If I declare an instance of a class in say, submain.cpp and then try to refrence to that class in main.cpp, it gives me an error.

Well how the hell am I supposed to do anything, if I can't even use one class in another file?

For example:
submain.cpp
class Blah
{
void DoStuff();
};

...

Blah blahinstance

And in main.cpp

blahinstance.DoStuff;

But it doesn't work! How can I use a class in another file, in another function without making it a global variable? Someone please help!