• NAME
• DESCRIPTION
• Class Data in a Can
• 캔안에 들어 있는 클래스 데이타
• Class Data as Package Variables
• 패키지 변수로 사용하는 클래스 데이타
• Putting All Your Eggs in One Basket
• Inheritance Concerns
• The Eponymous Meta-Object
• Indirect References to Class Data
• Monadic Classes
• Translucent Attributes
• Class Data as Lexical Variables
• Privacy and Responsibility
• File-Scoped Lexicals
• More Inheritance Concerns
• Locking the Door and Throwing Away the Key
• Translucency Revisited
• NOTES
• SEE ALSO
• AUTHOR AND COPYRIGHT
• ACKNOWLEDGEMENTS
• HISTORY

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NAME

perltooc - Tom's OO Tutorial for Class Data in Perl

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DESCRIPTION

When designing an object class, you are sometimes faced with the situation of wanting common state shared by all objects of that class. 오브젝트 클래스를 디자인 할때 당신은 가끔 그 클래스의 오브젝트들이 공통으로 공유되는 값을 원할때와 대면하게 될것이다. Such class attributes act somewhat like global variables for the entire class, but unlike program-wide globals, class attributes have meaning only to the class itself. 그러한 클래스 속성들은 클래스 전체에 대해 전역변수와 같이 행동하는데 프로그램상의 전역이라고 보기보다는 클래스에 한해서라고 볼수 있겠다. Here are a few examples where class attributes might come in handy: 클래스 속성들이 유용 할때의 예를 몇가지 들어 보도록 하겠다.

to keep a count of the objects you've created, or how many are still extant. 당신이 몇개의 갯수의 오브젝트를 생성했는지, 아니면 몇개가 현존하고 있는지 알기 위해
to extract the name or file descriptor for a logfile used by a debugging method. 디버깅에 사용될 로그 파일의 이름이나 파일 디스크립터를 추출할때.
to access collective data, like the total amount of cash dispensed by all ATMs in a network in a given day. 한 네트워크 내에 존재하는 모든 ATM 기계들의 전체 지출된 현금 금액을 알기 위한 총괄적인 데이터를 접근할때.
to access the last object created by a class, or the most accessed object, or to retrieve a list of all objects. 클래스를 이용해 마지막으로 생성된 오브젝트를 접근하기 위해, 아니면 가장 많이 접근된 오브젝트나, 모든 오브젝트에 대한 리스트를 가져오기 위해.

Unlike a true global, class attributes should not be accessed directly. 본래의 전역변수와는 달리, 클래스 속성들 직접적으로 접근되어선 안된다. Instead, their state should be inspected, and perhaps altered, only through the mediated access of class methods. These class attributes accessor methods are similar in spirit and function to accessors used to manipulate the state of instance attributes on an object. They provide a clear firewall between interface and implementation. 대신에 클래스 속성들의 상태를 검사를 하거나, 혹은 변경하기 위해선 클래스 메소드>의 중개를 통해서만 이루어 져야 한다. 클래스 속성 접근자들은 본질적으로나 기능적으로 오브젝트의 인스탄스 속성들의 상태를 바꿀때 사용하는 접근자와 같다. 클래스 속성자들은 인테페이스 (interface) 와 구현 (implementation) 사이에 명백한 방화벽을 제공한다. You should allow access to class attributes through either the class name or any object of that class. If we assume that $an_object is of type Some_Class, and the &Some_Class::population_count method accesses class attributes, then these two invocations should both be possible, and almost certainly equivalent. 당신은 클래스 속성들의 접근을 클래스 이름이나 해당 클래스의 오브젝트들을 통해서 허용해야 한다. 만약에 우리가 $as_object가 Some_Class라는 타입이라고 가정하고 &Some_Class::population_count가 클래스 속성을 접근하는 메소드라면 그러면 이 두개의 함수 호출은 가능해야 하며 거의 같아야만 한다.

