without an example of the code, I really don't know.
i guess you could make sure you have <iostream> included?
edit:
what also might have happened is that your files aren't pulling the information correctly from the included files.
i am in the process of making a MUD, and this happened to me. i had MULTIPLE header files included in my source files, and was getting random errors. i put all the header files into one "database.h" header, and the issue resolved itself.