The header file <iostream.h> should be included before "expr.h" is compiled. If you include <iostream.h> before "expr.h" in "expr.cpp" and "asgn3.cpp", there should be no problem. I think a better solution is to include <iostream.h> directly in "expr.h". I will be changing the online version of "expr.h" to have this include statement. - Dave Simpson CMPT 212 instructor