    Some_Class->population_count()

    $an_object->population_count()

The question is, where do you store the state which that method accesses? Unlike more restrictive languages like C++, where these are called static data members, Perl provides no syntactic mechanism to declare class attributes, any more than it provides a syntactic mechanism to declare instance attributes. Perl provides the developer with a broad set of powerful but flexible features that can be uniquely crafted to the particular demands of the situation. 질문이 있다면, 당신은 어디에 상태값을 저장할것이면 어떤 메소드를 이용해 그 값들을 접근할 것인가? 이러한것들을 정적인 데이타 멤버라고 부르는 좀더 제약적인 C++ 언어와는 다르게, 펄은 클래스 속성을 선언하기 위해 따로 문법을 제공하지 않으며 인스탄스 속성을 정의할때 사용하는 문법 이상은 지원하지 않는다. 개발자에게 펄은 강력하면서도 유연성있는 폭 넓은 기능들을 제공하는데 이 각 기능들은 특정 상황에 요구되는 곳에 세심하게 만들어졌다. A class in Perl is typically implemented in a module. A module consists of two complementary feature sets: a package for interfacing with the outside world, and a lexical file scope for privacy. Either of these two mechanisms can be used to implement class attributes. That means you get to decide whether to put your class attributes in package variables or to put them in lexical variables. 펄에게 있어 클래스는 보통 모듈과 같이 구현될수 있다. 모듈은 서로 보완을 해주는 두개의 기능으로 나눌수가 있겠는데: 바깥세상과의 인터페이싱을 위한 패키지와 프라이버시를 위한 파일내의 렉시칼 영역. 이 두개의 메카니즘을 이용하여 클래스 속성들이 구현될수 있겠다. 이로 인해 당신은 클래스 속성들을 패키지 변수로 넣던지 렉시칼 변수로 넣을지 결정할수 있겠다. And those aren't the only decisions to make. If you choose to use package variables, you can make your class attribute accessor methods either ignorant of inheritance or sensitive to it. If you choose lexical variables, you can elect to permit access to them from anywhere in the entire file scope, or you can limit direct data access exclusively to the methods implementing those attributes. # TODO: 오역이라 재해석이 필요함 context를 확실히 하고 번역 시도. 그것들뿐만 아니라 결정할것들이 있다. 만약 패키지 변수를 사용 하기로 선택했다면, 상속을 무시하던지 민감하게 생각해서 클래스 속성 접근자 메소들를 만들수 있겠다. 만약 렉시컬 변수를 선택했다면 전체 파일 영역에서 선출해서 변수들을 접근할수 있는 권한을 줄수 있으며 아니면 메소드들이 직접 속성들을 접근 할수 있는 부분을 확실히 제한을 두어 구현할수 있겠다.

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Class Data in a Can

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캔안에 들어 있는 클래스 데이타

One of the easiest ways to solve a hard problem is to let someone else do it for you! In this case, Class::Data::Inheritable (available on a CPAN near you) offers a canned solution to the class data problem using closures. So before you wade into this document, consider having a look at that module. 어려운 문제를 푸는데에 있어 가장 쉬운 방법이 있다면 그건 다른 사람에게 그 문제를 풀게끔 나둘수 있겠다. 이경우, Class::Data::Inheritance (당신에게 가까운 CPAN에 있습니다)는 클래스 데이타에 관한 문제를 클로우저(Closure)를 이용하여 바로 따서 사용할수 있는 해결법을 제공한다.

