[Dev-C++] Operator overloading - String concatenation

[Abhijeet S <abh...@ms...>]

Hi guys,
this is the program I have written.. this works fine on Turbo C++ except 
that in the end it gives a null pointer assignment after giving the output. 
in DevC++ it just accepts the input and crashes.. can anyone tell me why.. I 
tried using the while loop structure too but same error also tried finding 
out the length of the sting and then using that in the for loop
anyways.. I guess I'm going in the wrong direction so can anyone help me out 
?

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

using namespace std;

class strin
{
  private:
      char *str;

  public:

    void getstr()
    {
      cout<<"Enter the string : ";
      cin>>str;
     }

    void putstr()
    {
	 cout<<"String is : "<<str<<endl;
    }

    strin operator +(strin s2)
    {
      int i,j;
      strin temp;


     for(i=0;str[i]!='\0';i++)
       temp.str[i] = str[i];

     for(j=0;s2.str[j]!='\0';j++)
      temp.str[i++] = s2.str[j];
     temp.str[i] = '\0';
     return temp;
    }

};


int main()
{
  strin s1,s2,s3;
  s1.getstr();
  s1.putstr();
  s2.getstr();
  s2.putstr();
  s3 = s1 + s2;
  s3.putstr();
  getch();
}

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