IRIX Base Documentation 2002 November/SGI IRIX Base Documentation 2002 November.iso/usr/share/catman/u_man/cat1/perlobj.z/perlobj

Text File | 2002-10-03 | 26.7 KB | 727 lines

PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



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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.  Second, if you still find the following
     reference work too complicated, a tutorial on object-oriented programming
     in Perl can be found in the _p_e_r_l_t_o_o_t manpage.

     If you're still with us, then 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 class 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 to
     something "blessed" into a class, generally the class that the subroutine
     is defined in.  Here is a typical constructor:

         package Critter;
         sub new { bless {} }

     That word new isn't special.  You could have written a construct this
     way, too:

         package Critter;
         sub spawn { bless {} }

     In fact, this might even be preferable, because the C++ programmers won't
     be tricked into thinking that new works in Perl as it does in C++.  It
     doesn't.  We recommend that you name your constructors whatever makes
     sense in the context of the problem you're solving.  For example,
     constructors in the Tk extension to Perl are named after the widgets they
     create.

     One thing that's different about Perl constructors compared with those in
     C++ is that in Perl, they have to allocate their own memory.  (The other
     things is that they don't automatically call overridden base-class
     constructors.)  The {} allocates an anonymous hash containing no
     key/value pairs, and returns it  The _b_l_e_s_s() takes that reference and
     tells the object it references that it's now a Critter, and returns the



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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     reference.  This is for convenience, because the referenced object itself
     knows that it has been blessed, and the 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:

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

     If you care about inheritance (and you should; see the section on
     _M_o_d_u_l_e_s: _C_r_e_a_t_i_o_n, _U_s_e, _a_n_d _A_b_u_s_e in the _p_e_r_l_m_o_d_l_i_b manpage), then you
     want to use the two-arg form of bless so that your constructors may be
     inherited:

         sub new {
             my $class = shift;
             my $self = {};
             bless $self, $class;
             $self->initialize();
             return $self;
         }

     Or if you expect people to call not just CLASS->new() but also $obj-
     >new(), then use something like this.  The _i_n_i_t_i_a_l_i_z_e() method used will
     be of whatever $class we blessed the object into:

         sub new {
             my $this = shift;
             my $class = ref($this) || $this;
             my $self = {};
             bless $self, $class;
             $self->initialize();
             return $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 be accessed
     only through the class's methods.





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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     A constructor may re-bless 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 belong to
     only one class at a time.  (Although of course it's free to inherit
     methods from many classes.)  If you find yourself having to do this, the
     parent class is probably misbehaving, though.

     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 uses the reference 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 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.

     All classes implicitly inherit from class UNIVERSAL as their last base
     class.  Several commonly used methods are automatically supplied in the
     UNIVERSAL class; see the section on _D_e_f_a_u_l_t _U_N_I_V_E_R_S_A_L _m_e_t_h_o_d_s for more
     details.

     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 causes Perl to do the lookup again.

     If neither the current class, its named base classes, nor the UNIVERSAL
     class contains the requested method, these three places are searched all
     over again, this time looking for a method named _A_U_T_O_L_O_A_D().  If an
     AUTOLOAD is found, this method is called on behalf of the missing method,
     setting the package global $AUTOLOAD to be the fully qualified name of
     the method that was intended to be called.






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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     If none of that works, Perl finally gives up and complains.

     Perl classes do method inheritance only.  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.  The
     only problem with this is that you can't sure that you aren't using a
     piece of the hash that isn't already used.  A reasonable workaround is to
     prepend your fieldname in the hash with the package name.

         sub bump {
             my $self = shift;
             $self->{ __PACKAGE__ . ".count"}++;
         }


     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 (reference) or package (string) it is being invoked on.  There
     are just two types of methods, which we'll call class and instance.
     (Sometimes you'll hear these called static and virtual, in honor of the
     two C++ method types they most closely resemble.)

     A class 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 class methods.  Many
     class methods simply ignore their first argument, because they already
     know what package they're in, and don't care what package they were
     invoked via.  (These aren't necessarily the same, because class methods
     follow the inheritance tree just like ordinary instance methods.)
     Another typical use for class methods is to look up an object by name:

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

     An instance 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.

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



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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     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 class or instance methods.
     We'll use the two methods defined above, the class method to lookup an
     object reference and the instance 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 parentheses of

         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.




