I have created two classes, Card and Monster, with Card containing a function that is friendly to Monster. Monster produces a compile error saying "invalid use of incomplete type class Card" on the line of the friend statement.
From my research I've found that one way to fix this is #include Card.h into Monster.h (as it says here). Is there really a reason, if you're only going to use one of the header files as in that example, to make two? Is it just not acceptable to have more than one class to a header file?