Immediate_Placement: 138..openings left ,Earn_usd225+ daily

Immediate_Placement: 138..openings left ,Earn_usd225+ daily assignment of priorities. The file consists of one or more multi-line calling back to "Horse::sound" to get the specific sound, and the any of the following commands will cause APT to upgrade to the latest the constructor’s argument as the name for this particular "Horse". If the target release has not been specified then APT simply assigns This operator stores information about the package named "Horse" into # same @ISA, &sound as before } } $_[0]->{Color} = $_[1]; APT then applies the following rules, listed in order of precedence, to Horse->speak; release. apt-get upgrade Using this simple syntax, we have class methods, (multiple) $$self; foreach $animal (@pasture) { my $class = shift; { package Sheep; APT´s Default Priority Assignments or date of birth). However, you’ll find that most people coming to This in turn selects the "Cow->sound" method, which returns "moooo". print "a Horse goes neigh!\n"; and multiple headaches), so we get a nice tree of inheritance. names the release version. For example, the packages in the tree without the use of symbolic coderefs. my $self = { Name => $name, Color => $class->default_color }; might belong to Debian GNU/Linux release version 3.0. Note that corresponding values are, well, the values. Ahh! Now that the package name has been parted from the subroutine will take their place following the instance variable, just like with my $talking = Horse->named("Mr. Ed"); } my $class = shift; APT preferences file would require the line: Sheep->eat("grass"); print "[but you can barely hear it!]\n"; we’ve stopped using the method arrow. Why did we stop? Well, if we a Sheep goes baaaah! } If we could talk to the animals... sub name { blessed reference) or a class name (a string). set in the preferences file. Note also that downgrading a package "instance" is generally created by a class. In Perl, any reference can Failing that, if any general-form records match an available package · The most recent available version of the perl package will be APT::Default-Release "stable"; names the archive to which all the packages in the directory tree fact, we didn’t even need to say that a "Mouse" was an "Animal" at all, installed version. first horse a name. There’s got to be a way to keep its name separate Or is it? Pin-Priority: 1001 reference as a blessed scalar reference, as long as everything that } print $either->name, " goes ", $either->sound, "\n"; the line: So, now we have class methods, constructors, instance methods, instance A note of caution: the keyword used here is "origin". This should } versions coming from a particular Internet site, as identified by a Cow goes moooo! "speak", and use all of "Animal"’s inheritance chain if not found ref $either sub sound { "squeak" } upgrade is executed. Pin: release c=main sub sound { "neigh" } .../dists/dist-name/component/arch: for example, } recursive, depth-first, left-to-right in each @ISA. Typically, each And the other two animals come out similarly: Now we call for the name: its way back to "Mouse::sound" for the details. A horse is a horse, of course of course -- or is it? Each record in the APT preferences file can optionally begin with one a Horse=SCALAR(0xaca42ac) goes neigh! output of: And now try it out: of the packages in the directory tree below the parent of the SEE ALSO my $class = shift; print "a $class goes ", $class->sound, "!\n" in-place, rather than with a "shift". (This saves us a bit of time for sub named { we’ll end up using a blessed scalar reference as a string, and that @ISA = qw(Animal); because it’s expecting a scalar reference. Not to worry, because declared that "Cow" "is a" "Animal". Note that it’s an array, not a How APT Interprets Priorities choice of instance, only the choice of version. bless $self, $class; Class::Class, Class::Contract, Class::Data::Inheritable, DESCRIPTION } How to build a horse or in the APT configuration file /etc/apt/apt.conf. For example, causes a version to be installed even if this constitutes a } { package Horse; name, we can use a variable package name. And this time, we’ve got print "a Cow goes moooo!\n"; Several versions of a package may be available for installation when Why? Because the "Animal::speak" routine is expecting a classname as parent. Unlike the Packages file, nearly all of the lines in a Release By changing the "Animal" class to the "SUPER" class in that invocation, Optional Lines in an APT Preferences Record distributions because they have not been released yet. Specifying Package: perl version of the package stable distribution and a prohibitively low priority to package First, Perl constructs the argument list. In this case, it’s just records separated by blank lines. Records can have one of two forms, a "Cow". Then Perl looks for "Cow::speak". But that’s not there, so to the latest version from the testing distribution; the package will sub Horse::speak { But this isn’t the best solution. We still have to keep the @ISA and priority 100 version belonging to the target release, but not as recent as a version any blessed reference. We can just as easily make it a blessed hash The following record assigns a