Language basics Timeline Eiffel 11986 1988 (OOSC 1 st edition) - - PowerPoint PPT Presentation

• 3 -

Language basics

Timeline

Eiffel 11986 Eiffel 2 1988 (OOSC 1st edition) Eiffel 3 1990-1991 (Eiffel: The Language) Eiffel 4 1997 (“Precursor” mechanism, agents) ECMA 2005 ECMA/ISO 2006 (revised) http://www.ecma- international.org/publications/standards/ Ecma-367.htm See roadmap: http://eiffelsoftware.origo.ethz.ch/index.php/Language_road_map

Eiffel mechanisms

Classes, objects, ... Single and multiple inheritance Inheritance facilities: redefinition, undefinition, renaming Genericity, constrained and unconstrained Safe covariance Disciplined exception handling, based on principles of Design by Contract Full GC Agents (power of functional programming in O-O!) Unrestricted streaming: files, databases, networks...

What is not in Eiffel

• Goto
• Functions as arguments
• Pointer arithmetic
• Special increment syntax, e.g. x++, ++x
• In-class feature overloading

Syntax conventions

Semicolon used as a separator (not terminator) It’s optional almost all the time. Just forget about it! Style rules are an important part of Eiffel: Every feature should have a header comment Class should have an indexing clause Layout, indentation Choice of names for classes and features

Kinds of feature

Command Query Feature

Function

No result

Feature

Memory Computation

Client view (specification) Internal view (implementation)

Returns result

Attribute Procedure

Memory Computation

Routine Feature Feature

Uniform access principle

A call such as my_account. balance could use an attribute or a function

It doesn‘t matter to the client whether you look up or compute

Uniform Access

balance = deposits.total – withdrawals.total

deposits withdrawals balance deposits withdrawals

(A1) (A2)

Attributes (fields/ data members)

bounding_rectangle: RECTANGLE

• - Smallest rectangle including whole of current figure

Attributes with contracts (ECMA)

bounding_rectangle: RECTANGLE

• - Smallest rectangle including whole of current figure

require bounded attribute ensure Result.height = height Result.width = width Result.lower_left = lower_left Result.contains (Current) end

Self-initializing attributes (ECMA)

bounding_rectangle: FIGURE

• - Smallest rectangle including whole of current figure
• - (Computed only if needed)

require bounded attribute create Result.set (lower_left, width, height) ensure Result.height = height Result.width = width Result.lower_left = lower_left Result.contains (Current) end

… As before …

Another example: once per object

class STOCKS feature db: DATABASE history (ts: TICKER_SYMBOL): HISTORY

• - List of previous valuations of ts

attribute if {h: HISTORY} db.retrieved (ts) then Result := h else create Result

• - Produces empty list

end end end

A simple class

class POINT feature x, y: REAL

• - Point cartesian coordinates

move (a, b: REAL) is

• - Move by a horizontally and by b vertically.

do x := x + a y := y + b end scale (factor: REAL) is

• - Scale by factor.

do x := factor * x y := factor * y end

Class POINT (cont’d)

distance (p: POINT): REAL is

• - Distance to p

do Result := sqrt ((x – p.x)^2 + (y – p.y)^2) end ro: REAL is

• - Distance to origin (0, 0)

do Result := sqrt (x^2 + y^2) end theta: REAL is

• - Angle to horizontal axis

do … end end

Use of the class in a client (1/ 5)

class GRAPHICS feature p, q: POINT

• - Graphic points

… some_routine is

• - Use p and q.

local u, v: REAL do

• - Creation instructions

create p create q end end

0.0 0.0 p

(POINT)

0.0 0.0 q

(POINT)

Use of the class in a client (2/ 5)

class GRAPHICS feature p, q: POINT

• - Graphic points

… some_routine is

• - Use p and q.

local u, v: REAL do

• - Creation instructions

create p create q p.move (4.0, -2.0)

• - Compare with Pascal, C, Ada:
• - Move (p, 4.0, -2.0)

end end

4.0

• 2.0

p

(POINT)

0.0 0.0 q

(POINT)

Use of the class in a client (3/ 5)

class GRAPHICS feature p, q: POINT

• - Graphic points

… some_routine is

• - Use p and q.

local u, v: REAL do

• - Creation instructions

create p create q p.move (4.0, -2.0)

• - Compare with Pascal, C, Ada:
• - Move (p, 4.0, -2.0)

p.scale (0.5) end end

2.0

• 1.0

p

(POINT)

0.0 0.0 q

(POINT)

Use of the class in a client (4/ 5)

class GRAPHICS feature p, q: POINT

• - Graphic points

… some_routine is

• - Use p and q.

local u, v: REAL do

• - Creation instructions

create p create q p.move (4.0, -2.0)

• - Compare with Pascal, C, Ada:
• - Move (p, 4.0, -2.0)

p.scale (0.5) u := p.distance (q) v := p.x p := q end end

2.0

• 1.0

p

(POINT)

0.0 0.0 q

(POINT)

Use of the class in a client (5/ 5)

class GRAPHICS feature p, q: POINT

• - Graphic points

… some_routine is

• - Use p and q.

local u, v: REAL do

• - Creation instructions

create p create q p.move (4.0, -2.0)

• - Compare with Pascal, C, Ada:
• - Move (p, 4.0, -2.0)

p.scale (0.5) u := p.distance (q) v := p.x p := q p.scale (-3.0) end end

2.0

• 1.0

p

(POINT)

0.0 0.0 q

(POINT)

Example class

class SECURITY feature company_name: STRING nyse_ticker: STRING company: COMPANY traded: EXCHANGE latest_valuation: INTEGER public: BOOLEAN ... Other attributes and routines ... end

Creating an object

With the class SECURITY as given: my_security: SECURITY ...

create my_security

Effect of such a creation instruction: Allocate new object of the type declared for my_security. Initialize its fields to default values (0 for numbers, false for booleans, null for characters, void for references). Attach it to the instruction’s target, here my_security.

