OOP Class Lawrence D

1
OOP ClassLawrence DAntonio

• Lecture 4
• An Overview of C, Part 2

2
What is a class?

• A class is a set of objects sharing common
  features.
• A class defines an objects attributes and
  behavior. Methods are provided to act on an
  object and to pass messages between objects.
• A class is the basic unit of abstraction.
• A class is the basic unit of modularity.
• A class can be concrete or abstract.

3
Class Design as Type Design

• Scott Meyers, Item 19
• How should objects of your new type be created
  and destroyed?
• How should object initialization differ from
  object assignment?
• What does it mean for objects of your new type to
  be passed by value?

4
Class Design as Type Design 2

• What are the restrictions on legal values for
  your new type?
• Does your new type fit into an inheritance graph?
• What kind of type conversions are allowed for
  your new type?
• What operators and functions make sense for the
  new type?

5
Class Design as Type Design 3

• What standard functions should be disallowed?
• Who should have access to members of your new
  type?
• What is the undeclared interface of your new
  type?
• How general is your new type?
• Is a new type really what you need?

6
What is an object?

• An object is an instance of a class.
• An object has state, behavior, identity.

7
What is an object?

• Coad-Yourdon
• An abstraction of something in a problem domain,
  reflecting the capabilities of the system to keep
  information about it, interact with it, or both
  an encapsulation of attribute values and their
  exclusive services.

8
What is an object?

• OMG
• An object is a thing. It is created as the
  instance of an object type. Each object has a
  unique identity that is distinct from and
  independent of any of its characteristics. Each
  object offers one or more operations.

9
What is an object?

• Firesmith
• An object is defined as a software abstraction
  that models all relevant aspects of a single
  tangible or conceptual entity or thing from the
  application domain or solution space. An object
  is one of the primary entities in an
  object-oriented application, typically
  corresponds to a software module, and consists of
  a set of related attribute types, messages,
  exceptions, operations, and optional component
  objects.

10
What is an object?

• Booch
• From the perspective of human cognition, an
  object is any of the following
• A tangible and/or visible thing.
• Something that may be apprehended intellectually.
• Something toward which thought or action is
  directed.

11
What is an object?

• Booch continued.
• An object has state, behavior, and identity the
  structure and behavior of similar objects are
  defined in their common class the terms instance
  and object are interchangeable.

12
What is an object?

• Shlaer-Mellor
• An object is an abstraction of a set of
  real-world things such that
• All the things in the set have the same
  characteristic.
• All instances are subject to and conform to the
  same set of rules and policies.

13
What is an object?

• Jacobson
• An object is characterized by a number of
  operations and a state which remembers the effect
  of these operations.

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What is encapsulation?

• Internal details of objects are concealed from
  the objects users (information hiding).
• Both data and implementation may be hidden. The
  object is a black box.
• Access to members is controlled through the class
  definition.
• The accessible part of a class is called its
  interface.

15
Data encapsulation example

• class Clock
• private
• int hours // 1-12 private
• int minutes // 0-59
• public
• Clock(int h, int m)
• if (h lt 1 h gt 12)
• throw(Hours must be between 1 and
• 12?)
• if (m lt 0 m gt 59)
• throw(Minutes must be between 0 and
• 59?)
• h hours
• m minutes
• //...

16
Class Invariants

• The above is an example of Programming by
  Contract.
• The class guarantees that
• These are called class invariants

17
Data Members

• Data members can be declared as
• const a declaration that an object is read
  only. The object may be stored in a CPU register.
• volatile a declaration that an objects value
  may be changed asynchronously. The object may not
  be stored in a CPU register.

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Data Members 2

• static A data member shared by all objects of a
  class. There is only one copy of a static member,
  it is not part of the object memory layout.
• mutable A data member that is allowed to be
  modified, even if it a member of a const object.

19
Is the following code legal?

• struct X
• static int a 2
• main()
• X my_x
• Xa 4
• my_xa 5

20
Not legal! Cannot initialize static data member a
within class. static variables are similar to
extern variables.
21
Is the following code legal?

