Tell Less, Say More: The Power of Implicitness

W hen we are asked to describe something, our natural impulse is to reach for an exhaustive definition. If what we are asked to describe is a software concept—and if we are using a programming language— we will usually start writing a record, structure, or class definition, and we won't come back until we have all the properties in place. A bank account is the combination of the account holder's name, a current balance amount, an account number , an allowed minimum balance, and so on; a paragraph (in a text processing system) is the combination of a list of words, a default font, a justification rule, and so on. But we won't feel comfortable until we have removed every single " and so on " and filled in all the details. How does object technology affect this? The first part of the answer is well known. It's that the properties used in the description are no longer confined to attributes or data members, indicating what physically constitutes an object: Now they also include routines or methods , indicating applicable operations. So we will not just have a record or structure declaration, which in a Pascal-like language would read type ACCOUNT = record holder: PERSON; ... Other attribute declarations ... end but will obtain a true class by adding the routines, as in the following Eiffel form: class ACCOUNT feature holder: PERSON change_holder (new_holder: PERSON) is do .. end ... Other attribute and routine declarations end This example illustrates the principal and well-known difference, which also defines why a C++ class is not the same thing as a C structure. But there is another difference, no less important even if it is less obvious. The first form is explicit; the second is implicit. Implicitness here means that we do not claim to have listed all the properties— or care. In any class declaration there remains a possible " and so on. " We accept that the description may be incomplete, yet we can use it as it is. This is the open-closed principle, which in my opinion is one of the central innovations of object technology: the ability to use a software component as it is, while retaining the possibility of adding to it later through inheritance. Unlike the records or structures of other approaches, a class of object technology is both closed and open: closed because we can start …