I'm currently studying for a CS course's final exam and I've run into a minor (maybe major?) issue regarding the syntax of C++ #ifndef.

I've looked at the syntax for #infndef when using it as an #include guard, and most on the web seem to say:

    #ifndef HEADER_H
    #include "header.h"
    ...
    #endif

But my class's tutorial slides show examples as:

    #ifndef __HEADER_H__
    #include "header.h"
    ...
    #endif

I was wondering what (if any) the difference was between the two. The exam will most likely ask me to write an #include guard, and I know conventional wisdom is to just go with what the prof / tutor says, but if there's a difference during compilation I'd like to know.

Thanks all!