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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     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 wish merely
     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');

     If you're trying to control where the method search begins _a_n_d you're
     executing in the class itself, then you may use the SUPER pseudo class,
     which says to start looking in your base class's @ISA list without having
     to name it explicitly:

         $self->SUPER::display('Height', 'Weight');

     Please note that the SUPER:: construct is meaningful _o_n_l_y within the
     class.

     Sometimes you want to call a method when you don't know the method name
     ahead of time.  You can use the arrow form, replacing the method name
     with a simple scalar variable containing the method name:

         $method = $fast ? "findfirst" : "findbest";
         $fred->$method(@args);


     DDDDeeeeffffaaaauuuulllltttt UUUUNNNNIIIIVVVVEEEERRRRSSSSAAAALLLL mmmmeeeetttthhhhooooddddssss

     The UNIVERSAL package automatically contains the following methods that
     are inherited by all other classes:

     isa(CLASS)
         isa returns _t_r_u_e if its object is blessed into a subclass of CLASS

         isa is also exportable and can be called as a sub with two arguments.
         This allows the ability to check what a reference points to. Example

             use UNIVERSAL qw(isa);

             if(isa($ref, 'ARRAY')) {
                 #...
             }






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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     can(METHOD)
         can checks to see if its object has a method called METHOD, if it
         does then a reference to the sub is returned, if it does not then
         _u_n_d_e_f is returned.

     VERSION( [NEED] )
         VERSION returns the version number of the class (package).  If the
         NEED argument is given then it will check that the current version
         (as defined by the $VERSION variable in the given package) not less
         than NEED; it will die if this is not the case.  This method is
         normally called as a class method.  This method is called
         automatically by the VERSION form of use.

             use A 1.2 qw(some imported subs);
             # implies:
             A->VERSION(1.2);


     NNNNOOOOTTTTEEEE:::: can directly uses Perl's internal code for method lookup, and isa
     uses a very similar method and cache-ing strategy. This may cause strange
     effects if the Perl code dynamically changes @ISA in any package.

     You may add other methods to the UNIVERSAL class via Perl or XS code.
     You do not need to use UNIVERSAL in order to make these methods available
     to your program.  This is necessary only if you wish to have isa
     available as a plain subroutine in the current package.

     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 passes a reference to
     the object under destruction as the first (and only) argument.  Beware
     that the reference is a read-only value, and cannot be modified by
     manipulating $_[0] within the destructor.  The object itself (i.e.  the
     thingy the reference points to, namely ${$_[0]}, @{$_[0]}, %{$_[0]} etc.)
     is not similarly constrained.

     If you arrange to re-bless the reference before the destructor returns,
     perl will again call the DESTROY method for the re-blessed object after
     the current one returns.  This can be used for clean delegation of object
     destruction, or for ensuring that destructors in the base classes of your
     choosing get called.  Explicitly calling DESTROY is also possible, but is
     usually never needed.

     Do not confuse the foregoing with how objects _C_O_N_T_A_I_N_E_D in the current
     one are destroyed.  Such objects will be freed and destroyed
     automatically when the current object is freed, provided no other
     references to them exist elsewhere.



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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     WWWWAAAARRRRNNNNIIIINNNNGGGG

     While indirect object syntax may well be appealing to English speakers
     and to C++ programmers, be not seduced!  It suffers from two grave
     problems.

     The first problem is that an indirect object is limited to a name, a
     scalar variable, or a block, because it would have to do too much
     lookahead otherwise, just like any other postfix dereference in the
     language.  (These are the same quirky rules as are used for the
     filehandle slot in functions like print and printf.)  This can lead to
     horribly confusing precedence problems, as in these next two lines:

         move $obj->{FIELD};                 # probably wrong!
         move $ary[$i];                      # probably wrong!

     Those actually parse as the very surprising:

         $obj->move->{FIELD};                # Well, lookee here
         $ary->move->[$i];                   # Didn't expect this one, eh?

     Rather than what you might have expected:

         $obj->{FIELD}->move();              # You should be so lucky.
         $ary[$i]->move;                     # Yeah, sure.

     The left side of ``->'' is not so limited, because it's an infix
     operator, not a postfix operator.

     As if that weren't bad enough, think about this: Perl must guess _a_t
     _c_o_m_p_i_l_e _t_i_m_e whether name and move above are functions or methods.
     Usually Perl gets it right, but when it doesn't it, you get a function
     call compiled as a method, or vice versa.  This can introduce subtle bugs
     that are hard to unravel.  For example, calling a method new in indirect
     notation--as C++ programmers are so wont to do--can be miscompiled into a
     subroutine call if there's already a new function in scope.  You'd end up
     calling the current package's new as a subroutine, rather than the
     desired class's method.  The compiler tries to cheat by remembering
     bareword requires, but the grief if it messes up just isn't worth the
     years of debugging it would likely take you to to track such subtle bugs
     down.