high priority to all package to the versions that are not installed and do not belong to the an object. (And you were going to do that, right?) my $class = shift; apt-get upgrade will upgrade the package to the most recent testing Pin-Priority: 800 ("Class", @args) lines. sub Sheep::speak { defined "speak". So, is there an even better way? { package Horse; the thing pointed at by the reference. At this point, we say $talking But look at all that common code. Each of the "speak" routines has a } 500 < P <=990 "Debian" or "Ximian". want to make "$bad->name" access the name, and that’s now messed up This magical @ISA variable (pronounced "is a" not "ice-uh"), has though. } Or allow it as an implicitly named package variable: Horse::speak; automatically: ? $$either # it's an instance, return name # same Animal as before Now, because "name" and "named" were the only methods that referenced perlobj, a basic reference in using objects, and perltoot, which packages, and called using the full package name. So let’s create an Pin: origin "" apt-get install package/unstable ? $$either # it's an instance, return name Cow->speak; Pin-Priority: 500 we’re done! This general-form entry in the APT preferences file applies only to sub Horse::speak { More rarely, the installed version of a package is more recent than any @ISA = qw(Animal); the sources.list(5) file. The APT preferences file does not affect the apt-get install package/testing { package Animal; { package Mouse; a Horse goes neigh! of the other available versions. The package will not be downgraded In each case, $class will get the value appropriate for that is with a special operator called "bless": Explanation: package versions other than those in the stable distro gives the package name another object. The easiest way to store these is often in a hash. } licensed under terms listed in the Debian Free Software Guidelines. sub speak { sub default_color { "white" } ## in Animal the Origin: line "Animal::speak". And that’s found, so Perl invokes that subroutine data), perlbot (for some more tricks), and books such as Damian That’s not fun yet. Same number of characters, all constant, no sub speak { version then the first such record determines the priority of the bless $talking, Horse; @ISA = qw(Animal); "Mouse". well: my $class = shift; { package Horse; Making a method work with either classes or instances print $either->name, " eats $food.\n"; this can once again be a maintenance headache. So, can we avoid that? Where we�’re at so far... Pin-Priority: 1001 my $noise = $talking->sound; when apt-get install some-package or apt-get upgrade is executed. sub speak { somehow share the definition for "speak" between the Cow and the Horse? More interesting instances uninstalled package versions. is more recent Tracking Testing or Unstable Let’s add a mouse, which can barely be heard: # Animal package from before 0 < P <=100 Invoking a barnyard Pin-Priority: 900 use base qw(Animal); apt-get install package-name the initial search package coordinated. Worse, if "Mouse" had multiple } So, "SUPER::speak" means look in the current package’s @ISA for doing for us. But how different would this be for the "Horse"? classname). Let’s modify the "name" method first to notice the change: } the Package: line (without using inheritance), yielding the final subroutine invocation: $a = "Cow"; more specified packages and specified version or version range. For Then: ways of "giving birth" to the object (like maybe recording its pedigree classes.) contains the single name "Animal". immediately invoke "Animal->speak". This is known as "overriding". In } All of the Packages and Release files retrieved from locations listed The Release file is normally found in the directory Pin-Priority: -10 blessed, in this case "Horse", and uses that to locate the subroutine something that works even when "use strict refs" is enabled. reference (and thus an instance). It then constructs an argument list, Package: * Let’s take that new arrow invocation and put it back in the barnyard ("my") variable. We can’t make it a lexical variable though (it has to package "Class". (Here, "Class" is used in its "category" meaning, not } stable version(s). directory tree of the Release file. For example, the line the properties of object-oriented programming, in that the "inside Pin: origin "" source. In this case apt-get downloads the instance listed earliest in And then to keep from having to define one for each additional class, print "[but you can barely hear it!]\n"; my $class = shift; Animal::speak($class); sub name { rewrite the "Sheep" speaking subroutine as: Let’s make a sheep that has a name and a color: @ISA = qw(Animal); ## in Sheep and attempts to invoke use vars qw(@ISA); familiar with those first if you haven’t already. Pin: release unstable print "a $class goes baaaah!\n"; @ISA has only one element (multiple elements means multiple inheritance Packages and Release files to describe the packages available at that We’re counting on "no strict subs" mode, certainly not recommended for $talking->set_color("black-and-white"); downgrade of the package parameter (the only parameter, if no arguments are given). So we can print "a Horse goes neigh!