OO设计五个原则.pdf

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The Open-Closed Principle

This is the first of my

Engineering Notebook

that will appear in this column will focus on the use of C++ and OOD, and will address

issues of software engineering. I will strive for articles that are pragmatic and directly use-

ful to the software engineer in the trenches. In these articles I will make use of Booch’s

notation for documenting object oriented designs. The sidebar provides a brief lexicon of

Booch’s notation.

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coined the now famous open-closed principle. To paraphrase him:

ENTITIES

(

CLASSES

,

MODULES

ETC

.)

BE

FOR

EXTENSION

,

BUT

modules that

never change

. When requirements change, you extend the behavior of such

modules by adding new code, not by changing old code that already works.

Modules that conform to the open-closed principle have two primary attributes.

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extend the behavior of a module is to make changes to that module. A module that cannot

be changed is normally thought to have a fixed behavior. How can these two opposing

attributes be resolved?

In C++, using the principles of object oriented design, it is possible to create abstractions

that are fixed and yet represent an unbounded group of possible behaviors. The abstrac-

sented by all the possible derivative classes. It is possible for a module to manipulate an

functions. the

class

will be using objects of the derivative

class. If we want

objects to use a

different server class, then a new derivative of the

class can be cre-

ated. The

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we might solve this problem as shown in Listing 1. Here we see a set of data structures

that have the same first element, but are different beyond that. The first element of each is

a type code that identifies the data structure as either a circle or a square. The function

code and then calling the appropriate function (either

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tions all over the application; each one doing something a little different. Adding a new

shape to such an application means hunting for every place that such

(or

unlikely that all the

tured as the one in

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statements would be combined with logical operators, or that the

clauses of the

Thus the problem of finding and understanding all the places where the new shape needs

to be added can be non-trivial.

the open-closed principle. In this case an abstract

class is created. This abstract

class has a single pure-virtual function called

derivatives of the