It is not the sign underscore... It is just used when you can't or don't want to see the keyboard to pass your hand over and feel it. It automatically tells your feelings that this is number 5 which is in the middle of the keyboard... [edit]It is something like F and J on some keyboards that have the same symbol so you can easily know where your pointer (finger pointer :P) must be placed... But like firedraco says it has nothing to do with C++...
key number 5 and the '_' sign?????