Thanks by all the conversation, I'm learning through this too 
But could you explain just one more thing? If I write
void assignToBoard(analogMultiplexer &obj2) {
it compiles. The analogMultiplexer class is outside, in a .h file. Is this the problem? I can't have classes in the same page of the code? Why? I remember that I can do it in a general c++ code.
Thank you!