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Usually, it is a good idea to draw a class-hierarchy diagram, such as the following, to see how your classes fit together. All arrows represent subclass-to-superclass relations:
*-----------------------*
| container |
| percent_loaded |
*-----------------------*
^ ^
| |
*--------* *--------*
| |
*-------------------* *-------------------* *--------------------*
| box | | cylinder | | railroad_car |
| volume function | | volume function | | age function |
| height | | radius | | year_built |
| width | | length | | |
| length | | | | |
*-------------------* *-------------------* *--------------------*
^ ^ ^ ^ ^ ^
| | | | | |
| *----------------------------------------* | | |
| | | *---------------------* | |
| | | | *----------* |
| | | | | |
*----------* *----------* *----------* *----------*
| box_car | | tank_car | | engine | | caboose |
*----------* *----------* *----------* *----------*
Such a class-hierarchy diagram helps you to see how to distribute member variables and member functions among the classes in the hierarchy. Such a class-hierarchy diagram can also expose semantic risks of the sort described in Segment 224.