On Saturday, June 23rd, Damian Conway had a little free-for-all workshop that he gave at College of DuPage in Wheaton, IL. Although the whole day was fascinating, the most useful part for me was his discussion of ``Ten criteria for knowing when to use object-oriented design''. Apparently, Damian was once a member of Spinal Tap, because his list goes to eleven.
Damian said that this list, in expanded form, is going to be part of the standard Perl distribution soon.
- Design is large, or is likely to become large
- When data is aggregated into obvious structures, especially if there's a lot of data in each aggregate
For instance, an IP address is not a good candidate: There's only 4 bytes of information related to an IP address. An immigrant going through customs has a lot of data related to him, such as name, country of origin, luggage carried, destination, etc.
- When types of data form a natural hierarchy that lets us use
Inheritance is one of the most powerful feature of OO, and the ability to use it is a flag.
- When operations on data varies on data type
GIFs and JPGs might have their cropping done differently, even though they're both graphics.
- When it's likely you'll have to add data types later
OO gives you the room to expand in the future.
- When interactions between data is best shown by operators
Some relations are best shown by using operators, which can be overloaded.
- When implementation of components is likely to change, especially in the same program
- When the system design is already object-oriented
- When huge numbers of clients use your code
If your code will be distributed to others who will use it, a standard interface will make maintenence and safety easier.
- When you have a piece of data on which many different operations
Graphics images, for instance, might be blurred, cropped, rotated, and adjusted.
- When the kinds of operations have standard names (check, process, etc)
Objects allow you to have a DB::check, ISBN::check, Shape::check, etc without having conflicts between the types of check.