     The infix arrow notation using ``->'' doesn't suffer from either of these
     disturbing ambiguities, so we recommend you use it exclusively.

     SSSSuuuummmmmmmmaaaarrrryyyy

     That's about all there is to it.  Now you need just to go off and buy a
     book about object-oriented design methodology, and bang your forehead
     with it for the next six months or so.





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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     TTTTwwwwoooo----PPPPhhhhaaaasssseeeedddd GGGGaaaarrrrbbbbaaaaggggeeee CCCCoooolllllllleeeeccccttttiiiioooonnnn

     For most purposes, Perl uses a fast and simple reference-based garbage
     collection system.  For this reason, there's an extra dereference going
     on at some level, so if you haven't built your Perl executable using your
     C compiler's -O flag, performance will suffer.  If you _h_a_v_e built Perl
     with cc -O, then this probably won't matter.

     A more serious concern is that unreachable memory with a non-zero
     reference count will not normally get freed.  Therefore, this is a bad
     idea:

         {
             my $a;
             $a = \$a;
         }

     Even thought $a _s_h_o_u_l_d go away, it can't.  When building recursive data
     structures, you'll have to break the self-reference yourself explicitly
     if you don't care to leak.  For example, here's a self-referential node
     such as one might use in a sophisticated tree structure:

         sub new_node {
             my $self = shift;
             my $class = ref($self) || $self;
             my $node = {};
             $node->{LEFT} = $node->{RIGHT} = $node;
             $node->{DATA} = [ @_ ];
             return bless $node => $class;
         }

     If you create nodes like that, they (currently) won't go away unless you
     break their self reference yourself.  (In other words, this is not to be
     construed as a feature, and you shouldn't depend on it.)

     Almost.

     When an interpreter thread finally shuts down (usually when your program
     exits), then a rather costly but complete mark-and-sweep style of garbage
     collection is performed, and everything allocated by that thread gets
     destroyed.  This is essential to support Perl as an embedded or a
     multithreadable language.  For example, this program demonstrates Perl's
     two-phased garbage collection:

         #!/usr/bin/perl
         package Subtle;









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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



         sub new {
             my $test;
             $test = \$test;
             warn "CREATING " . \$test;
             return bless \$test;
         }

         sub DESTROY {
             my $self = shift;
             warn "DESTROYING $self";
         }

         package main;

         warn "starting program";
         {
             my $a = Subtle->new;
             my $b = Subtle->new;
             $$a = 0;  # break selfref
             warn "leaving block";
         }

         warn "just exited block";
         warn "time to die...";
         exit;

     When run as /_t_m_p/_t_e_s_t, the following output is produced:

         starting program at /tmp/test line 18.
         CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
         CREATING SCALAR(0x8e57c) at /tmp/test line 7.
         leaving block at /tmp/test line 23.
         DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
         just exited block at /tmp/test line 26.
         time to die... at /tmp/test line 27.
         DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.

     Notice that "global destruction" bit there?  That's the thread garbage
     collector reaching the unreachable.

     Objects are always destructed, even when regular refs aren't and in fact
     are destructed in a separate pass before ordinary refs just to try to
     prevent object destructors from using refs that have been themselves
     destructed.  Plain refs are only garbage-collected if the destruct level
     is greater than 0.  You can test the higher levels of global destruction
     by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
     -DDEBUGGING was enabled during perl build time.

     A more complete garbage collection strategy will be implemented at a
     future date.





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PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))                                                          PPPPEEEERRRRLLLLOOOOBBBBJJJJ((((1111))))



     In the meantime, the best solution is to create a non-recursive container
     class that holds a pointer to the self-referential data structure.
     Define a DESTROY method for the containing object's class that manually
     breaks the circularities in the self-referential structure.

SSSSEEEEEEEE AAAALLLLSSSSOOOO
     A kinder, gentler tutorial on object-oriented programming in Perl can be
     found in the _p_e_r_l_t_o_o_t manpage.  You should also check out the _p_e_r_l_b_o_t
     manpage for other object tricks, traps, and tips, as well as the
     _p_e_r_l_m_o_d_l_i_b manpage for some style guides on constructing both modules and
     classes.












































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