C++: preprocessor used as a constant data field

#include <iostream.h>
#include <stdlib.h>
#define SAL 4000
//preprocessor_5.cpp
int main(void)

{
double sal, sal_cal;
cout<<"Use as a constant\n";
cout<<"----proprocessor starts------------\n";
cout << " enter commision ";
cin >> sal;
sal_cal = SAL + sal;
cout << sal_cal << endl;
cout<<"----proprocessor ends------------\n";
return 0;
}