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Class Data as Package Variables

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패키지 변수로 사용하는 클래스 데이타

Because a class in Perl is really just a package, using package variables to hold class attributes is the most natural choice. This makes it simple for each class to have its own class attributes. Let's say you have a class called Some_Class that needs a couple of different attributes that you'd like to be global to the entire class. The simplest thing to do is to use package variables like $Some_Class::CData1 and $Some_Class::CData2 to hold these attributes. But we certainly don't want to encourage outsiders to touch those data directly, so we provide methods to mediate access. 펄에서의 클래스는 결국 패키지나 마찬가지라서, 패키지 변수를 이용하여 클래스 속성을 담는 방법이 가장 자연스러운거 같다. 이를 통해 각 클래스들이 간단하게 자기들만의 클래스 속성을 가질수 있는거 같다. Some_Class라는 클래스를 가지고 있다고 가정하자 이 클래스는 파일 전체 내에서 전역변수로 사용할수 있는 여러 속성들을 필요로 한다. 가장 쉬운 방법으로 할수 있는 방법은 $Some_Class::CData1나 $Some_Class::CData2같은 패키지 변수들을 이용해 속성값들을 줄수 있겠다. 우리는 외부에서 이러한 데이타들을 직접적으로 접근하는 것을 별로 원치 않을 것이다, 그래서 데이타 접급을 중계해주는 메소드들을 제공해 주도록 한다. In the accessor methods below, we'll for now just ignore the first argument--that part to the left of the arrow on method invocation, which is either a class name or an object reference. 아래에 있는 접근자 메소드(accessor methods)들을 보면, 우선 첫번째 인자는 무시하도록 하자--메소드 실행시 화살표의 왼쪽 부분에 위치하는 클래스 이름이나 오브젝트 참조값(object reference)들을 말이다.

    package Some_Class;

    sub CData1 {

        shift;  # XXX: ignore calling class/object

        $Some_Class::CData1 = shift if @_;

        return $Some_Class::CData1;

    }

    sub CData2 {

        shift;  # XXX: ignore calling class/object

        $Some_Class::CData2 = shift if @_;

        return $Some_Class::CData2;

    }

This technique is highly legible and should be completely straightforward to even the novice Perl programmer. By fully qualifying the package variables, they stand out clearly when reading the code. Unfortunately, if you misspell one of these, you've introduced an error that's hard to catch. It's also somewhat disconcerting to see the class name itself hard-coded in so many places. 이 테크닉은 가장 명료하고도 초심자 펄 프로그래머 한테도 매우 무난한 방법이 되겠다. 패키지 변수 이름 전체를 명시해줌으로써 코드를 읽는데 있어 바로 눈에 띄일것이다. 안타깝게도 만약 변수들중에 오타를 내면 매우 잡기 힘든 에러를 초래하게 될것이며 하드코드된 클래스 이름들을 보고 꽤 당황하게 될것이다. Both these problems can be easily fixed. Just add the use strict pragma, then pre-declare your package variables. (The our operator will be new in 5.6, and will work for package globals just like my works for scoped lexicals.) 이 문제들은 쉽게 고쳐질수 문제들이 되겠다. 간단히 use strict pragma를 더해주고 미리(선행으로?) 패키지 변수들을 선언해 줌으로써 말이다. ( 5.6에서 새롭게 our 연산자가 제공 되며, 렉시칼 영역에서 사용되는 my 연산자와 같이 패키지 전역에서 사용할수 있겠다.)

    package Some_Class;

    use strict;

    our($CData1, $CData2);      # our() is new to perl5.6

    sub CData1 {

        shift;  # XXX: ignore calling class/object

        $CData1 = shift if @_;

        return $CData1;

    }

    sub CData2 {

        shift;  # XXX: ignore calling class/object

        $CData2 = shift if @_;

        return $CData2;

    }

As with any other global variable, some programmers prefer to start their package variables with capital letters. This helps clarity somewhat, but by no longer fully qualifying the package variables, their significance can be lost when reading the code. You can fix this easily enough by choosing better names than were used here. 다른 전역 변수들과 마찬가지로, 여러 프로그래머중에는 패키지변수 이름의 시작을 대문자로 주는것을 선호하기도 한다. 이를 통해 어느정도의 명료함을 주지만, 더이상 패키지 변수 로서의 자격을 상실해 코드를 읽을시 의미를 잃어 버릴수도 있겠다. 이런 문제들은 예제에 사용된 것들보다 더 적절한 이름들을 선택함으로 쉽게 해결할수 있겠다.

