1. Topic 11: Object-oriented Perl CSE3395 Perl Programming Camel3 chapter 12, pages 308-346 perlobj, perltoot, perlbot, perlmod manpages

2. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 2 In this topic  Classes as modules  Defining methods ► object methods ► class methods ► constructors ► destructors  Creating objects ► bless keyword  Inheritance ► @ISA array ► polymorphism  Comparison with other OO languages ► Java, C++  Classes as modules  Defining methods ► object methods ► class methods ► constructors ► destructors  Creating objects ► bless keyword  Inheritance ► @ISA array ► polymorphism  Comparison with other OO languages ► Java, C++

3. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 3 Additional information about OO  Advanced Perl Programming ► chapters 6-8  Object-oriented Perl ► by Damian Conway ► Manning Press, first edition 2000  manpages ► perlmod –about packages and modules ► perltoot –Tom’s object-oriented tutorial; simpler introduction to OO Perl ► perlobj –official reference, complete but terse ► perlbot –Bag of Object Tricks; some more complex OO stuff  Advanced Perl Programming ► chapters 6-8  Object-oriented Perl ► by Damian Conway ► Manning Press, first edition 2000  manpages ► perlmod –about packages and modules ► perltoot –Tom’s object-oriented tutorial; simpler introduction to OO Perl ► perlobj –official reference, complete but terse ► perlbot –Bag of Object Tricks; some more complex OO stuff

4. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 4 Object-orientation  A class is a description of a type of thing  An object is an instance of a class  A method is a function that operates on a class or object ► class methods (static methods) apply to the class as a whole ► object methods (instance methods) apply to one object  A constructor is a method called at object creation  A destructor is a method called at object destruction  Inheritance is the is-a relationship between two classes ► describes default behaviour of a class ► derived class inherits from base class  Polymorphism is the selection of the correct derived method based on the class of the object  A class is a description of a type of thing  An object is an instance of a class  A method is a function that operates on a class or object ► class methods (static methods) apply to the class as a whole ► object methods (instance methods) apply to one object  A constructor is a method called at object creation  A destructor is a method called at object destruction  Inheritance is the is-a relationship between two classes ► describes default behaviour of a class ► derived class inherits from base class  Polymorphism is the selection of the correct derived method based on the class of the object

5. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 5 Object-orientation in Perl  A class is a description of a type of thing ► in Perl, class is a module  An object is an instance of a class ► in Perl, object is a (usually anonymous) value ► object is identified by reference to value ► object looks like a scalar  A method is a function that operates on a class or object ► in Perl, method is a subroutine in the module  A constructor is a method called at object creation ► in Perl, constructor is a kind of object method which returns a (blessed) object  A class is a description of a type of thing ► in Perl, class is a module  An object is an instance of a class ► in Perl, object is a (usually anonymous) value ► object is identified by reference to value ► object looks like a scalar  A method is a function that operates on a class or object ► in Perl, method is a subroutine in the module  A constructor is a method called at object creation ► in Perl, constructor is a kind of object method which returns a (blessed) object Camel3 page 310; perlobj manpage

6. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 6 Using an OO module  use Class; ► use Car;  Everything else depends on class’ documentation  To call a class method: Class->cmethod(args) ► $vehicle = Car->new("ABC123"); ► new() is class method, doesn’t apply to any specific Car (yet) ► new() returns an object (i.e., constructor) –$vehicle is (a reference to) a Car object ► nothing special about name “new”, unlike Java/C++  To call an object method: $object->omethod(args) ► $odometer = $vehicle->odometer(); ► odometer() is object method, applies to a specific Car  use Class; ► use Car;  Everything else depends on class’ documentation  To call a class method: Class->cmethod(args) ► $vehicle = Car->new("ABC123"); ► new() is class method, doesn’t apply to any specific Car (yet) ► new() returns an object (i.e., constructor) –$vehicle is (a reference to) a Car object ► nothing special about name “new”, unlike Java/C++  To call an object method: $object->omethod(args) ► $odometer = $vehicle->odometer(); ► odometer() is object method, applies to a specific Car Camel3 pages 311-313; perlobj, perltoot manpages

7. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 7 Writing an OO module  Create a package named for the class ► package Car;  For every method, create a subroutine ► sub new {... } ► sub odometer {... } ► class and object methods declared the same way  End the method with true value ► 1;  Save in module called Class.pm  Create a package named for the class ► package Car;  For every method, create a subroutine ► sub new {... } ► sub odometer {... } ► class and object methods declared the same way  End the method with true value ► 1;  Save in module called Class.pm

8. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 8 Writing a class method  To write a class method, declare subroutine ► sub new {  First parameter ( $_[0] ) is class name (string) ► usually same as current class, but could be derived class if it inherited method from current class ► my $classname = shift;  Remaining parameters are arguments from caller ► my ($plate) = @_;  To write a class method, declare subroutine ► sub new {  First parameter ( $_[0] ) is class name (string) ► usually same as current class, but could be derived class if it inherited method from current class ► my $classname = shift;  Remaining parameters are arguments from caller ► my ($plate) = @_;

9. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 9 Writing a constructor  Constructor usually a class method  To create an object: ► allocate the object’s storage, usually an anonymous hash –my $auto = { }; ► initialize the object with its default values –$auto->{"plate"} = $plate; –$auto->{"ident"} = create_identification(); –$auto->{"odo"} = 0; ► mark the object as belonging to a class using bless keyword –bless $auto, $classname; –bless allows Perl to know which module contains the right subroutine when invoking an object method ► return the reference to caller –return $auto;  Constructor usually a class method  To create an object: ► allocate the object’s storage, usually an anonymous hash –my $auto = { }; ► initialize the object with its default values –$auto->{"plate"} = $plate; –$auto->{"ident"} = create_identification(); –$auto->{"odo"} = 0; ► mark the object as belonging to a class using bless keyword –bless $auto, $classname; –bless allows Perl to know which module contains the right subroutine when invoking an object method ► return the reference to caller –return $auto; Camel3 pages 317-318

10. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 10 Timeout # Typical constructor (example of object method). package Car; # Constructor new(plate) # Create and return a new car with specified number plate. sub new { my $classname = shift; my ($plate) = @_; my $auto = { plate => $plate, ident => create_identification() odo => 0, # Lifetime odometer trip => 0 # Trip odometer }; bless $auto, $classname; return $auto; } # Typical constructor (example of object method). package Car; # Constructor new(plate) # Create and return a new car with specified number plate. sub new { my $classname = shift; my ($plate) = @_; my $auto = { plate => $plate, ident => create_identification() odo => 0, # Lifetime odometer trip => 0 # Trip odometer }; bless $auto, $classname; return $auto; }

11. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 11 Writing an object method  To write a object method, declare subroutine ► sub odometer {  First parameter ( $_[0] ) is object (reference to blessed value) ► my $self = shift; ► no special self or this pointer/reference  Remaining parameters are arguments from caller ► my ($trip, $reset) = @_;  To write a object method, declare subroutine ► sub odometer {  First parameter ( $_[0] ) is object (reference to blessed value) ► my $self = shift; ► no special self or this pointer/reference  Remaining parameters are arguments from caller ► my ($trip, $reset) = @_;

12. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 12 Timeout # Typical object method. package Car; use Carp; # For croak() function below. # Object method odometer(trip, reset) # Return either the lifetime (0) or trip (1) odometer, # and optionally reset it (trip odometer only). sub odometer { my $self = shift; my ($trip, $reset) = @_; # croak() is like built-in die() but reports error from # perspective of caller. croak("Tried to reset lifetime odometer") if $reset && !$trip; my $result = $self->{ $trip ? "trip" : "odo" }; $self->{"trip"} = 0 if $reset; return $result; } # Typical object method. package Car; use Carp; # For croak() function below. # Object method odometer(trip, reset) # Return either the lifetime (0) or trip (1) odometer, # and optionally reset it (trip odometer only). sub odometer { my $self = shift; my ($trip, $reset) = @_; # croak() is like built-in die() but reports error from # perspective of caller. croak("Tried to reset lifetime odometer") if $reset && !$trip; my $result = $self->{ $trip ? "trip" : "odo" }; $self->{"trip"} = 0 if $reset; return $result; }

13. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 13 Destructors  Destructor is called automatically when object falls out of scope ► i.e., its reference count falls to zero ► not called explicitly  In Perl, destructor is method called DESTROY (all capitals)  Seldom needed because Perl manages memory allocation  Some uses ► to keep track of class meta-data ► to dump persistent objects to disk ► to close a file or network connection  Destructor is called automatically when object falls out of scope ► i.e., its reference count falls to zero ► not called explicitly  In Perl, destructor is method called DESTROY (all capitals)  Seldom needed because Perl manages memory allocation  Some uses ► to keep track of class meta-data ► to dump persistent objects to disk ► to close a file or network connection Camel3 pages 330-331

14. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 14 Inheritance  A class can derive from one or more parent classes by setting the @ISA array ► package SportsCar; @ISA = qw(Car StatusSymbol); ► Any method not found in class SportsCar will now fall back to the same-named methods in Car and StatusSymbol ► This is how Exporter.pm module works –@ISA = qw(Exporter); # Inherit export() ► Can delegate to superclass (parent) with special SUPER:: class name –skips current class when dispatching method –$self->SUPER::method()  A class can derive from one or more parent classes by setting the @ISA array ► package SportsCar; @ISA = qw(Car StatusSymbol); ► Any method not found in class SportsCar will now fall back to the same-named methods in Car and StatusSymbol ► This is how Exporter.pm module works –@ISA = qw(Exporter); # Inherit export() ► Can delegate to superclass (parent) with special SUPER:: class name –skips current class when dispatching method –$self->SUPER::method() Camel3 pages 321-326; perlobj manpage

15. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 15 Polymorphism  All method calls are dispatched at runtime in Perl  Method dispatch is based on ► class that object was blessed into ► name of method  All methods are by default polymorphic  All method calls are dispatched at runtime in Perl  Method dispatch is based on ► class that object was blessed into ► name of method  All methods are by default polymorphic

16. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 16 Timeout package Random; # new(n): create n-sided Random object. sub new { my $class = shift; my $self = { range => $_[0] }; return bless $self, $class; } # toss(): return result from 1 to object's range. sub toss { my $self = shift; $self->{"tosses"}++; # Count tosses. return int (rand $self->{"range"}) + 1; } sub DESTROY { my $self = shift; print "Did ", $self->{"tosses"}, " tosses on ", $self->{"range"}, "-sided\n" } 1; package Random; # new(n): create n-sided Random object. sub new { my $class = shift; my $self = { range => $_[0] }; return bless $self, $class; } # toss(): return result from 1 to object's range. sub toss { my $self = shift; $self->{"tosses"}++; # Count tosses. return int (rand $self->{"range"}) + 1; } sub DESTROY { my $self = shift; print "Did ", $self->{"tosses"}, " tosses on ", $self->{"range"}, "-sided\n" } 1;

17. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 17 Timeout package Die; use Random; @ISA = qw(Random); # Inherit from Random. # Introduce roll() as synonym for toss() sub roll { my $self = shift; return $self->toss(@_); } 1; package Die; use Random; @ISA = qw(Random); # Inherit from Random. # Introduce roll() as synonym for toss() sub roll { my $self = shift; return $self->toss(@_); } 1;

18. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 18 Timeout package Coin; use Random; @ISA = qw(Random); # new(): make a coin object. Coins are 2-sided. sub new { my $class = shift; $class->SUPER::new(2); # Delegate to parent class, 2 sides. } # toss(): return "heads" or "tails" sub toss { my $self = shift; return $self->SUPER::toss(@_) == 1 ? "heads" : "tail"; } 1; package Coin; use Random; @ISA = qw(Random); # new(): make a coin object. Coins are 2-sided. sub new { my $class = shift; $class->SUPER::new(2); # Delegate to parent class, 2 sides. } # toss(): return "heads" or "tails" sub toss { my $self = shift; return $self->SUPER::toss(@_) == 1 ? "heads" : "tail"; } 1;

19. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 19 Timeout #!/usr/bin/perl -w use Die; use Coin; $c = Coin->new(); $d6 = Die->new(6); $d10 = Die->new(10); # Do it 100 times. foreach (1.. 100) { print $c->toss(), " ", $d6->roll() + $d10->roll(), "\n"; } #!/usr/bin/perl -w use Die; use Coin; $c = Coin->new(); $d6 = Die->new(6); $d10 = Die->new(10); # Do it 100 times. foreach (1.. 100) { print $c->toss(), " ", $d6->roll() + $d10->roll(), "\n"; }

20. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 20 OO in Perl, Java and C++  Underlying representation ► Perl: anonymous hash/array/scalar, looked up by reference ► C++, Java: opaque structure similar to a C struct  Object methods and class methods ► Perl: equivalent syntax, distinguished by use ► C++, Java: class methods denoted by static keyword  Class data ► Perl: package-level global variables ► C++, Java: variables defined with static keyword  Underlying representation ► Perl: anonymous hash/array/scalar, looked up by reference ► C++, Java: opaque structure similar to a C struct  Object methods and class methods ► Perl: equivalent syntax, distinguished by use ► C++, Java: class methods denoted by static keyword  Class data ► Perl: package-level global variables ► C++, Java: variables defined with static keyword

21. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 21 OO in Perl, Java and C++  Instance data ► Perl: stored as key/value in object hash ► C++, Java: stored in opaque structure  Constructors ► Perl: a class method, called explicitly; must allocate object and initialize it; can have any name ► C++, Java: not strictly a method, called implicitly; need only initialize the object; new keyword has special meaning for object construction  Destructors ► Perl, C++, Java: an object method, invoked implicitly when object falls out of scope  Instance data ► Perl: stored as key/value in object hash ► C++, Java: stored in opaque structure  Constructors ► Perl: a class method, called explicitly; must allocate object and initialize it; can have any name ► C++, Java: not strictly a method, called implicitly; need only initialize the object; new keyword has special meaning for object construction  Destructors ► Perl, C++, Java: an object method, invoked implicitly when object falls out of scope

22. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 22 OO in Perl, Java and C++  Inheritance ► Perl, C++: multiple, using @ISA (Perl) and :public Base (C++); no difference between inheritance and interfaces ► Java: single, using extends ; multiple inheritance approximated with interfaces  Polymorphism ► Perl, Java: applies to all method calls, runtime dispatch ► C++: disabled (compile-time dispatch) unless method declared with virtual keyword  Access control ► Perl: none, relies on programmer to honour documented interface ► C++, Java: enforced at compile time with public, private and protected keywords; C++ has friend keyword  Inheritance ► Perl, C++: multiple, using @ISA (Perl) and :public Base (C++); no difference between inheritance and interfaces ► Java: single, using extends ; multiple inheritance approximated with interfaces  Polymorphism ► Perl, Java: applies to all method calls, runtime dispatch ► C++: disabled (compile-time dispatch) unless method declared with virtual keyword  Access control ► Perl: none, relies on programmer to honour documented interface ► C++, Java: enforced at compile time with public, private and protected keywords; C++ has friend keyword

23. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 23 OO in Perl, Java and C++  Method overloading ► Perl: none, only one method with each name ► C++/Java: all parameter types contribute to method signature  Operator overloading ► Perl, C++: yes, with use overload (Perl) and operator+ (etc.) methods (C++); infix/prefix notation converted to method calls ► Java: none  Method overloading ► Perl: none, only one method with each name ► C++/Java: all parameter types contribute to method signature  Operator overloading ► Perl, C++: yes, with use overload (Perl) and operator+ (etc.) methods (C++); infix/prefix notation converted to method calls ► Java: none

24. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 24 Covered in this topic  Classes as modules  Defining methods ► object methods ► class methods ► constructors ► destructors  Creating objects ► bless keyword  Inheritance ► @ISA array ► polymorphism  Comparison with other OO languages ► Java, C++  Classes as modules  Defining methods ► object methods ► class methods ► constructors ► destructors  Creating objects ► bless keyword  Inheritance ► @ISA array ► polymorphism  Comparison with other OO languages ► Java, C++

25. Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 25 Going further  Operator overloading ► Camel3 chapter 13, pages 347-362  Autoloading ► generation of methods on the fly ► Camel3 pages 296-298  Closures ► another way of doing OO ► Camel3 pages 259-263, 339-342  Operator overloading ► Camel3 chapter 13, pages 347-362  Autoloading ► generation of methods on the fly ► Camel3 pages 296-298  Closures ► another way of doing OO ► Camel3 pages 259-263, 339-342