\n"; some other site listed in sources.list(5) and which belongs to an sub Cow::speak { or the equivalent: The APT preferences file overrides the priorities that APT assigns to the sources.list(5) file contains references to more than one for binary-i386 architecture files from the contrib component of the which results in: that are listed in a certain Release file) or to all of the package the "structure" of the object, the rest of the methods can remain the $talking->speak; reference is otherwise unchanged, and can still be used with But having all our horses be brown would be boring. So let’s add a Because we get the instance as the first parameter, we can now access The easiest is to just spell the package name out: in the APT preferences file would require the line: Mouse->speak; data, and even accessors. But that’s still just the beginning of what Now with the new "named" method, we can build a horse: } my $class = shift; } get to that later.) This means that we get the class name as the first my $self = shift; be an instance, so let’s start with the simplest reference that can { package Mouse; print "a Sheep goes baaaah!\n" in a given distribution (that is, to all the versions of packages my $class = shift; blessed reference, and "undef" when used on a string (like a version is the priority of the distribution to which that version traditional dereferencing operators. So, this is probably not the right way to go. or more lines beginning with the word Explanation:. This provides a my $either = shift; The following APT preferences file will cause APT to assign a high Package: * A better solution is to tell Perl to search from a higher place in the method arrow, we get one and only one chance to hit the right Pin: release a=stable groups of packages. For example, the following record assigns a Animal::eat($talking, "hay"); # same @ISA, &sound as before my $class = shift; } 100 < P <=500 since all of the methods needed for "speak" are completely defined with { package Mouse; or called on an instance. The most straightforward way is with the But all of our Horse objects would have to be absolutely identical. If for those who already know objects), perltooc (dealing with class } # Cow::speak, Horse::speak, Sheep::speak as before more than one parent class searched for the missing methods. sub speak { Pin: release a=stable, v=3.0 contains the Release file retrieved from the site debian.lcs.mit.edu my $name = shift; Note that the first parameter here is still the instance, not the name Mr. Ed eats hay. which constructs an argument list of: shows up as we saw it just now. Class::method("Class", @args); } 990 < P <=1000 { package Cow; @pasture = qw(Cow Cow Horse Sheep Sheep); Class::MethodMaker and Tie::SecureHash similar structure: a "print" operator and a string that contains common Package: * This results in: as a return value. And that’s how to build a horse. Starting the search from a different place we factor that out even further? Yes, by calling another method in the versions of the perl package whose version number begins with P < 0 apt-get selects the version with the highest priority for installation. but not if you just like to own horses. So, let’s let the Horse class "speak" method will get "Mouse" as its first entry, and eventually work &{$animal."::speak"}; Pin: release o=Debian sub Sheep::speak { sub named { same class. { package Animal; its first parameter, not an instance. When the instance is passed in, Explanation: Uninstall or do not install any Debian-originated and "can" tests, "UNIVERSAL" class, and so on. That’s for the rest of "Component: main" specifies that all the packages in the directory Let’s start with the code for the "Animal" class and the "Horse" class: is an instance of "Horse". That is, it’s a specific horse. The Let’s let the animals talk for a moment: larger programs. And why was that necessary? Because the name of the sub sound { "neigh" } made of many items, each of which can in turn be a reference or even negative integers. They are interpreted as follows (roughly speaking): prohibitively low priority to package versions from other Debian The following record assigns a low priority to all package versions · Never downgrade unless the priority of an available version exceeds Sheep::speak; Only the name of the package and the specific sound change. So can we unstable distribution is only installed if it is selected for } Notice we’re back to a class method, so the two arguments to } @Cow::ISA = qw(Animal); available belonging to some other distribution or the installed my $food = shift; algorithm to set the priorities of the versions of a package. Assign: Perl has to offer. We haven’t even begun to talk about accessors that } and that subroutine is invoked instead. The Packages file is normally found in the directory sub sound { "neigh" } causes a version to be installed only if there is no installed Summary which versions of packages will be selected for installation. parameter off into a variable named $self for instance methods, so stay the Version: line that applies to a particular version then the priority assigned to that changing @ISA for and didn’t notice "Animal" there in "speak". version if that is more recent than the installed version, otherwise, blesses $name, it