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.

>>20
Learn how compilation in C and C++ works - I tried writing a short explanation but there's no easy way to do this. Also, it seems that you're wrongly using the term object. "Object file" doesn't have anything at all to do with OOP and C++, and I've no idea why you'd call derp an object.