introduction to programming -...

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Introduction to Programming – Lecture 12

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Chair of Software Engineering

Introduction to Programming

Bertrand Meyer

Last revised 24 November 2004


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Lecture 12:The dynamic model


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Changing values: assignment

target := source

source is an expression:

Call to a query:positionupper_left.position

Arithmetic or boolean expression:a + (b * c)(a < b) and (c = d)

target may be: An attributeResult in a functionA “local variable” of a routine (not yet seen)

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Assignment

Replaces a value by another

x

y

2

0p.set (2, 1)

0

1

p


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Setting fields (in routines of the class)

classPOSITION

feature – Access

x: REAL-- Horizontal position

y: REAL-- Vertical position

feature – Element change

set (xval, yval: REAL) is-- Set coordinates to (`xval', `yval').

requirex_positive: xval >= 0y_positive: yval >= 0

do

x := xvaly := yval

ensurex_set: x = xvaly_set: y = yval

endend


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Do not confuse assignment with equality

x := y

if x = y then...

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Effect of an assignment

Reference types: reference assignmentExpanded types: value copy

3class TWO_VALUES feature

item: INTEGERright: TWO_VALUESset (n: INTEGER; r: TWO_VALUES) is

-- Reset both fieldsdo

item := nright := r

endend

t: TWO_VALUES

...

create t

...

t.set (25, Void)

item := nright := r


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Assignment

class METRO_STATION featurelocation: POSITIONname: STRINGlength: REAL

set_all (p: POSITION; l: REAL; n: STRING) isdo

location := plength := lname := n

endend


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A linked list of strings

Haldenegg

item right

Central

item right

Haupt-bahnhof

item right

(LINKABLE) (LINKABLE) (LINKABLE)

first_element

last_element

active

count 3

Parade-platz

item right

: inserting at the end

(LINKABLE)

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Inserting an item at the end

extend (v: STRING) is-- Add v to end.-- Do not move cursor.

localp: LINKABLE [STRING]

dop := new_cell (v)if is_empty then

first_element := pactive := p

elselast_element.put_right (p)if after then active := p end

endcount := count + 1

end


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LINKABLE cells

class LINKABLE featureitem: STRING

-- Value in this cell

right: LINKABLE-- Cell, if any, to which this one is chained

put_right (other: like Current) is-- Put other to the right of current cell.

doright := other

ensurechained: right = other

endend

Haldenegg

item right


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A linked list of strings

Haldenegg

item right

Central

item right

Haupt-bahnhof

item right

(LINKABLE) (LINKABLE) (LINKABLE)

first_element

last_element

active

count 3

Parade-platz

item right

: inserting at the end

(LINKABLE)

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Local variables (in routines)

A form of entity (they are also called “local entities”)

Just declare them on entry to a routine:

r (...) is-- Header comment

require...

localx: REALm: METRO_STATION

do... Can use x and m here ...

ensure...

end

Local variables include Result for a function


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Our “maximum” computation

highest_name: STRING is-- Alphabetically greatest station name of line

dofrom

fancy_line.start ; Result := ""until

fancy_line.afterloop

Result := greater (Result, fancy_line.item.name)

fancy_line.forthend

end


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Initialization

Automatic initialization rules:0 for numbers (integers, reals)“Null” character for charactersFalse for booleans

Void for references

These rules apply to:Fields (from class attributes), on object creationLocal variables, on start of routine execution

(includes Result)

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Is this OK?

In class METRO_STATION:

is_greater (m: METRO_STATION): BOOLEAN is-- Does current station -- have a higher name than m?

doif name > m.name then

Result := Trueend

end


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The trouble with reference assignment

A comfortable mode of reasoning:

-- Here SOME_PROPERTY holds of a

“Apply SOME_OPERATION to b”

-- Here SOME_PROPERTY still holds of a

This applies to “expanded” values, e.g. integers

-- Here P (a) holds

OP (b)

-- Here P (a) still holds of a


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Dynamic aliasing

a, b: LINKABLE [STRING]

create a....

b := a

a.put ("Haldenegg")

-- Here a.item has value "Haldenegg"

b.put ("Paradeplatz")

-- Here a.item has value ?????

Haldenegg

item right

a b

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On the other hand...

-- I heard that the boss’s cousin earns less-- than 50,000 francs a year

“Raise Caroline’s salary by 1 franc”

-- ?????

