The C++ Queue is a container adapter that gives the programmer a FIFO (first- in, first-out) data structure.
| Constructors | construct a new queue | 
| back | returns a reference to last element of a queue | 
| empty | true if the queue has no elements | 
| front | returns a reference to the first element of a queue | 
| pop | removes the first element of a queue | 
| push | adds an element to the end of the queue | 
| size | returns the number of items in the queue |