Einführung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer

Chair of Software Engineering

Einführung in die ProgrammierungIntroduction to Programming

Prof. Dr. Bertrand Meyer

Lecture 12: More on references and the dynamic model

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Purpose of this lectureFew really new concepts, but gain a better understanding of the tricky matter of references

Reminder on garbage collection and associated concepts

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AssignmentReplaces a value by another

x

y

2

0 p.set_coordinates (2, 1)

0

1

p

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Setting fields (in routines of the class)class

POSITIONfeature –– Access

x: REAL-- Horizontal position

y: REAL-- Vertical position

feature –– Element changeset_coordinates (xval, yval: REAL)

-- Set coordinates to (`xval', `yval').require

x_positive: xval >= 0y_positive: yval >= 0

do

ensurex_set: x = xvaly_set: y = yval

endend

x := xvaly := yval

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class LINKED_CELL feature item : INTEGER right : LINKED_CELL set_fields (n : INTEGER ; r : LINKED_CELL) -- Reset both fields. do item := n right := r endend

Effect of an assignmentReference types: reference assignmentExpanded types: value copy ite

mright

item := nright := r

t, u : LINKED_CELL

create t ; create u

t.set_fields (25, u)

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See LINKABLE in EiffelBase

t 0

u

t.set_fields (25, u)

25

0

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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 endextend (v : STRING)

-- Add v to end.-- Do not move cursor.

localp : LINKABLE [STRING]

docreate p.make (v)if is_empty then

first_element := pactive := p

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

endlast_element := pcount := count + 1

end

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Exercise (uses loops)Reverse a list!

Halden-eggitem right

Central

Haupt-bahnhof

last_element

first_element

count3

(LINKABLE)

(LINKED_LIST)

Parade-

platz

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Reversing a list

1 2 3 4 5

1 2 3 4 5

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Reversing a list

frompivot := first_elementfirst_element := Void

until pivot = Void loopi := first_elementfirst_element := pivotpivot := pivot.rightfirst_element.put_right (i )

end

first_element pivot

right

i

1 2 3 4 5

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Reversing a list



end

first_element pivot

right

i

1 2 3 4 5

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Reversing a list



end

first_element pivot

right

i

1 2 3 4 5

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Reversing a list



end

first_element pivot

right

i

1 2 3 4 5

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ii pivotpivot

Reversing a list



end

first_element

right

1 2 3 4 5

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The loop invariant



end

first_element pivot

right

i

1 2 3 4 5

Invariant: from first_elementfollowing right, initial itemsin inverse order; from pivot,rest of items in original order

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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....

a.put ("Haldenegg")

b := a

-- 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 := a.twin

Object duplication (deep): d := a.deep_twin

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 := a.twin

d := a.deep_twin

“Almaviva”namelandlord

loved_one

O1

“Figaro”O2 “Susanna”O3

b

“Almaviva”O4c

“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.

Needs to be improved,

see “ “object test”

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Objects and references

States of a reference:

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

VOID ATTACHED

create pp := q (where q is attached)

p := Voidp := q (where q is void)

p ATTACHED

p VOID

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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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Operator features

expanded class INTEGER feature

plus alias "+" (other : INTEGER): INTEGER-- Sum with other

do ... endtimes alias “*“ (other : INTEGER): INTEGER

-- Product by otherdo ... end

minus alias “-" : INTEGER-- Unary minus

do ... end...

end

Calls such as i.plus (j) can now be written i + j

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Reading assignment for next weekChapters on

Syntax (11) Inheritance (16)

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What we have seen

Playing with references: list reversalDynamic aliasing and the difficulties of

pointers & referencesOverall inheritance structureCopy, clone and storage operationsPersistence closureInfix & prefix operators

Einführung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer

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