Putting All Your Eggs in One Basket

Just as the mindless enumeration of accessor methods for instance attributes grows tedious after the first few (see perltoot), so too does the repetition begin to grate when listing out accessor methods for class data. Repetition runs counter to the primary virtue of a programmer: Laziness, here manifesting as that innate urge every programmer feels to factor out duplicate code whenever possible. 얼마 안있어 아무생각없이 열거된 인스탄스(intance)속성들의 접근자(accessor) 메소드들은 짜증을 키울것이며 또한 클래스 데이타의 접근자 메소드들에 대한 리스팅을 할때 그런 반복성으로 인해 성질이 나기 시작할것이다. 이런식의 되풀이는 프로그래머로써의 주된 장점에 반대되는 것이다: 즉 게으름인데, 모든 프로그래머들은 선천적으로 가능하면 중복된 코드를 피하는것을 볼수 있겠다. Here's what to do. First, make just one hash to hold all class attributes. 여기에 해결방법이 있다. 첫번째로 해쉬 하나를 생성하여 모든 클래스 속성들을 담는다.

    package Some_Class;

    use strict;

    our %ClassData = (          # our() is new to perl5.6

        CData1 => "",

        CData2 => "",

    );

Using closures (see perlref) and direct access to the package symbol table (see perlmod), now clone an accessor method for each key in the %ClassData hash. Each of these methods is used to fetch or store values to the specific, named class attribute. 여러 클로우저(closure)의 사용과 패키지 심볼 테이블에 직접 접근(direct access)을 통해, 각 %ClassData해쉬에 있는 키를 위해 접근자 메소드들을 클론을 한다. 이 각 메소드들을 통해 특정 클래수 속성 값을 가져오거나 저장할수 있겠다.

    for my $datum (keys %ClassData) {

        no strict "refs";       # to register new methods in package

        *$datum = sub {

            shift;      # XXX: ignore calling class/object

            $ClassData{$datum} = shift if @_;

            return $ClassData{$datum};

        }

    }

It's true that you could work out a solution employing an &AUTOLOAD method, but this approach is unlikely to prove satisfactory. Your function would have to distinguish between class attributes and object attributes; it could interfere with inheritance; and it would have to careful about DESTROY. Such complexity is uncalled for in most cases, and certainly in this one. 사실 &AUTOLOAD를 도입하여 해답을 구사할수 있겠지만, 이 방식으로는 만족성을 충당할수 없는듯하다. 작성된 함수는 값이 클래수 속성인지 오브젝트 속성인지 구별해야 하며; 상속을 방해 할수있으며; 섬멸자(DESTROY)를 조심히 다루어야 한다. 주로 이정도 어려운 구현방식은 요구되지않으며 여기에선 특히 더욱 그렇다. You may wonder why we're rescinding strict refs for the loop. We're manipulating the package's symbol table to introduce new function names using symbolic references (indirect naming), which the strict pragma would otherwise forbid. Normally, symbolic references are a dodgy notion at best. This isn't just because they can be used accidentally when you aren't meaning to. It's also because for most uses to which beginning Perl programmers attempt to put symbolic references, we have much better approaches, like nested hashes or hashes of arrays. But there's nothing wrong with using symbolic references to manipulate something that is meaningful only from the perspective of the package symbol table, like method names or package variables. In other words, when you want to refer to the symbol table, use symbol references. 여기서 왜 루프 안에는 strict ref를 무효화 했는지 궁금해 할껏이다. 우리는 여기서 심볼 테이블인 Clustering all the class attributes in one place has several advantages. They're easy to spot, initialize, and change. The aggregation also makes them convenient to access externally, such as from a debugger or a persistence package. The only possible problem is that we don't automatically know the name of each class's class object, should it have one. This issue is addressed below in The Eponymous Meta-Object.

