Object graph  In an object-oriented program, groups of objects
form a network through their relationships with each
other—either through a direct reference to another object or
through a chain of intermediate references. These groups of
objects are referred to as object graphs. Object graphs may be
small or large, simple or complex. An array object that contains
a single string object represents a small, simple object graph.
A group of objects containing an application object, with
references to the windows, menus and their views, and other
supporting objects, may represent a large, complex object graph.
Sometimes you may want to convert an object graph—usually just a
section of the full object graph in the application—into a form
that can be saved to a file or transmitted to another process or
machine and then reconstructed. This process is known as
archiving.  Some object graphs may be incomplete—these are often
referred to as partial object graphs. Partial object graphs have
placeholder objects that repre