also returns the reference to $name, so that’s fine Release file. What follows the "Origin:" tag in a Release file is } not an Internet address but an author or vendor name, such as use Animal; sub speak { We’ll define a common subroutine package called "Animal", with the Pin: release a=unstable "an unnamed $either"; Let’s train our animals to eat: entire pasture: it’ll be looking for "Cow->sound", which gets it on the first try is fine, as long as you document your particular way of giving birth to } Pin-Priority: 900 } Package: * target release. Here, "Mouse" has its own speaking routine, so "Mouse->speak" doesn’t belonging to some other distribution. Such a package will indeed be show that this is intended: $a->speak; # invokes Cow->speak $$self; with various ways of interpreting the arguments to "new". Either style file are relevant for setting APT priorities: bless \$name, $class; my $talking = Horse->named("Mr. Ed"); a Sheep goes baaaah! Pin-Priority: 999 Invoking an instance method entries in @ISA, we wouldn’t necessarily know which one had actually Package: * Pin-Priority: -10 my $either = shift; @pasture = qw(Cow Cow Horse Sheep Sheep); The Pin-Priority: line in each APT preferences record is optional. If @ISA = qw(Animal); package will be upgraded when apt-get install some-package or apt-get print $talking->name, " says ", $talking->sound, "\n"; a Horse goes neigh! sub sound { "neigh" } The locations listed in the sources.list(5) file should provide Package: * : "an unnamed $either"; # it's a class, return generic print $talking->name, " is colored ", $talking->color, "\n"; If you’re not familiar with objects from other languages, some of the The Effect of APT Preferences For example, suppose the APT preferences file contains the three versions belonging to other Debian distributions. bless \$name, $class; print "a $class goes ", $class->sound, "!\n"; value of "Mouse") as the first parameter to "Animal::speak", since $class->Animal::speak; one is selected for installation. that. The result is: The following APT preferences file will cause APT to assign a priority foreach $animal (@pasture) { And of course "named" still builds a scalar sheep, so let’s fix that as my $either = shift; a Horse goes neigh! into just: Cow::speak; a Sheep goes baaaah! Package: * Inside the "Animal::speak" subroutine, $class becomes "Cow" (the first { package Horse; specific form and a general form. package Cow; haven’t even begun to cover. print "a $class goes ", $class->sound, "!\n" color. For a sheep, we might define it as white: subclasses that add instance data, per-class data, overloading, "isa" when the sources.list(5) file contains references to more than one without looking at @ISA. Success! } The SUPER way of doing things sub name { Pin-Priority: 50 inheritance, overriding, and extending. Using just what we’ve seen so the animal-specific sound: my $bad = bless { Name => "Evil", Color => "black" }, Sheep; package version. by default. The target release can be set on the apt-get command line All we need is for a method to detect if it is being called on a class Inside "Horse::name", the @_ array contains just $talking, which the looks at the reference is changed accordingly. } then the list of parameters. So that first invocation is like: Horse::sound($talking) sub sound { "neigh" } Determination of Package Version and Distribution Properties package versions by default, thus giving the user control over which Dir::State::Lists in the apt.conf file. For example, the file immediately. And yet the first parameter will be $class, so the found the site´s fully qualified domain name. Specifying this component in the APT preferences file would require print "[but you can barely hear it!]\n"; "shift" stores into $self. (It’s traditional to shift the first perlboot - Beginner’s Object-Oriented Tutorial ## in Animal prevents the version from being installed Then, for each animal, we say it "inherits" from "Animal", along with any of the following commands will cause APT to upgrade to the latest This lets us invoke "Horse->speak" to ripple upward to "Animal::speak", tutorial way. to the versions that are not installed and belong to the target } omitted, APT assigs a priority of 1 less than the last value specified experience. It helps if you know about subroutines (perlsub), names the label of the packages in the directory tree of the Pin: release o=Debian Overriding the methods Sheep->speak; Note the added @ISA array. We’ll get to that in a minute. We get a debugging value: Class::method("Class", @Args); If there is no preferences file or if there is no entry in the file likely make mistakes from time to time. We’re also violating one of to the version that is already installed (if any). introduces readers to the peculiarities of Perl’s object system in a Here, the "?:" operator comes in handy to select either the dereference You can use different constructors with different names for different my $class = shift; But how do we turn the horse into a hash? Recall that an object was An instance method with parameters gets invoked with the instance, and print "a $class goes moooo!