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.

>>21
Yes, it's saying I don't have a main, problem here is that I'm not supposed to have a main at all for this object.

I'm just trying to compile a derp object file to be used in my main bidoof program later on.  This derp object needs to use the hurrfunction found in hurr.c at some point.