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.

>>31
I realize that now, I meant that derp.h and derp.cpp would become the object derp.o upon compilation, which would then be used in bidoof.cpp, which has the main function.  But you're right, this is quite new to me, and I have a lot to read and try. 

I'm actually converting a working C program into a C++ program (with limitations, like hurr.c) and needed to use objects.  Was pretty blind to the compiler before, but this thread has made me investigate it further.  Pretty much a homework problem, but I didn't want to just ask you guys to write everything for me.  I still have more to do but this was one step forward.