
  else if ((*(head+index)>=65) && (*(head+index)<=90))
   *(head+index)+=32;
  index++;
 }
}

int string::substring(char a[])
{
 int index=0,i,flag=-1;
 while (*(head+index))
 {
  if(*(head+index) == *a)
  {
   flag=index;
   i=1;
   while (*(a+i))
   {
    if (*(head+index+i) != *(a+i))
    {
     flag=-1;
     break;
    }
    else
     i++;
   }
  }
  if (flag != -1)
   return flag;
  else
   index++;
 }
 return flag;
}

void main(void)
{
 char sub[30],j;
 int i,po;
 string s1("the 1st and most IMPORTANT thing");
 clrscr();
 s1.print();
 cout<<endl<<"The length of string (including null character)=
"<<s1.length();
 cout<<"\nEnter the index for a single character value: ";
 cin>> po;  /* NOTE : IF INITIALIZE PO ABOVE INSTEAD OF GETTING IT FROM THE
USER. PROGRAM WORKS FINE*/
  cout<<"\nThe character at position "<<po<<" is \""<<s1.atindex(po)<<"\"";
 s1.upper();
 cout<<"\nThe string converted to upper case: ";
 s1.print();
 s1.lower();
 cout<<"\nThe string converted to lower case: ";
 s1.print();
 s1.proper();
 cout<<"\nThe string converted to proper case: ";
 s1.print();
 cout<<"\nEnter a substring to be searched in the main string: ";
 gets(sub);
 i=s1.substring(sub);
 if (i != -1)
  cout<<"The position of \""<<sub<<"\" is at index = "<<i;
 else
  cout<<"Substring not found!!!";
 getch();
}