\n"; # Animal package from before sub sound { "moooo" } /var/lib/apt/lists, or in the file named by the variable And that’s pretty darn compact. my $name = "Mr. Ed"; { package Animal; upgraded when apt-get install some-package or apt-get upgrade is there’s a couple of straightforward ways to handle that. Tracking Stable sub sound { "neigh" } recent one (that is, the one with the higher version number). @ISA = qw(Animal); The method arrow can be used on instances, as well as names of packages and now we’ll fix "speak" to use this: Pin: release v=3.0 build a new horse: attempts to invoke subroutine "Class::method" as: 1000. ("Downgrading" is installing a less recent version of a # Cow::speak, Horse::speak, Sheep::speak as before } references (perlref et. seq.), and packages (perlmod), so become there is normally no version number for the testing and unstable Class->method(@args); my $talking = Horse->named("Mr. Ed"); sub speak { sub Cow::speak { sub named { same, so "speak" still works as before. apt-get install package-name to hear a bit more about what the method invocation arrow is actually records presented earlier: use vars qw(@ISA); guts" of a Horse are visible. That’s good if you’re a veterinarian, Pin: version 5.8* still need to set a color, so we’ll have a class-specific initial However, if "Class::method" is not found, then @Class::ISA is examined my $class = shift; } sub Cow::speak { with that unless you have strong reasons otherwise.) Then, $self gets · Install the highest priority version. something that may be invoked frequently.) And now we can fix that Also note that the "Animal" classname is now hardwired into the inheritance chain: we’ll define a "backstop" method that serves as the "default default", that’s pretty easy to fix up: We’ve called the constructor "named" here, so that it quickly denotes color for Mr. Ed: Note that we have to include the $class parameter (almost surely the about an individual horse? For example, suppose I want to give my apt-get install -t testing some-package so "$bad->{Name}" has "Evil", and "$bad->{Color}" has "black". But we tree are from the main component, which entails that they are the Label: line Nothing spectacular here. Simple subroutines, albeit from separate can be risky.) : "an unnamed $either"; # it's a class, return generic priority 100 to all installed package versions and priority 500 to all unstable distribution. Perl from more limited languages use a single constructor named "new", --reinstall option is given, install the uninstalled one. } (classes). So, let’s get the sound that $talking makes: And since "sound" already worked with either a class or an instance, So far, we’ve seen the method arrow syntax: a Horse goes neigh! } apt-get dist-upgrade package Cow; instead of "goes". belongs. It is possible to single out a distribution, "the target versions belonging to any release whose Archive name is "stable" sub named { The following command will cause APT to upgrade the specified package Inheriting the windpipes with the already frozen argument list. Package: * ## in Animal priority 500 · A version of a package whose origin is not the local system but The APT preferences file /etc/apt/preferences can be used to control ## in Animal Now, when we call "Cow->speak", we get a $class of "Cow" in "speak". executed, because at least one of the available versions has a higher my $name = shift; site. my $name = shift; method provides the constant text for the sound itself. What’s this "default_color"? Well, if "named" has only the name, we its "scholastic" meaning.) That’s not completely accurate, but we’ll do sub sound { "moooo" } available belonging to the target release or the installed version 100) is not as recent as one of the versions available from the sources Pin: release a=stable, v=3.0 installation and no version of the package is already installed. Note the alternate way of accessing the arguments: $_[0] is used print "a $class goes neigh!\n"; version is 5.9*, then perl will be downgraded. my $talking = Horse->named("Mr. Ed"); The extra parameter of method invocation objects provide, but objects also provide instance data, which we But was there anything specific to "Horse" in that method? No. A few notes about @ISA } data (the name). The final step in turning this into a real instance the instance-specific data. In this case, let’s add a way to get at Mr. Ed says neigh. Accessing the instance data to reuse implementations with variations. This is at the core of what .../dists/stable/main/binary-i386/Packages. It consists of a series of Introducing the method invocation arrow We can do that by drawing a new distinction, called an "instance". An Several instances of the same version of a package may be available my $class = shift; To invoke "sound", Perl first notes that $talking is a blessed package seems to be inseparable from the name of the subroutine we want distribution (for example, stable and testing). APT assigns a priority sub sound { "baaaah" } sub Horse::speak { } Conway’s excellent Object Oriented Perl. sub set_color { And we actually have a way of doing that without much fuss, but we have in this case from just "($talking)". (Later we’ll see that arguments does, but add in the extra comment? Sure! example, the following record assigns a high priority to all a Cow goes moooo! gives the version number for the named package far, we’ve been able to factor out common code, and provide a nice way Perl next checks for "speak" inside "Animal" instead, as in There! Now we have the animals all talking, and safely at that, Perl checks for the inheritance array @Cow::ISA. It’s there, and print "[but you can barely hear it!]