Inheritance Concerns

Suppose you have an instance of a derived class, and you access class data using an inherited method call. Should that end up referring to the base class's attributes, or to those in the derived class? How would it work in the earlier examples? The derived class inherits all the base class's methods, including those that access class attributes. But what package are the class attributes stored in? The answer is that, as written, class attributes are stored in the package into which those methods were compiled. When you invoke the &CData1 method on the name of the derived class or on one of that class's objects, the version shown above is still run, so you'll access $Some_Class::CData1--or in the method cloning version, $Some_Class::ClassData{CData1}. Think of these class methods as executing in the context of their base class, not in that of their derived class. Sometimes this is exactly what you want. If Feline subclasses Carnivore, then the population of Carnivores in the world should go up when a new Feline is born. But what if you wanted to figure out how many Felines you have apart from Carnivores? The current approach doesn't support that. You'll have to decide on a case-by-case basis whether it makes any sense for class attributes to be package-relative. If you want it to be so, then stop ignoring the first argument to the function. Either it will be a package name if the method was invoked directly on a class name, or else it will be an object reference if the method was invoked on an object reference. In the latter case, the ref() function provides the class of that object.

    package Some_Class;

    sub CData1 {

        my $obclass = shift;

        my $class   = ref($obclass) || $obclass;

        my $varname = $class . "::CData1";

        no strict "refs";       # to access package data symbolically

        $$varname = shift if @_;

        return $$varname;

    }

And then do likewise for all other class attributes (such as CData2, etc.) that you wish to access as package variables in the invoking package instead of the compiling package as we had previously. Once again we temporarily disable the strict references ban, because otherwise we couldn't use the fully-qualified symbolic name for the package global. This is perfectly reasonable: since all package variables by definition live in a package, there's nothing wrong with accessing them via that package's symbol table. That's what it's there for (well, somewhat). What about just using a single hash for everything and then cloning methods? What would that look like? The only difference would be the closure used to produce new method entries for the class's symbol table.

    no strict "refs";

    *$datum = sub {

        my $obclass = shift;

        my $class   = ref($obclass) || $obclass;

        my $varname = $class . "::ClassData";

        $varname->{$datum} = shift if @_;

        return $varname->{$datum};

    }

The Eponymous Meta-Object

It could be argued that the %ClassData hash in the previous example is neither the most imaginative nor the most intuitive of names. Is there something else that might make more sense, be more useful, or both? As it happens, yes, there is. For the ``class meta-object'', we'll use a package variable of the same name as the package itself. Within the scope of a package Some_Class declaration, we'll use the eponymously named hash %Some_Class as that class's meta-object. (Using an eponymously named hash is somewhat reminiscent of classes that name their constructors eponymously in the Python or C++ fashion. That is, class Some_Class would use &Some_Class::Some_Class as a constructor, probably even exporting that name as well. The StrNum? class in Recipe 13.14 in The Perl Cookbook does this, if you're looking for an example.) This predictable approach has many benefits, including having a well-known identifier to aid in debugging, transparent persistence, or checkpointing. It's also the obvious name for monadic classes and translucent attributes, discussed later. Here's an example of such a class. Notice how the name of the hash storing the meta-object is the same as the name of the package used to implement the class.

    package Some_Class;

    use strict;

    # create class meta-object using that most perfect of names

    our %Some_Class = (         # our() is new to perl5.6

        CData1 => "",

        CData2 => "",

    );

    # this accessor is calling-package-relative

    sub CData1 {

        my $obclass = shift;

        my $class   = ref($obclass) || $obclass;

        no strict "refs";       # to access eponymous meta-object

        $class->{CData1} = shift if @_;

        return $class->{CData1};

    }

    # but this accessor is not

    sub CData2 {

        shift;                  # XXX: ignore calling class/object

        no strict "refs";       # to access eponymous meta-object

        __PACKAGE__ -> {CData2} = shift if @_;

        return __PACKAGE__ -> {CData2};

    }

In the second accessor method, the PACKAGE notation was used for two reasons. First, to avoid hardcoding the literal package name in the code in case we later want to change that name. Second, to clarify to the reader that what matters here is the package currently being compiled into, not the package of the invoking object or class. If the long sequence of non-alphabetic characters bothers you, you can always put the PACKAGE in a variable first.

