Eiffel

Eiffel

Naeem EsfahaniUniversity of Tehran

"Man cannot discover new oceans unless he has the courage to lose sight of the shore." -- Andre Gide

I prefer to have as little as possible to do with Bertrand Meyer

-- Bjarne Stroustrup, 1989

Overview

• Introduction & History

• Design By Contract™

• Elements

• Concepts

• Syntax and Semantics

Introduction & History

Bertrand Meyer

• BS from Ecole polythecnique

• MS from Stanford University

• PhD from Univesite du Nancy

• Professor of Software Engineering at ETH in Zurich 2001

Bertrand Meyer

• Nine years in a large company

• Three years on the faculty at the University of California, Santa Barbara

• computer languages, object-oriented system design, architectural reviews, technology assessment

History

• Designed at Eiffel Software in 1985 as an internal tool

• A modern, OO environment integrating the concepts of modern software engineering

• There was simply nothing available

History

• First demonstrated in public at the first OOPSLA conference in October of 1986

• Release it as a commercial product at the end of 1986

• In 1988 in a book by Bertrand Meyer

• Version 2.3, released in the Summer of 1990

History

• Eiffel 3, was written entirely in Eiffel– Melting Ice Technology for fast recompilation – Fully graphical environment– Considerable advances in libraries– Optimization of the generated code

• The latest milestones is:

Eiffel 5 and EiffelStudio

Application

• Leading to a set of successful industrial projects in the US, Canada, Europe and the Far East

• Adoption by numerous universities as the primary teaching language

Where the Name does come from?

• Homage to Gustave Eiffel, the man who built the eponymous Tower in Paris

• The Eiffel Tower was completed on time and within budget

• Small number of robust, elegant design patterns, combined and varied repeatedly to yield a powerful, efficient structure

Where the Name does come from?

• The Eiffel Tower was initially conceived as a temporary structure and it was able to endure far beyond its original goals

• What better symbol could there be of the engineering principles behind Eiffel?

Design By Contract™

Assertions

• A boolean expression that is evaluated when it is reached during execution

• If true, execution continues

• else, execution may halt or an exception

• Eiffel has support for writing assertion

Assertions

Preconditions:When a routine is called

Postconditions:When a routine returns

General assertions:When execution reaches them

Class invariants:Maintained by all instances of a class

Pre and postconditions

• Preconditions and postconditions are associated with routines of a class

Pre and postconditions

• The preconditions must be true when the routine is called

• Postconditions must have been established when the routine terminates

Class Invariant

• Conditions that must be true for all objects of the class at ‘stable’ times

• When method starts to execute:– Its preconditions are met and the class invariant is

satisfied

• After execution:– The postcondition must be met and the class

invariant must be satisfied

Class Invariant

Elements

Classes

• The basic and only construct in Eiffel

• The attributes of entities and the operations that entities can perform

• A class represents all objects of that type

Libraries

• Classes that frequently appear– Reuse

• EiffelBase: – INTEGER, REAL, STRING, LISTs, ARRAYs

• Libraries are not defined as part of the Eiffel language

Features

• Each class has a set of features which represent the attributes and routines of a class

• routines are either procedures or functions; attributes can be fields, constants or functions

Features

• Procedures (Commands) alter the state of the object

• Functions (Queries) are return an answer to a query about the state of an object

• This is by convention, rather than being enforced in Eiffel

Concepts

Inheritance

• Build new classes out of existing classes, and to reuse features already defined in those classes, you use inheritance

• Eiffel has multiple inheritance– solves clashing by having a rename clause– Redefinition

• Inheritance is one of the fundamental mechanisms for reuse

• Polymorphism

Class Syntax

Genericity

• You can reuse like inheritance

• Genericity is important in making programs type safe without resorting to type casts

• constrained genericity– allows generic parameters to be constrained

• allow you to write general algorithmic patterns

Objects

• A name in an Eiffel program is declared as having a type. The declaration

x : T

• Still does not refer to any objects: x is not bound to an object

• Expressed as the object void. We can see if an object is bound to void by writing x=void

• Name x can be bound to any object that type conforms to type T

Objects

• Three ways to achieve this binding:

Assignment instruction: The assignment x:=y binds x to the object to which y is currently bound

A creation instruction: The creation instruction !!x creates a new object of type T,and binds x to it. This is similar to new in C++.

Routine call

Objects

c: C!! c.make• An object of type C is created and attached to

the reference c

!D! c.make• An object of type D, where D conforms to C is

created and attached to c.

• If you have a creation routine declared for the class, the creation routine must be called like constructor

Objects

• Creation routines can be called as normal routines

• Note that the !! syntax is somewhat cryptic, and a recent change to the language has changed this for a create command keyword

• Eiffel has no delete operator. This is because, as with Java, Eiffel is garbage collected

Etc

• Class variables: variables which do not have one copy per class of objects. The Eiffel equivalent for doing this is once routines

• Eiffel does not have...– Gotos are not needed, as the Eiffel style is to write small

routines– Global variables are a sign of poor structuring use once

routines instead– Type casts to make up for a flawed type system– Pointers with their associated problems

• Code must be structured in classes

Syntax and Semantics

Some constructs

• Comments are introduced by --

• Grouped entities are terminated by the keyword end, no begin keyword

• No semicolons, optionally be placed between instructions

Style

• Keywords are shown in bold

• User named entities in italic

• Class names are given in uppercase

• Eiffel is not case sensitive, so it is up to the programmer to follow style conventions.

• More than one words separate the constituent words with underscore ‘_’

Access specifiers

• Any set of features introduced by the feature keyword can be exported to other specific classes

• ANY and NONE

• There is no strict equivalent to private, as Eiffel believes it is not sensible to restrict visibility in subclasses.

Access specifiers

Basic Types

• BOOLEAN not, or, and, implies, or else, and then

• INTEGER +, -, *, //, \\, ˆ, <, >, <=, >=

• REAL +, -, *, /, ˆ, <, >, <=, >=

Conditional

Iterations

• There is only one form of iteration in Eiffel

Procedures

Functions

• The value to be returnedresult := expression

Redefinition

Renaming

Counterclass

          COUNTER

feature -- Access item: INTEGER     -- Counter's value.

feature -- Element change increment is  -- Increase counter by one.

   do             item := item + 1   end

decrement is -- Decrease counter by one.   do             item := item - 1   end

reset is -- Reset counter to zero.   do             item := 0   end

end

Questions

If Time!

• Native C and C++

• Simplicity

• First release instead prototype

• Melting ice