Objected Oriented Perl

1. Objected Oriented Perl An introduction � because I don�t have the time or patience for an in-depth OOP lecture series�

2. Classes in Perl A class is defined by storing code which defines it in a separate file, and then useing that file The file must be named with the name of the class (starting with an capital letter), followed by the extension .pm After the shebang in your �main� file, this line of code: use Classname; You can then create instances of the class anywhere in your file.

3. Defining an Object In Perl, an object is simply a reference containing the members of a class. typically, a reference to a hash, but can be any kind of reference The reference becomes an object when it is �blessed� � when you tell Perl the reference belongs to a certain class.

4. Simple Example package Student; $obj = {Name => �Paul�, ID => 123}; bless ($obj, Student); $obj is now an object of the class Student �package Student� is the first line of your .pm file. It identifies all following code as belonging to this class/package/module

5. Constructors Unlike C++, a Constructor in Perl is simply another subroutine. Typically named �new�, but you can give it any name you want. (see Perl Quotes page) package Student; sub new { my $ref = {Name => ��, ID => 0}; bless ($ref, Student); return $ref; } In this example, don�t actually have to give $ref any elements. You can define them all in a later subroutine, if you choose.

6. Calling the Constructor As you may be able to guess, TMTOWTDI $student = new Student; $student = Student->new; $student = Student::new(Student); First two methods get translated to 3rd method internally by perl. This has beneficial consequences�

7. Arguments to Constructor (actually, this applies to arguments to any class method) Every time the constructor is called, first argument to function is the name of the class. Remaining arguments are caller-defined $obj = new Student (�Paul�, 123); $obj = Student->new(�Paul�, 123); $obj = Student::new(Student, �Paul�, 123); So, when defining constructor, often see this: sub new{ my $class = shift; my ($name, $ID) = @_; my $ref = {Name => $name, ID => $ID}; bless ($ref, $class); return $ref; }

8. More Methods Within the .pm file, any subroutines you declare become methods of that class. For all object methods, first argument is always the object that the method is being called on. This is also beneficial� sub setName{ my $ref = shift; my $name = shift; $ref->{Name} = $name; } To call this method: $obj->setName(�Paul Lalli�); Perl translates this to: Student::setName($obj, �Paul Lalli�);

9. One more thing� In one of the oddest things I�ve ever learned about Perl, you need to place the following statement at the end of your .pm file: 1; This is because the use keyword needs to take something that returns a true value. Perl returns the last statement evaluated.

10. ?Be Kind� to One Another ? Note that class variables are not strictly �private� in the C++ sense. There is nothing preventing the user of your class from modifying the data members directly, bypassing your interface functions. Perl�s general philosophy is �If someone wants to shoot himself in the foot, who are you to stop him?� When using other people�s classes, it�s almost always a better idea to use the functions they�ve given you, and pretend you can�t get at the internal data. There are, of course, methods you can use to prevent users from doing this. Really not worth the trouble Significantly beyond scope of this course

11. Standard Modules Perl distributions come with a significant number of pre-installed modules that you can use in your own programs. These files are found in system-specified directories. To find where the files are located on your system, examine the @INC array: print �@INC\n�; Gives a listing of all directories that are looked at when Perl finds a use statement.

12. Standard Module Example We�ll look at a pre-defined class that implements Complex-number support in Perl. This module is defined in a subdirectory of the include path, called Math. use Math::Complex; The constructor for this class is called make, not new. $z = Math::Complex->make(4,3); Creates an instance of the class Complex. $z can be thought to hold the value 4 + 3i

13. More Math::Complex Complex.pm is also good enough to overload the basic mathematical operators to work with members of the complex class Overloading operators is beyond the scope of this lecture � read the Camel chapter 13 for more info It also defines a constant i = sqrt(-1); use Math::Complex; $z = Math::Complex->make(3,5); print �$z\n�; $z = $z + 4; print �$z\n�; $z = $z � 3*i; print �$z\n�;

14. Privacy? Bah� If you examine the Complex.pm file, you�ll see that the internal structure of the class is (at least partially) represented by an array reference called cartesian. Perl will let you modify this directly: $z = Math::Complex->make (3,5); ${$z->cartesian}[0] = 40; Don�t do that. Instead, use the functions provided by the class: Re() and Im() $z->Re(40); #set real part of $z to 40 $img = $z->Im(); #get img. part of $z

15. Pragmatic modules Some modules do not define classes, but rather effect how your perl script is �compiled�. These modules are called �pragmatic modules� or just �pragmas�. By convention, they should be all lower-case. (whereas class modules start with a capital) You�ve already seen two of these pragmas: warnings and strict.

16. use and no Certain pragmas take a list of options, declaring which policies of the pragma to import. use warnings qw(syntax numeric); This will only give warnings that fall into the syntax or numeric categories See Camel page 863 for a full list Pragmas can be turned off using no use warnings; { no warnings �digit�; #something that would give warnings }

17. More Standard Pragmas use integer; print �10/3=�, 10/3; #prints 3 { no integer; print �10/3=�, 10/3; #3.333333333 } use constant PI => 4 * atan2 1, 1; use constant DEBUGGING => 0; See chapter 31 of Camel for more

18. Help on Standard Modules For documentation for all these modules, you have several resources: Camel, chapters 31& 32 Unix program perldoc ex, perldoc Math::Complex CPAN � http://www.cpan.org

19. And Now a Taste of Things to Come Next week, we�ll look at the Perl debugger, as well as some helpful debugging techniques -w, warn(), perldebug, Data::Dumper Following that, we will be working extensively with two different Perl Modules: CGI.pm ? part of the core distribution ? Tk.pm ? not. ? Tk is used for GUI w/ Perl. You may have to download your own copy of perl to use it, as none of us have rights to install a CPAN module on RCS or CSLab