    sub CData2 {

        shift;                  # XXX: ignore calling class/object

        no strict "refs";       # to access eponymous meta-object

        my $class = __PACKAGE__;

        $class->{CData2} = shift if @_;

        return $class->{CData2};

    }

Even though we're using symbolic references for good not evil, some folks tend to become unnerved when they see so many places with strict ref checking disabled. Given a symbolic reference, you can always produce a real reference (the reverse is not true, though). So we'll create a subroutine that does this conversion for us. If invoked as a function of no arguments, it returns a reference to the compiling class's eponymous hash. Invoked as a class method, it returns a reference to the eponymous hash of its caller. And when invoked as an object method, this function returns a reference to the eponymous hash for whatever class the object belongs to.

    package Some_Class;

    use strict;

    our %Some_Class = (         # our() is new to perl5.6

        CData1 => "",

        CData2 => "",

    );

    # tri-natured: function, class method, or object method

    sub _classobj {

        my $obclass = shift || __PACKAGE__;

        my $class   = ref($obclass) || $obclass;

        no strict "refs";   # to convert sym ref to real one

        return \%$class;

    }

    for my $datum (keys %{ _classobj() } ) {

        # turn off strict refs so that we can

        # register a method in the symbol table

        no strict "refs";

        *$datum = sub {

            use strict "refs";

            my $self = shift->_classobj();

            $self->{$datum} = shift if @_;

            return $self->{$datum};

        }

    }

Indirect References to Class Data

A reasonably common strategy for handling class attributes is to store a reference to each package variable on the object itself. This is a strategy you've probably seen before, such as in perltoot and perlbot, but there may be variations in the example below that you haven't thought of before.

    package Some_Class;

    our($CData1, $CData2);              # our() is new to perl5.6

    sub new {

        my $obclass = shift;

        return bless my $self = {

            ObData1 => "",

            ObData2 => "",

            CData1  => \$CData1,

            CData2  => \$CData2,

        } => (ref $obclass || $obclass);

    }

    sub ObData1 {

        my $self = shift;

        $self->{ObData1} = shift if @_;

        return $self->{ObData1};

    }

    sub ObData2 {

        my $self = shift;

        $self->{ObData2} = shift if @_;

        return $self->{ObData2};

    }

    sub CData1 {

        my $self = shift;

        my $dataref = ref $self

                        ? $self->{CData1}

                        : \$CData1;

        $$dataref = shift if @_;

        return $$dataref;

    }

    sub CData2 {

        my $self = shift;

        my $dataref = ref $self

                        ? $self->{CData2}

                        : \$CData2;

        $$dataref = shift if @_;

        return $$dataref;

    }

As written above, a derived class will inherit these methods, which will consequently access package variables in the base class's package. This is not necessarily expected behavior in all circumstances. Here's an example that uses a variable meta-object, taking care to access the proper package's data.

        package Some_Class;

        use strict;

        our %Some_Class = (     # our() is new to perl5.6

            CData1 => "",

            CData2 => "",

        );

        sub _classobj {

            my $self  = shift;

            my $class = ref($self) || $self;

            no strict "refs";

            # get (hard) ref to eponymous meta-object

            return \%$class;

        }

        sub new {

            my $obclass  = shift;

            my $classobj = $obclass->_classobj();

            bless my $self = {

                ObData1 => "",

                ObData2 => "",

                CData1  => \$classobj->{CData1},

                CData2  => \$classobj->{CData2},

            } => (ref $obclass || $obclass);

            return $self;

        }

        sub ObData1 {

            my $self = shift;

            $self->{ObData1} = shift if @_;

            return $self->{ObData1};

        }

        sub ObData2 {

            my $self = shift;

            $self->{ObData2} = shift if @_;

            return $self->{ObData2};

        }

        sub CData1 {

            my $self = shift;

