@hobovsky
I have defined that operator for my type and it still does not work.

Edit: it did not work because I had other mistakes in my code (mainly to do with const qualifiers of functions). Once my code started working I removed the operator<< definition and started getting errors connected it. Thank you for the suggestion.

One of a few people who made this function accept the vector by reference instead of by copy.

I should have noticed that.
My bad.

Edit: done.
Thanks for the reply!

You have to respect the order the clients come in.

Can Vasya sell a ticket to every person and give change if he initially has no money and sells the tickets strictly in the order people queue?