// header.h:
const int SIZE =20;
int nConv;
char *pHeltal;
char *pBuffer;

---------------
// athaserminzkat.cpp
#include<iostream>
#include "header.h"
using namespace std;

int main( )
{     
  nConv   = 0;    
  pHeltal = new char[SIZE];   
  pBuffer = new char[SIZE];     
  cout<<"Indtast et heltal"<<endl;   
  cin>>pBuffer;   
  nConv = atoi(pBuffer);   
  nConv *=nConv;   
  itoa(nConv,pHeltal,10);   
  cout<<pHeltal<<endl;   
  delete [] pHeltal;   
  delete [] pBuffer;   

  system("PAUSE");
  return 0;
}