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

Einführung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer. Lecture 12: More on references and the dynamic model. Purpose of this lecture. Few really new concepts, but gain a better understanding of the tricky matter of references

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

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  1. Einführung in die ProgrammierungIntroduction to ProgrammingProf. Dr. Bertrand Meyer Lecture 12: More on references and the dynamic model

  2. Purpose of this lecture • Few really new concepts, but gain a better understanding of the tricky matter of references • Reminder on garbage collection and associated concepts

  3. p Assignment • Replaces a value by another x 0 2 p.set_coordinates(2, 1) y 1 0

  4. Setting fields (in routines of the class) • class • POSITION • feature–– Access • x: REAL • -- Horizontal position • y: REAL • -- Vertical position • feature–– Element change • set_coordinates(xval, yval: REAL) • -- Set coordinates to(`xval', `yval'). • require • x_positive: xval >= 0 • y_positive: yval >= 0 • do • ensure • x_set: x = xval • y_set: y = yval • end • end x :=xval y :=yval

  5. Effect of an assignment SeeLINKABLE in EiffelBase • Reference types: reference assignment • Expanded types: value copy item right 3 class LINKED_CELL feature item: INTEGER right:LINKED_CELL set_fields(n: INTEGER ; r: LINKED_CELL) -- Reset both fields. do item := n right := r end end  t 0 25 u 0 item := n right := r t, u: LINKED_CELL create t ;create u t.set_fields (25, u) t.set_fields (25, u)

  6. : inserting at the end A linked list of strings 3 4 count right item last_element first_element Parade-platz active (LINKABLE) Haupt-bahnhof Haldenegg Central right item right right item item (LINKABLE) (LINKABLE) (LINKABLE)

  7. Inserting an item at the end • extend (v: STRING) • -- Add v to end. • -- Do not move cursor. • local • p: LINKABLE [STRING] • do • create p.make (v) • if is_empty then • first_element := p • active := p • else • last_element.put_right (p) • if after then active := p end • end • last_element := p • count := count + 1 • end

  8. Exercise (uses loops) • Reverse a list! (LINKED_LIST) 3 count last_element first_element Haupt-bahnhof Parade-platz Halden-egg Central item right (LINKABLE)

  9. Reversing a list 1 2 3 4 5 1 2 3 4 5

  10. Reversing a list • from • pivot := first_elementfirst_element:=Void • untilpivot = Voidloop • i :=first_element • first_element:= pivot • pivot := pivot.right • first_element.put_right(i) • end 1 2 3 4 5 right first_element pivot i

  11. Reversing a list • from • pivot := first_elementfirst_element := Void • untilpivot = Voidloop • i :=first_element • first_element:= pivot • pivot := pivot.right • first_element.put_right(i) • end 1 2 3 4 5 right first_element pivot i

  12. Reversing a list • from • pivot := first_elementfirst_element := Void • untilpivot = Voidloop • i :=first_element • first_element:= pivot • pivot := pivot.right • first_element.put_right(i) • end 1 2 3 4 5 right first_element pivot i

  13. Reversing a list • from • pivot := first_elementfirst_element := Void • untilpivot = Voidloop • i :=first_element • first_element:= pivot • pivot := pivot.right • first_element.put_right(i) • end 1 2 3 4 5 right first_element pivot i

  14. Reversing a list • from • pivot := first_elementfirst_element := Void • untilpivot = Voidloop • i :=first_element • first_element:= pivot • pivot := pivot.right • first_element.put_right(i) • end 1 2 3 4 5 right pivot first_element pivot i i

  15. The loop invariant • from • pivot := first_elementfirst_element := Void • untilpivot = Voidloop • i :=first_element • first_element:= pivot • pivot := pivot.right • first_element.put_right(i) • end 1 2 3 4 5 right first_element pivot i Invariant: from first_elementfollowing right, initial itemsin inverse order; from pivot,rest of items in original order

  16. The trouble with reference assignment • A comfortable mode of reasoning: -- HereSOME_PROPERTYholds of a “Apply SOME_OPERATIONto b” -- Here SOME_PROPERTYstill holds ofa • This applies to “expanded” values, e.g. integers -- HereP (a) holds OP (b) -- HereP (a) still holds ofa

  17. Dynamic aliasing • a, b: LINKABLE [STRING] • create a.... • a.put("Haldenegg")‏ • b := a • -- Herea.itemhas value"Haldenegg" • b.put("Paradeplatz")‏ • -- Herea.itemhas value ????? b a Haldenegg right item

  18. 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

  19. Practical advice • Reference assignment is useful • It’s also potentially tricky • As much as possible, leave it to specialized libraries of general data structures

  20. 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)‏

  21. Shallow and deep cloning a • Initial situation: • Result of: b:=a c := a.twin • d := a.deep_twin O1 name “Almaviva” landlord loved_one O3 O2 “Figaro” “Susanna” b O4 “Almaviva” c d “Almaviva” O5 O7 O6 “Figaro” “Susanna”

  22. 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.

  23. B C D E Completing the inheritance structure ANY A Inheritsfrom NONE

  24. A related mechanism: Persistence Needs to be improved, see “ “object test” a.store (file)‏ .... b := retrieved(file) • Storage is automatic. • Persistent objects identified individually by keys. • These features come from the library class STORABLE.

  25. Objects and references • States of a reference: • Operations on references: createp p := q p := Void ifp = Voidthen... createp p := q(where q is attached)‏ p ATTACHED VOID ATTACHED p VOID p := Void p := q(where q is void)‏

  26. The object-oriented form of call • some_target.some_feature (some_arguments)‏ • For example: • Paris.display • Line6.extend (Station_Parade_Platz)‏ • x := a.plus (b)???????

  27. Infix and prefix operators • In • a − b • the − operator is “infix” (written between operands)‏ • In • − b • the − operator is “prefix” (written before the operand)‏

  28. Operator features • expanded class INTEGERfeature • plus alias"+" (other : INTEGER): INTEGER • -- Sum with other • do ... endtimes alias“*“ (other : INTEGER): INTEGER • -- Product by other • do ... endminus alias“-" : INTEGER • -- Unary minus • do ... end • ... • end • Calls such as i.plus (j) can now be written i+ j

  29. Reading assignment for next week • Chapters on • Syntax (11) • Inheritance (16)

  30. Playing with references: list reversal • Dynamic aliasing and thedifficulties of pointers &references • Overall inheritance structure • Copy, clone and storage operations • Persistence closure • Infix & prefix operators What we have seen

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