Skunkware 5/Skunkware 5.iso/man/cat.1/perlobj.1

Text File | 1995-07-25 | 15KB | 397 lines

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          perlobj - Perl objects

     DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
          First of all, you need to understand what references are in
          Perl.  See the _p_e_r_l_r_e_f manpage for that.

          Here are three very simple definitions that you should find
          reassuring.

          1.  An object is simply a reference that happens to know
              which class it belongs to.

          2.  A class is simply a package that happens to provide
              methods to deal with object references.

          3.  A method is simply a subroutine that expects an object
              reference (or a package name, for static methods) as the
              first argument.

          We'll cover these points now in more depth.

          AAAAnnnn OOOObbbbjjjjeeeecccctttt iiiissss SSSSiiiimmmmppppllllyyyy aaaa RRRReeeeffffeeeerrrreeeennnncccceeee

          Unlike say C++, Perl doesn't provide any special syntax for
          constructors.  A constructor is merely a subroutine that
          returns a reference that has been "blessed" into a class,
          generally the class that the subroutine is defined in.  Here
          is a typical constructor:

              package Critter;
              sub new { bless {} }

          The {} constructs a reference to an anonymous hash
          containing no key/value pairs.  The _b_l_e_s_s() takes that
          reference and tells the object it references that it's now a
          Critter, and returns the reference.  This is for
          convenience, since the referenced object itself knows that
          it has been blessed, and its reference to it could have been
          returned directly, like this:

              sub new {
                  my $self = {};
                  bless $self;
                  return $self;
              }

          In fact, you often see such a thing in more complicated
          constructors that wish to call methods in the class as part
          of the construction:





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              sub new {
                  my $self = {}
                  bless $self;
                  $self->initialize();
                  $self;
              }

          Within the class package, the methods will typically deal
          with the reference as an ordinary reference.  Outside the
          class package, the reference is generally treated as an
          opaque value that may only be accessed through the class's
          methods.

          A constructor may rebless a referenced object currently
          belonging to another class, but then the new class is
          responsible for all cleanup later.  The previous blessing is
          forgotten, as an object may only belong to one class at a
          time.  (Although of course it's free to inherit methods from
          many classes.)

          A clarification:  Perl objects are blessed.  References are
          not.  Objects know which package they belong to.  References
          do not.  The _b_l_e_s_s() function simply uses the reference in
          order to find the object.  Consider the following example:

              $a = {};
              $b = $a;
              bless $a, BLAH;
              print "\$b is a ", ref($b), "\n";

          This reports $b as being a BLAH, so obviously _b_l_e_s_s()
          operated on the object and not on the reference.

          AAAA CCCCllllaaaassssssss iiiissss SSSSiiiimmmmppppllllyyyy aaaa PPPPaaaacccckkkkaaaaggggeeee

          Unlike say C++, Perl doesn't provide any special syntax for
          class definitions.  You just use a package as a class by
          putting method definitions into the class.

          There is a special array within each package called @ISA
          which says where else to look for a method if you can't find
          it in the current package.  This is how Perl implements
          inheritance.  Each element of the @ISA array is just the
          name of another package that happens to be a class package.
          The classes are searched (depth first) for missing methods
          in the order that they occur in @ISA.  The classes
          accessible through @ISA are known as base classes of the
          current class.

          If a missing method is found in one of the base classes, it
          is cached in the current class for efficiency.  Changing
          @ISA or defining new subroutines invalidates the cache and



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          causes Perl to do the lookup again.

          If a method isn't found, but an AUTOLOAD routine is found,
          then that is called on behalf of the missing method.

          If neither a method nor an AUTOLOAD routine is found in
          @ISA, then one last try is made for the method (or an
          AUTOLOAD routine) in a class called UNIVERSAL.  If that
          doesn't work, Perl finally gives up and complains.

          Perl classes only do method inheritance.  Data inheritance
          is left up to the class itself.  By and large, this is not a
          problem in Perl, because most classes model the attributes
          of their object using an anonymous hash, which serves as its
          own little namespace to be carved up by the various classes
          that might want to do something with the object.

          AAAA MMMMeeeetttthhhhoooodddd iiiissss SSSSiiiimmmmppppllllyyyy aaaa SSSSuuuubbbbrrrroooouuuuttttiiiinnnneeee

          Unlike say C++, Perl doesn't provide any special syntax for
          method definition.  (It does provide a little syntax for
          method invocation though.  More on that later.)  A method
          expects its first argument to be the object or package it is
          being invoked on.  There are just two types of methods,
          which we'll call static and virtual, in honor of the two C++
          method types they most closely resemble.