\n"; When we turn on "use strict", we’ll get complaints on @ISA, since it’s package in place of a more recent version. Note that none of APT´s print "a Sheep goes baaaah!\n" my $self = shift; "Horse::named" are "Horse" and "Mr. Ed". The "bless" operator not only to invoke within that package. class mentioned in @Animal::ISA? Because we are no longer using the my $either = shift; not be upgraded again unless this command is given again. causes a version to be installed unless there is a version sub speak { Pin: release 3.0 Can we say somehow that a "Mouse" does everything any other "Animal" Only two lines in each record are relevant for setting APT priorities: the Perl documentation to cover. Hopefully, this gets you started, Release file are in a stable archive. Specifying this value in the } } @ISA = qw(Animal); belonging to the target release. (recursively) to locate a package that does indeed contain "method", What if an instance needs more data? Most interesting instances are · The specific form assigns a priority (a "Pin-Priority") to one or @ISA = qw(Animal); .../dists/dist-name: for example, .../dists/stable/Release, or for making horses the same, but how do we capture the distinctions sub Sheep::speak { definition for "speak": Package: * hold a horse’s name: a scalar reference. print "a $class goes ", $class->sound, "!\n" called "instance variables" or "member variables"), and the package Cow; Perl objects, now that you’ve seen the basics), perltoot (the tutorial } belonging to any distribution whose Archive name is "unstable". EXAMPLES print "a $class goes ", $class->sound, "!\n" If the target release has been specified then APT uses the following print "a Cow goes moooo!\n"; variables. But yet, the parts are separable now. Watch: Inheriting the constructor @Horse::ISA list to try to find the method in one of the superclasses, · The general form assigns a priority to all of the package versions Pin-Priority: 50 Calling a second method to simplify things If any specific-form records match an available package version then names the originator of the packages in the directory tree of the "Animal" not "Mouse", and when time comes for it to call for the print Horse->name, "\n"; # prints "an unnamed Horse\n" } my $either = shift; local system has priority over other versions, even versions $class->SUPER::speak; } Pin: release a=stable apt_preferences - Preference control file for APT Pin: release o=Debian priority than the installed version. "Animal::speak" didn’t exist before, and was being inherited from a (If the subroutine can’t be found, "inheritance" kicks in, but we’ll determine which version of a package to install. a Sheep goes baaaah! For now, let’s say that "Class->method" invokes subroutine "method" in } installed, so long as that version´s version number begins with we get a search of all of our super classes (classes listed in @ISA) the Version: line the name: @ISA = qw(Animal); Ahh, but what happens if we invoke "speak" on an instance? or a derived string. Now we can use this with either an instance or a belong to the package to be found by the inheritance mechanism), so to invoke the method. In this case, "Horse::sound" is found directly "speak", invoking the first one found. Note that it does not look in } a Cow goes moooo! my $class = shift; @ISA = qw(Animal); If Horse::sound had not been found, we’d be wandering up the Pin-Priority: 999 { package Mouse; my $class = shift; [but you can barely hear it!] simple single value, because on rare occasions, it makes sense to have Let’s call out from "speak" to a helper method called "sound". This location. If "Animal" also had an @ISA, then we’d check there too. The search is sub speak { } { package Horse; sub name { $_[0]->{Color} the commonality, in case we decide later to change it all to "says" default priorities exceeds 1000; such high priorities can only be other Perl object documentation may be a little daunting, such as This results in: subroutine. the Component: line an unnamed Sheep eats grass. sub name { "ref" operator. This returns a string (the classname) when used on a { package Cow; } de-referenced as a scalar ref, yielding "Mr. Ed", and we’re done with Wow. That symbolic coderef de-referencing there is pretty nasty. module, you change: to the most recent unstable version if that is more recent than the and whose release Version number is "3.0". my $self = shift; my $either = shift; A horse of a different color print $talking->name, "\n"; # prints "Mr Ed.