• struct X
• static int a
• main()
• X my_x
• Xa 4
• my_xa 5

22
Not legal! This is a linker error. Xa was
used but never defined.
23
Is the following code legal?

• struct X
• static int a
• int Xa
• main()
• X my_x
• Xa 4
• my_xa 5

24
Legal. Xa was defined before being
used. Note it is okay that Xa was not
initialized.
25
Is the following code legal?

• struct X
• static int a
• static const int b 3
• int Xa 2
• main()
• X my_x
• Xa 4
• my_xa Xb

26
Legal. static const members can be declared and
defined at the same time.
27
Is the following code legal?

• void func(volatile stdlistltintgt li)
• int n li.front()
• Not legal!
• Only volatile member functions can be called on a
  volatile object.
• listfront() not a volatile function

28
What is a method?

• A method is a member function that acts upon an
  object. A method is generally called for one
  object (exception static members).
• Commonly found methods are constructors,
  destructors, assignment, mutators, accessors.

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Static Members

• include ltiostreamgt
• using namespace std
• class X
• public
• int a
• void f(int b) cout ltlt Xf()\n
• int main()
• int Xptiptr Xa //pointer to data member
• void (X ptfptr) (int) Xf //pointer to
  member function
• X xobject
• xobject.ptiptr 10
• (xobject.ptfptr) (20)

30
What is message passing?

• Messages are transfers of data or requests for
  another object to take an action.

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Message passing example

• Adapter pattern

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What is polymorphism?

• Different types of objects respond to the same
  message and use the appropriate method.

Parametric
Universal
Subtype
Polymorphism
Overloading
Ad-hoc
Coercion
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Polymorphic Objects

• A function (or operator) is polymorphic if it has
  an argument that can accept different types.
• A variable is polymorphic if it can have
  different types in different contexts.
• A type is polymorphic if its operations can apply
  to arguments of different types.

34
Overloading

• The same name is used to denote different
  functions.
• These functions are distinguished by different
  signatures.
• Some languages (such as C) allow the programmer
  to define their own overloaded functions and
  operators.

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Overloading Example

• int square(int x) return xx
• long square(long x) return xx
• float square(float x) return xx
• double square(double x) return xx

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Alternative Method

• templatelttypename Tgt
• T square(T x) return xx
• This works on all data types for which operator
  is defined.
• int x square(4) //Calls square(int)
• double y square(4.2) //Calls square(double)
• float z square(3) //Calls square(int)

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Implementation

• How is overloading done?
• Through name mangling. The compiler modifies the
  names of each overloaded function.
• Example
• void foo(int,int)
• void foo(double,double)
• In Assembler, these would be renamed
• foo_Fii
• foo_Fdd

38
Is this code legal?

• include ltstdlib.hgt
• struct C1 enum E red, blue
• struct C2 enum E red, blue
• extern "C" int printf(const char , ...)
• void f(C1E x) printf("f(C1E)\n")
• void f(C2E x) printf("f(C2E)\n")
• int main()
• f(C1red)
• f(C2red)
• return EXIT_SUCCESS

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Yes, this is legal. The nested enums C1E and
C2E are different types. So the overloaded
functions have different signatures.
40
Is this legal?

• class X
• public
• int f()
• double f()
• No, you cant overload only on return type.

41
Is this legal?

• struct A
• static int f()
• int f()
• No, its not legal. You cant overload by static.

42
Is this legal?

• typedef int I
• void f(float, int)
• void f(float, I)
• Not legal. A typedef of an int is still an int.

43
Is this legal?

• f(char)
• f(char10)
• Not legal. The arguments are considered the same
  type (pointer to char).

44
Is this legal?

• g(char()20)
• g(char()40)
• Yes, its legal. You can distinguish
  multidimensional arrays by their second (or
  higher) dimensions.

45
Is this legal?

• int f(int)
• int f(const int)
• Not legal. You cant overload by constness of
  argument.

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Is this legal?

• void f(int ) stdcout ltlt int \n
• void f(const int ) stdcout ltlt const int
  \n
• main()
• f(3)
• return 0
• Legal. const is used within a type specification.
  