Metaphors:“The beautiful daughter of Leda”“Menelas’s spouse”“Paris’s lover”

= Helen of Troy


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Practical advice

Reference assignment is useful

It’s also potentially tricky

As much as possible, leave it to specialized libraries of general data structures


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Variants of assignment and copy

Reference assignment (a and b of reference types):

b := a

Object duplication (shallow): c := clone (a)

Object duplication (deep): d := deep_clone (a)

Also: shallow field-by-field copy (no new object is created):

e.copy (a)

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Shallow and deep cloningInitial situation:

Result of:

b := a

c := clone (a)

d := deep_clone (a)

“Almaviva”namelandlord

loved_one

O1

“Figaro”O2 “Susanna”O3

b

“Almaviva”O4

c

“Almaviva”O5

“Figaro”O6 “Susanna”O7

d

a


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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.


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Completing the inheritance structure

A B

D E

C

ANY

NONE

Inheritsfrom

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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.


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Setter procedures

set_attribute procedure, e.g.

set_temperature (u: REAL) is-- Set temperature value to u.

dotemperature := u

ensuretemperature_set: temperature = u

end

Client will use e.g. x.set_temperature (21.5).

Client cannot directly assign to attribute


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Other uses of a setter procedure

set_temperature (u: REAL) is-- Set temperature value to u.

requirenot_under_minimum: u >= -273not_above_maximum: u <= 2000

dotemperature := uupdate_database

ensuretemperature_set: temperature = u

end

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Information hiding

class A

feature

f ...g ...

feature {NONE}

h ...

feature {B, C}

j ...

feature {A, B, C}

k

end

In clients, with the declaration a1: A, we have:

a1.f, a1.g: valid in any client

a1.h: invalid anywhere (including in A’s own text).

a1.j: valid only in B, C and their descendants(not valid in A!)

a1.k: valid in B, C and their descendants, as well as in A and its descendants


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An example of selective export

LINKABLE exports its features to LINKED_LISTDoes not export them to the rest of the worldClients of LINKED_LIST don’t need to know about LINKABLE cells.

Haldenegg

item right

Central

item right

Haupt-bahnhof

item right

first_element

active

count 3


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Exporting selectively

classLINKABLE [G]

feature {LINKED_LIST}

put_right (...) is do ... end

right: G is do ... end

...end

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Objects and references

States of a reference:

Operations on references: create pp := qp := Voidif p = Void then ...

VOID ATTACHED

create p

p := q (where q is attached)

p := Void

p := q (where q is void)

p ATTACHED

p VOID


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What to do with unreachable objects

Reference assignmentsmay make some objects useless.

Two possible approaches:Manual “free” (C++).Automatic garbage collection (Eiffel, Oberon, Java, .NET)

“Almaviva”namelandlord

loved_one

aO1

“Figaro”O2

“Susanna”O3


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The C programmer’s view

Newsgroup posting by Ian Stephenson, 1993 (as cited in Object-Oriented Software Construction, 2nd

edition):

I say a big NO! Leaving an unreferenced object around is BAD PROGRAMMING. Object pointers ARE like ordinary pointers — if you allocate an object you should be responsible for it, and free it when its finished with. (Didn't your mother always tell you to put your toys away when you'd finished with them?)

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Arguments for automatic collection

Manual reclamation is dangerous for reliability.

Wrong “frees” are among the most difficult bugs to detect and correct.

Manual reclamation is tedious.

Modern garbage collectors have acceptable performance overhead.

GC is tunable: disabling, activation, parameterization....


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Properties of a garbage collector (GC)

Consistency (never reclaim a reachable object). Completeness (reclaim every unreachable object –eventually).

Consistency (also called safety) is an absolute requirement. Better no GC than an unsafe GC.

But: safe automatic garbage collection is hard in C-based languages.


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Two kinds of type

Reference types; value of an entity is a reference. Example:b: POINT

Expanded types; value of an entity is an object. Example:d: expanded POINT

b

(POINT)

d

(POINT)

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Expanded classes

A class may also be declared as expanded class C

... The rest as usual ...

Then you can declare:a: C

with the same effect asb: expanded C

in the earlier syntax (still permitted, with same meaning).


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Basic types as expanded classes

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

n: INTEGER


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The object-oriented form of call

some_target.some_feature (some_arguments)

For example:

Paris.displayLine6.extend (Station_Parade_Platz)

x := a.plus (b) ???????

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Infix and prefix operators

In

a − bthe − operator is “infix”

(written between operands)

In

− bthe − operator is “prefix”

(written before the operand)


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Infix and prefix operators

expanded classINTEGER

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).


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End of lecture 12

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