CSC/ECE 517 Fall 2007/wiki3 7 cl

Bertrand Meyer is a prominent author in o-o design. He has developed a set of principles, many of which are embodied in the Eiffel language. Consider the principles of small interfaces, explicit interfaces, the uniform-access principle, the self-documentation principle, and the single-choice principle. What are good examples of each? Do other languages besides Eiffel support them? Is it difficult to follow these principles in certain o-o languages?

Bertrand Meyer's Set of Principles

Bertrand Meyer is the developer of the Eiffel programming language. As a developer, Dr. Meyer participates in the evolution of the Eiffel method and language which includes a set of principles that he developed. Some of the more important principles are the principle of small interfaces, explicit interfaces, the uniform-access principle, the self-documentation principle, and the single-choice principle.

Principle of Small Interfaces

The principle of small interfaces, also known as weak coupling, states that if any two modules communicate at all, they should exchange as little information as possible.

Principle of Explicit Interfaces

The principle of explicit interfaces states that whenever two modules A and B communicate, this must be obvious from the text of A or B or both.

Example of Principle of Explicit Interfaces

Uniform-access Principle

The uniform-access principle states that all services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation.

Example of Uniform-access Principle

Ruby Programming language:

class Foo
 attr_reader :x
 def initialize(x)
   @x = x
 end
 def x_times_5
   return @x*5
 end
end
y = Foo.new(2)
puts y.x
puts y.x_times_5

This code outputs:

2
10

Note: Even though x is an attribute and x_times_5 is a parameterless method call, they're accessed the same way.

Self-documentation Principle

The designer of a module should strive to make all information about the module part of the module itself.

Single-choice Principle

The single-choice principle states that whenever a software system must support a set of alternatives, one and only one module in the system should know their exhaustive list.

Resources

Bertrand Meyer Wiki
List of Principles
Uniform-Access Principle
Single-Choice Principle