            $self = $self->_classobj() unless ref $self;

            my $dataref = $self->{CData1};

            $$dataref = shift if @_;

            return $$dataref;

        }

        sub CData2 {

            my $self = shift;

            $self = $self->_classobj() unless ref $self;

            my $dataref = $self->{CData2};

            $$dataref = shift if @_;

            return $$dataref;

        }

Not only are we now strict refs clean, using an eponymous meta-object seems to make the code cleaner. Unlike the previous version, this one does something interesting in the face of inheritance: it accesses the class meta-object in the invoking class instead of the one into which the method was initially compiled. You can easily access data in the class meta-object, making it easy to dump the complete class state using an external mechanism such as when debugging or implementing a persistent class. This works because the class meta-object is a package variable, has a well-known name, and clusters all its data together. (Transparent persistence is not always feasible, but it's certainly an appealing idea.) There's still no check that object accessor methods have not been invoked on a class name. If strict ref checking is enabled, you'd blow up. If not, then you get the eponymous meta-object. What you do with--or about--this is up to you. The next two sections demonstrate innovative uses for this powerful feature.

Monadic Classes

Some of the standard modules shipped with Perl provide class interfaces without any attribute methods whatsoever. The most commonly used module not numbered amongst the pragmata, the Exporter module, is a class with neither constructors nor attributes. Its job is simply to provide a standard interface for modules wishing to export part of their namespace into that of their caller. Modules use the Exporter's &import method by setting their inheritance list in their package's @ISA array to mention ``Exporter''. But class Exporter provides no constructor, so you can't have several instances of the class. In fact, you can't have any--it just doesn't make any sense. All you get is its methods. Its interface contains no statefulness, so state data is wholly superfluous. Another sort of class that pops up from time to time is one that supports a unique instance. Such classes are called monadic classes, or less formally, singletons or highlander classes. If a class is monadic, where do you store its state, that is, its attributes? How do you make sure that there's never more than one instance? While you could merely use a slew of package variables, it's a lot cleaner to use the eponymously named hash. Here's a complete example of a monadic class:

    package Cosmos;

    %Cosmos = ();

    # accessor method for "name" attribute

    sub name {

        my $self = shift;

        $self->{name} = shift if @_;

        return $self->{name};

    }

    # read-only accessor method for "birthday" attribute

    sub birthday {

        my $self = shift;

        die "can't reset birthday" if @_;  # XXX: croak() is better

        return $self->{birthday};

    }

    # accessor method for "stars" attribute

    sub stars {

        my $self = shift;

        $self->{stars} = shift if @_;

        return $self->{stars};

    }

    # oh my - one of our stars just went out!

    sub supernova {

        my $self = shift;

        my $count = $self->stars();

        $self->stars($count - 1) if $count > 0;

    }

    # constructor/initializer method - fix by reboot

    sub bigbang {

        my $self = shift;

        %$self = (

            name         => "the world according to tchrist",

            birthday     => time(),

            stars        => 0,

        );

        return $self;       # yes, it's probably a class.  SURPRISE!

    }

    # After the class is compiled, but before any use or require

    # returns, we start off the universe with a bang.

    __PACKAGE__ -> bigbang();

Hold on, that doesn't look like anything special. Those attribute accessors look no different than they would if this were a regular class instead of a monadic one. The crux of the matter is there's nothing that says that $self must hold a reference to a blessed object. It merely has to be something you can invoke methods on. Here the package name itself, Cosmos, works as an object. Look at the &supernova method. Is that a class method or an object method? The answer is that static analysis cannot reveal the answer. Perl doesn't care, and neither should you. In the three attribute methods, %$self is really accessing the %Cosmos package variable. If like Stephen Hawking, you posit the existence of multiple, sequential, and unrelated universes, then you can invoke the &bigbang method yourself at any time to start everything all over again. You might think of &bigbang as more of an initializer than a constructor, since the function doesn't allocate new memory; it only initializes what's already there. But like any other constructor, it does return a scalar value to use for later method invocations. Imagine that some day in the future, you decide that one universe just isn't enough. You could write a new class from scratch, but you already have an existing class that does what you want--except that it's monadic, and you want more than just one cosmos. That's what code reuse via subclassing is all about. Look how short the new code is:

    package Multiverse;

    use Cosmos;