Specific creation procedures

class SECURITY create make_by_name, make_by_ticker, make_by_company feature -- Initialization make_by_name (n: STRING)

• - Initialize to company of name n.

do company_name := n ... end make_by_ticker (n: STRING) do ... end make_by_company (c: COMPANY) do ... end feature ... The rest as before ...

If there is a creation clause

Creation instructions must be “creation calls”, such as create my_security.make_by_name ("Microsoft") create my_security.make_by_ticker ("MSFT") create my_security.make_by_company (Microsoft_company)

If there is no creation clause

An absent creation clause, as in class SECURITY

• - No creation clause

feature … The rest as before … end is understood as one that would only list default_create, as in class SECURITY create default_create feature … The rest as before … end Procedure default_create is defined in ANY as doing nothing; any class can redefine it to provide proper default initializations.

To allow both forms

To make both forms valid: create my_security as well as create my_security.make_by_ticker ("MSFT") it suffices to make default_create (redefined or not) one of the creation procedures: class SECURITY create make_by_ticker, ..., default_create feature ... The rest as before ...

Associated convention

The notation

create x

is understood (if permitted) as an abbreviation for

create x.default_create

To prohibit instantiating a class

class NOT_CREATABLE create

• - Nothing here!

feature

... The rest as before ...

end

Creating an object of a specific type

If D is a descendant of A: a1: A … create {D} a1… Avoids local variables, as in d1: D … create d1…. a1 := d1 Particularly useful for factories

Types

Reference types; value of an entity is a reference Example: b: SECURITY Expanded types; value of an entity is an object. Example: d: expanded SECURITY

b

(SECURITY)

d

(SECURITY)

Expanded classes

You may declare a class as expanded class C ... The rest as usual ...

Subobjects

Expanded classes and entities support the notion of subobject

class RECTANGLE_R feature corner1, corner2, corner3, corner4: POINT ... end class RECTANGLE_E feature corner1, corner2, corner3, corner4: POINT_E … end

The meaning of expanded classes More than implementation notion: a system modeling tool. Two forms of client relation: Simple client Expanded client What is the difference between these two statements? A stock has a trading exchange A stock has an analyst valuation (Compare: a car has an originating factory and an engine)

Basic types as expanded classes

expanded class INTEGER ... expanded class BOOLEAN ... expanded class CHARACTER ... expanded class REAL ... expanded class DOUBLE ... n: INTEGER

Semantics of expanded types (ECMA)

An object has either reference or copy semantics

Infix and prefix operators (Eiffel 3)

expanded class INTEGER feature infix "+" (other: INTEGER): INTEGER is

• - Sum with other

do ... end infix "*" (other: INTEGER): INTEGER is

• - Product by other

do ... end prefix "–": INTEGER is

• - Unary minus

do ... end ... end Calls are then of the form i + j rather than i.plus (j).

Infix and prefix operators (ECMA)

expanded class INTEGER feature plus alias "+" (other: INTEGER): INTEGER is

• - Sum with other

do ... end multiplied alias "*" (other: INTEGER): INTEGER is

• - Product by other

do ... end minus alias "–": INTEGER is

• - Unary minus

do ... end ... end Calls are then of the form i + j rather than i.plus (j).

Effect of an assignment

Reference types: reference assignment Expanded types: value copy

3 class TWO_VALUES feature item: INTEGER right: TWO_VALUES set (n: INTEGER; r: TWO_VALUES) is

• - Reset both fields

do item := n right := r end end t: TWO_VALUES ... create t ... t.set (25, Void)

item := n right := r

Initial situation: Result of:

• b := a
• c := cloned (a)
• d := deep_cloned (a)

“Almaviva” name landlord loved_one a O1 “Figaro” O2 “Susanna” O3 b “Almaviva” O4 c “Almaviva” name landlord loved_one O5 “Figaro” O6 “Susanna” O7 d

Shallow and deep cloning

Where do these mechanisms come from? Class ANY in the Eiffel “Kernel Library” Every class that doesn’t explicitly inherit from another is considered to inherit from ANY As a result, every class is a descendant of ANY.

A related mechanism: Persistence

a.store (file) .... b ?= retrieved (file) Storage is automatic. Persistent objects identified individually by keys. These features come from the library class STORABLE.

The module-type merge

A class is both: A module A type Much of the conceptual power of the method comes from the fusion

• f these two notions.

From the module viewpoint: Set of available services (“features”). From the type viewpoint: Description of set of possible run-time objects (its instances). Connection: The services of the class, viewed as a module, are the

• perations applicable to the instances of the class, viewed as a

type.