C++ and mouthwash

Having a problem figuring out an error when I compile an object.

I have a .c file that I'm not supposed to modify, but need to use a function from inside a .h+.cpp object file.  The error has to deal with scope as the compiler (g++) complains.

I understand that normally I should use objectname::function, but since the function I need to use is contained outside the object, I'm not sure what the scope is.

Also, I've been trying to compile it like this:
g++ hurr.c derp.h derp.cpp -o derp.o

Perhaps this isn't correct but I'm not sure of another way while retaining the .c file and not simply copying its contents.

>>32
I'm actually converting a working C program into a C++ program (with limitations, like hurr.c) and needed to use objects.
Why on earth would you do such a thing? C++'s object system is worse than C's; the only thing it really gives you is syntactic sugar for dynamic dispatch.