The "key" struct is before the
void assignToBoard(key &obj1, analogMultiplexer &obj2) {
which YOU put in, but it is AFTER the
void assignToBoard(key &obj1, analogMultiplexer &obj2);
that the IDE puts in.
When you compile your code the IDE generates a .cpp file in your "build" folder. Examine that folder and you'll see the .cpp file. Look at that .cpp file and all will become clear.