• Q Which function is called?
• A f(const int )

47
Is this legal?

• void f(int) stdcout ltlt int \n
• void f(int ) stdcout ltlt int \n
• main()
• f(3)
• return 0
• Legal. The signatures are different.
• Q Which function is called?
• A f(int)

48
Is this legal?

• void f(double int) stdcout ltlt double \n
• void f(const int ) stdcout ltlt const int
  \n
• main()
• f(3)
• return 0
• Legal. The signatures are different.
• Q Which function is called?
• A f(const int )

49
Is this legal?

• void f(int)
• void f(int i 10)
• Not legal. Cant overload by default arguments.

50
Is this legal?

• void g(int (float))
• void g(int ()(float))
• Not legal. Both functions take the same argument
  (pointer to function of the same type).

51
Coercion

• A coercion is an implicit type conversion. This
  allows the programmer to omit a type cast.
• There are three types of coercions
• Compiler defined (such as promotions, derived ?
  base)
• Constructor
• User defined

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Compiler defined coercions

• Simple example
• double x
• x 2 //2 promoted to double

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Function call coercions

• void foo(double x) //...
• //foo() can be called with any type that can be
• //converted to double
• foo((short) 4)
• foo(a)
• foo(3L)
• foo(2.3F)

54
Is this legal?

• include ltiostreamgt
• void f(char a, int b)
• stdcout ltlt a ltlt b ltlt '\n'
• void f(int a, char b)
• stdcout ltlt a ltlt b ltlt '\n'
• main()
• f('a','b')
• return 0

55
Not legal, its ambiguous.
56
Coercion vs. Overloading example

• 3 4
• 3 4.0
• 3.0 4
• 3.0 4.0
• How does this work?

57
Explanations of example

• Four overloaded versions of operator
• Two overloaded versions of operator , one for
  integers, the other for doubles. The middle two
  calls in the example would use coercion.
• One version of operator , for doubles. The first
  three calls in the example would use coercion.

58
Derived to Base Conversion

• C will implicitly convert a pointer or
  reference to a derived class to a pointer or
  reference to the base class. This is because the
  derived class has a copy of the base class inside
  it.
• A pointer to the base class cannot be converted
  to a pointer to the derived class.

59
Is this legal?

• class A
• class B public A
• class C protected A
• class D private A
• main()
• A pa new B
• pa new C
• pa new D
• return 0

60

pa new B //Legal, B is an A pa new C
//Illegal, C is not an A pa new D //Illegal,
D is not an A In the last two cases, the base
class is inaccessible.
61
Is this legal?

• class A
• class C protected A
• public
• void foo() A pa this
• class D private A
• public
• void foo() A pa this
• main()
• C c c.foo()
• D d d.foo()
• return 0

62
Yes, its legal. Protected and private
inheritance exemplify a has-a relationship,
rather than an is-a relationship. Member
functions are allowed to convert pointer to
derived into pointer to base.
63
Pointer to Member Conversions

• What is the relationship between a pointer to
  member of a base class and a derived class?

64
Is this legal?

• struct A
• int a
• struct B public A
• int a
• main()
• A a
• B b
• int Apa Ba
• int Bpb Aa
• return 0

65
Complicated. int Apa Ba is illegal.
Cant convert pointer to derived member to a
pointer to base member. int Bpb Aa Is
legal. Can convert pointer to base member to a
pointer to derived member.
66
Conversion Constructors

• Constructors that take a single argument can be
  thought of as a type conversion.
• struct X X(int)
• So that any code that expects an object of class
  X can be passed an int that will then be
  converted to an X.

67
Is this legal?

• struct X X(int)
• void f(const X )
• void g(int)
• void g(X)
• main()
• f(3)
• g(4)
• g(X(5))
• return 0

68
f(3) //Legal, you can create a temp //object
X(3), which is passed to //f() g(4) //Legal,
calls g(int) g(X(5)) //Legal, calls g(X)
69
Is this legal?

• struct X X(int)
• void f(X )
• void g(int)
• void g(X)
• main()
• int a 2
• f(3)
• f(a)
• g(4)
• g(X(5))
• return 0

70
f(3) //Not legal, cant use temp X object to
initialize an X f(a) //Not legal, cant use
temp X object to initialize an X g(4)
//Legal, as before g(X(5)) //Legal, as before
71
Is this legal?

