225: Mainline

Although you must be careful when you make assertions such as a box car is a box, such assertions are often expedient, especially if the word is can be taken to mean usefully can be viewed as.

For example, the following are plausible reasons why you might want to view a box car as a box and a tank car as a cylinder:

• You may have various sorts of trucks or storage tanks that make use of the same member variables and member functions as do box cars and tank cars. By collecting those member variables and member functions into box and cylinder classes, you avoid needless duplication and simplify maintenance.
• You may already have defined fully debugged box and cylinder classes. To prevent the gratuitous introduction of bugs, you would want to use those class definitions as they stand, rather than, say, copying bits of those box and cylinder class definitions into the box_car and tank_car class definitions.
• You may have decided to purchase code for box and cylinder classes from a vendor, because you anticipate using the elaborate box and cylinder capabilities advertised by that vendor. Because the vendor supplies you with compiled code only, you cannot access the source code, so you cannot copy bits of the vendor's box and cylinder class definitions into your box_car and tank_car class definitions. You can only define new classes that use those compiled classes as base classes.