          A static method expects a class name as the first argument.
          It provides functionality for the class as a whole, not for
          any individual object belonging to the class.  Constructors
          are typically static methods.  Many static methods simply
          ignore their first argument, since they already know what
          package they're in, and don't care what package they were
          invoked via.  (These aren't necessarily the same, since
          static methods follow the inheritance tree just like
          ordinary virtual methods.)  Another typical use for static
          methods is to look up an object by name:

              sub find {
                  my ($class, $name) = @_;
                  $objtable{$name};
              }

          A virtual method expects an object reference as its first
          argument.  Typically it shifts the first argument into a
          "self" or "this" variable, and then uses that as an ordinary
          reference.








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              sub display {
                  my $self = shift;
                  my @keys = @_ ? @_ : sort keys %$self;
                  foreach $key (@keys) {
                      print "\t$key => $self->{$key}\n";
                  }
              }


          MMMMeeeetttthhhhoooodddd IIIInnnnvvvvooooccccaaaattttiiiioooonnnn

          There are two ways to invoke a method, one of which you're
          already familiar with, and the other of which will look
          familiar.  Perl 4 already had an "indirect object" syntax
          that you use when you say

              print STDERR "help!!!\n";

          This same syntax can be used to call either static or
          virtual methods.  We'll use the two methods defined above,
          the static method to lookup an object reference and the
          virtual method to print out its attributes.

              $fred = find Critter "Fred";
              display $fred 'Height', 'Weight';

          These could be combined into one statement by using a BLOCK
          in the indirect object slot:

              display {find Critter "Fred"} 'Height', 'Weight';

          For C++ fans, there's also a syntax using -> notation that
          does exactly the same thing.  The parentheses are required
          if there are any arguments.

              $fred = Critter->find("Fred");
              $fred->display('Height', 'Weight');

          or in one statement,

              Critter->find("Fred")->display('Height', 'Weight');

          There are times when one syntax is more readable, and times
          when the other syntax is more readable.  The indirect object
          syntax is less cluttered, but it has the same ambiguity as
          ordinary list operators.  Indirect object method calls are
          parsed using the same rule as list operators: "If it looks
          like a function, it is a function".  (Presuming for the
          moment that you think two words in a row can look like a
          function name.  C++ programmers seem to think so with some
          regularity, especially when the first word is "new".)  Thus,
          the parens of



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              new Critter ('Barney', 1.5, 70)

          are assumed to surround ALL the arguments of the method
          call, regardless of what comes after.  Saying

              new Critter ('Bam' x 2), 1.4, 45

          would be equivalent to

              Critter->new('Bam' x 2), 1.4, 45

          which is unlikely to do what you want.

          There are times when you wish to specify which class's
          method to use.  In this case, you can call your method as an
          ordinary subroutine call, being sure to pass the requisite
          first argument explicitly:

              $fred =  MyCritter::find("Critter", "Fred");
              MyCritter::display($fred, 'Height', 'Weight');

          Note however, that this does not do any inheritance.  If you
          merely wish to specify that Perl should _S_T_A_R_T looking for a
          method in a particular package, use an ordinary method call,
          but qualify the method name with the package like this:

              $fred = Critter->MyCritter::find("Fred");
              $fred->MyCritter::display('Height', 'Weight');


          DDDDeeeessssttttrrrruuuuccccttttoooorrrrssss

          When the last reference to an object goes away, the object
          is automatically destroyed.  (This may even be after you
          exit, if you've stored references in global variables.)  If
          you want to capture control just before the object is freed,
          you may define a DESTROY method in your class.  It will
          automatically be called at the appropriate moment, and you
          can do any extra cleanup you need to do.

          Perl doesn't do nested destruction for you.  If your
          constructor reblessed a reference from one of your base
          classes, your DESTROY may need to call DESTROY for any base
          classes that need it.  But this only applies to reblessed
          objects--an object reference that is merely _C_O_N_T_A_I_N_E_D in the
          current object will be freed and destroyed automatically
          when the current object is freed.

          SSSSuuuummmmmmmmaaaarrrryyyy

          That's about all there is to it.  Now you just need to go
          off and buy a book about object-oriented design methodology,



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          and bang your forehead with it for the next six months or
          so.

     SSSSEEEEEEEE AAAALLLLSSSSOOOO
          You should also check out the _p_e_r_l_b_o_t manpage for other
          object tricks, traps, and tips.

















































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