• struct X X(int)
• struct Y Y(X)
• void f(Y)
• main()
• f(3)
• return 0

72
Not legal. Basically, only one level conversions
are allowed. Not allowed to convert int to X to Y.
73
Is this legal?

• struct X X(int)
• void f(X)
• void g(int)
• void g(X)
• main()
• int a 2
• f(3)
• f(a)
• g(4)
• g(X(5))
• return 0

74
f(3) //Legal, temp X is passed by value f(a)
//Legal, temp X is passed by value g(4)
//Legal, as before g(X(5)) //Legal, as before
75
Parametric Polymorphism

• Parametric polymorphism parametrizes the object
  type (e.g., a list class, where the type of
  object stored is parametrized).

76
Template Functions

• templateltclass Tgt
• T max(T a, T b)
• return a gt b ? a b

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Is this legal?

• int a,b 6
• const int c 3
• double x,y 3.2
• a max(b,4)
• a max(b,c)
• x max(y,3.3)
• x max(b,y)

78
a max(b,4) //Legal, maxltint,intgt a max(b,c)
//Legal, maxltint,intgt x max(y,3.3) //Legal,
maxltdouble,doublegt x max(b,y) //Illegal, no
maxltint,doublegt A template function is called
only when there is an exact match for type
parameters (only trivial conversions, such as
const int to int are allowed).
79
Better max?

• templateltclass S, class Tgt
• T max(S a, T b)
• return a gt b ? a b
• main()
• int a, b 3
• double x, y 3.2
• a max(b,5)
• x max(y,5.4)
• x max(b,y)
• x max(y,b)
• return 0

80
a max(b,5) //Legal, returns 5 x
max(y,5.4) //Legal, returns 5.4 x max(b,y)
//Legal, 3.2 x max(y,b) //Legal, but
//returns 3.0!
81
Best max?

• templateltclass R, class S, class Tgt
• R max(S a, T b)
• return a gt b ? a b
• main()
• int a, b 3
• double x, y 3.2
• a max(b,5)
• x max(y,5.4)
• x max(b,y)
• x max(y,b)
• return 0

82
Doesnt compile. The function max() is supposed
to have 3 template parameters. But each call only
uses 2 parameters.
83
Try this max

• templateltclass R, class S, class Tgt
• R max(S a, T b)
• return a gt b ? a b
• main()
• int a, b 3
• double x, y 3.2
• a maxltintgt(b,5)
• x maxltdoublegt(y,5.4)
• x maxltdoublegt(b,y)
• x maxltdoublegt(y,b)
• return 0

84
Subtype polymorphism

• Subtype (or inclusion) polymorphism allows
  objects of a given type to be substituted for by
  objects of a subtype.

85
What is inheritance?

• One class (derived/child) relies on the
  definition of another class (base/parent).
• Single vs. multiple inheritance
• A method of sharing code or sharing interface
  among classes.
• Language may define a class tree (with single
  root) Java, Smalltalk
• Language may define a class forest C

86
What is typing?

• Static typing Data type determined at
  compile-time. Type must be declared.
• Dynamic typing Data type may be determined at
  run-time. Type need not be declared.
• Strong typing Variables are bound to a specific
  type.
• Weak typing A variables type may change.

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Varieties of typing

• Static and strong typing Java, Pascal, OCaml,
  Haskell
• Static and weak typing C/C
• Dynamic and strong typing Python
• Dynamic and weak typing PHP

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Dynamic typing example

• Python example
• class Cat def speak(self) print "meow!"
• class Dog def speak(self) print "woof!"
• class Bob def speak(self) print "hello world!"
• def command(pet) pet.speak()
• pets Cat(), Dog(), Bob()
• for pet in pets
• command(pet)

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Weak typing example

• var x 5
• var y "37"
• Print(x y)
• In Visual Basic this prints 42
• In JavaScript this prints 537

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What is exception handling?

• The mechanism used to report and recover from
  abnormal states.
• When an error is detected, execution is passed to
  a handler.