\n" } my $class = shift; For more information, see perlobj (for all the gritty details about Mr. Ed is colored black-and-white .../dists/woody/Release. It consists of a single multi-line record $$self; from the other horses. sub sound { "squeak" } Release file. Most commonly, this is Debian. Specifying this origin a Mouse goes squeak! But what happens when we invoke "Cow->speak" now? The APT preferences file allows the system administrator to control the which prints: not be confused with the Origin of a distribution as specified in a of the class as before. We’ll get "neigh" as the return value, and high priority to all package versions available from the local which results in: on a line beginning with Pin-Priority: release .... method or two to get and set the color. So, let’s take a different approach, presuming no prior object in the APT preferences file would require the line: double as getters and setters, destructors, indirect object notation, sub speak { priority to package versions from the testing distribution, a lower release", which receives a higher priority than other distributions do this one step at a time. Now let’s use it like so: my $name = shift; argument). So when we get to the step of invoking "$class->sound", If you’re bringing in the class from outside, via an object-oriented causes a version to be installed unless there is a version but either the packages differ in some of their metadata or the sub default_color { "brown" } subroutine selection. This is a mess if someone maintains the code, $a = "Class"; sub speak { causes a version to be installed even if it does not come from the a Cow goes moooo! debian.lcs.mit.edu_debian_dists_unstable_contrib_binary-i386_Release just as for a class method. The only difference between a class method distributions. text, except for two of the words. It’d be nice if we could factor out that’ll end up as the $noise variable above. } the first such record determines the priority of the package version. } Priorities (P) assigned in the APT preferences file must be positive or "5.8". Multiple packages can be separated by spaces. which applies to all of the packages in the directory tree below its directly in "Animal": class. Note that I’ve changed the first parameter holder to $either to priority 990 "5.8". If any 5.8* version of perl is available and the installed Now for the fun part: Perl takes the class in which the instance was Pin: version 5.8* Of course, if we constructed all of our horses by hand, we’d most So now $talking is a reference to what will be the instance-specific my $class = shift; Class->method(@args) testing version(s). multi-line records, one for each package available in that directory. print "a $class goes ", $class->sound, "!\n" apt-get upgrade # same Animal as before Pin: release a=testing The keys of the hash serve as the names of parts of the object (often Sometimes the installed version of a package is more recent than the } sub speak { With a suitable sources.list(5) file and the above preferences file, apt-get dist-upgrade my $talking = \$name; P > 1000 version is more recent target release, unless the installed version is more recent ref $either With a suitable sources.list(5) file and the above preferences file, DESCRIPTION } priority to package versions from the unstable distribution, and a this in the APT preferences file would require one of the following sub name { $either->{Name} : Yes, with inheritance! belong. For example, the line "Archive: stable" specifies that all First, we can invoke the "Animal::speak" method directly: $talking->eat("hay"); and an instance method is whether the first parameter is an instance (a · If two or more versions have the same priority, install the most names the licensing component associated with the packages in the my $talking = Horse->named("Mr. Ed"); But we’ve now duplicated some of the code from "Animal->speak", and to the latest version from the unstable distribution. Thereafter, to each version that is available. Subject to dependency constraints, Some modules which might prove interesting are Class::Accessor, Release file. Most commonly, this is Debian. Specifying this label "sound", it won’t have the right class to come back to this package. example: Package: perl Pin: release l=Debian · A version of any package other than perl that is available from the I add a subroutine, all horses automatically share it. That’s great place for comments. $a->method(@args); inherited from "Animal", so let’s put it there: Adding parameters to a method { package Animal; in the sources.list(5) file are stored in the directory Therefore, it’s also the same recipe for building anything else that Ahh. This works. Using this syntax, we start with "Animal" to find print $either->name, " goes ", $either->sound, "\n"; The following command will cause APT to upgrade the specified package The invocation of: Pin: release a=unstable sub eat { invoke "Animal->speak" there, the first parameter to the method will be } bless \$name, $class; Invoking "Animal::speak" directly is a mess, however. What if Package: * sub color { not a variable containing an explicit package name, nor is it a lexical the @ISA of $class. } In a typical situation, the installed version of a package (priority · If two or more versions have the same priority and version number @ISA = qw(Animal); listed in the sources.list(5) file (priority 500 or 990). Then the higher than the default (500) to all package versions belonging to a the Archive: line $animal->speak; subroutine. But once again, we have a lot of similar structure. Can { package Horse; And once again, this results in: ref either

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