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Diffstat (limited to 'pod/perlobj.pod')
| -rw-r--r-- | pod/perlobj.pod | 55 |
1 files changed, 30 insertions, 25 deletions
diff --git a/pod/perlobj.pod b/pod/perlobj.pod index 691ce8b9ee..1d13d90c9a 100644 --- a/pod/perlobj.pod +++ b/pod/perlobj.pod @@ -4,10 +4,13 @@ perlobj - Perl objects =head1 DESCRIPTION -First of all, you need to understand what references are in Perl. See -L<perlref> for that. +First of all, you need to understand what references are in Perl. +See L<perlref> 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 L<perltoot>. -Here are three very simple definitions that you should find reassuring. +If you're still with us, then +here are three very simple definitions that you should find reassuring. =over 4 @@ -44,7 +47,7 @@ constructor: The C<{}> constructs a reference to an anonymous hash containing no key/value pairs. The bless() 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 +This is for convenience, because the referenced object itself knows that it has been blessed, and its reference to it could have been returned directly, like this: @@ -65,7 +68,7 @@ that wish to call methods in the class as part of the construction: } If you care about inheritance (and you should; see -L<perlmod/"Modules: Creation, Use and Abuse">), +L<perlmod/"Modules: Creation, Use, and Abuse">), then you want to use the two-arg form of bless so that your constructors may be inherited: @@ -94,17 +97,17 @@ object into: 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. +be accessed only through the class's methods. 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 only -belong to one class at a time. (Although of course it's free to +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.) A clarification: Perl objects are blessed. References are not. Objects know which package they belong to. References do not. The bless() -function simply uses the reference in order to find the object. Consider +function uses the reference to find the object. Consider the following example: $a = {}; @@ -118,7 +121,7 @@ operated on the object and not on the reference. =head2 A Class is Simply a Package 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. 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 @@ -143,7 +146,7 @@ supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for more details.) If that doesn't work, Perl finally gives up and complains. -Perl classes only do method inheritance. Data inheritance is left +Perl classes do only 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 @@ -163,9 +166,9 @@ 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, since +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, since +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: @@ -224,7 +227,7 @@ 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 +especially when the first word is "new".) Thus, the parentheses of new Critter ('Barney', 1.5, 70) @@ -246,8 +249,8 @@ 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 I<START> looking for a method in a +Note however, that this does not do any inheritance. If you wish +merely to specify that Perl should I<START> looking for a method in a particular package, use an ordinary method call, but qualify the method name with the package like this: @@ -255,13 +258,13 @@ name with the package like this: $fred->MyCritter::display('Height', 'Weight'); If you're trying to control where the method search begins I<and> you're -executing in the class itself, then you may use the SUPER pseudoclass, +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 explicitly name it: +to name it explicitly: $self->SUPER::display('Height', 'Weight'); -Please note that the C<SUPER::> construct is I<only> meaningful within the +Please note that the C<SUPER::> construct is meaningful I<only> within the class. Sometimes you want to call a method when you don't know the method name @@ -344,9 +347,9 @@ 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 +re-blessed a reference from one of your base classes, your DESTROY may +need to call DESTROY for any base classes that need it. But this applies +to only re-blessed objects--an object reference that is merely I<CONTAINED> in the current object will be freed and destroyed automatically when the current object is freed. @@ -367,7 +370,7 @@ are equivalent, but AB and CD are different: =head2 Summary -That's about all there is to it. Now you just need to go off and buy a +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. @@ -413,7 +416,7 @@ 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 +multi-threadable language. For example, this program demonstrates Perl's two-phased garbage collection: #!/usr/bin/perl @@ -462,7 +465,7 @@ 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 +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 C<-DDEBUGGING> was enabled during perl build time. @@ -472,6 +475,8 @@ at a future date. =head1 SEE ALSO +A kinder, gentler tutorial on object-oriented programming in Perl can +be found in L<perltoot>. You should also check out L<perlbot> for other object tricks, traps, and tips, as well as L<perlmod> for some style guides on constructing both modules and classes. |