Unique "logical" identifier for an object?

From: Stuart Leonard (suncds_at_earthlink.net)
Date: 04/19/04

Date: Mon, 19 Apr 2004 21:45:46 GMT

I have a collection of "Invoice" objects within a vector. I need to provide
a "logical key" to each "Invoice" object, which consists of a vendor number
and an invoice number, which are both Strings. I need place a logical key
within each "Invoice" object, so that I can retreive the objects later in
the program by this "key".
This seems like a classic problem. Can anyone provide any ideas of how to
accomplish this?