    @ISA = qw(Cosmos);

    sub new {

        my $protoverse = shift;

        my $class      = ref($protoverse) || $protoverse;

        my $self       = {};

        return bless($self, $class)->bigbang();

    }

    1;

Because we were careful to be good little creators when we designed our Cosmos class, we can now reuse it without touching a single line of code when it comes time to write our Multiverse class. The same code that worked when invoked as a class method continues to work perfectly well when invoked against separate instances of a derived class. The astonishing thing about the Cosmos class above is that the value returned by the &bigbang ``constructor'' is not a reference to a blessed object at all. It's just the class's own name. A class name is, for virtually all intents and purposes, a perfectly acceptable object. It has state, behavior, and identity, the three crucial components of an object system. It even manifests inheritance, polymorphism, and encapsulation. And what more can you ask of an object? To understand object orientation in Perl, it's important to recognize the unification of what other programming languages might think of as class methods and object methods into just plain methods. ``Class methods'' and ``object methods'' are distinct only in the compartmentalizing mind of the Perl programmer, not in the Perl language itself. Along those same lines, a constructor is nothing special either, which is one reason why Perl has no pre-ordained name for them. ``Constructor'' is just an informal term loosely used to describe a method that returns a scalar value that you can make further method calls against. So long as it's either a class name or an object reference, that's good enough. It doesn't even have to be a reference to a brand new object. You can have as many--or as few--constructors as you want, and you can name them whatever you care to. Blindly and obediently using new() for each and every constructor you ever write is to speak Perl with such a severe C++ accent that you do a disservice to both languages. There's no reason to insist that each class have but one constructor, or that a constructor be named new(), or that a constructor be used solely as a class method and not an object method. The next section shows how useful it can be to further distance ourselves from any formal distinction between class method calls and object method calls, both in constructors and in accessor methods.

Translucent Attributes

A package's eponymous hash can be used for more than just containing per-class, global state data. It can also serve as a sort of template containing default settings for object attributes. These default settings can then be used in constructors for initialization of a particular object. The class's eponymous hash can also be used to implement translucent attributes. A translucent attribute is one that has a class-wide default. Each object can set its own value for the attribute, in which case $object->attribute() returns that value. But if no value has been set, then $object->attribute() returns the class-wide default. We'll apply something of a copy-on-write approach to these translucent attributes. If you're just fetching values from them, you get translucency. But if you store a new value to them, that new value is set on the current object. On the other hand, if you use the class as an object and store the attribute value directly on the class, then the meta-object's value changes, and later fetch operations on objects with uninitialized values for those attributes will retrieve the meta-object's new values. Objects with their own initialized values, however, won't see any change. Let's look at some concrete examples of using these properties before we show how to implement them. Suppose that a class named Some_Class had a translucent data attribute called ``color''. First you set the color in the meta-object, then you create three objects using a constructor that happens to be named &spawn.

    use Vermin;

    Vermin->color("vermilion");

    $ob1 = Vermin->spawn();     # so that's where Jedi come from

    $ob2 = Vermin->spawn();

    $ob3 = Vermin->spawn();

    print $obj3->color();       # prints "vermilion"

Each of these objects' colors is now ``vermilion'', because that's the meta-object's value for that attribute, and these objects do not have individual color values set. Changing the attribute on one object has no effect on other objects previously created.

    $ob3->color("chartreuse");

    print $ob3->color();        # prints "chartreuse"

    print $ob1->color();        # prints "vermilion", translucently

If you now use $ob3 to spawn off another object, the new object will take the color its parent held, which now happens to be ``chartreuse''. That's because the constructor uses the invoking object as its template for initializing attributes. When that invoking object is the class name, the object used as a template is the eponymous meta-object. When the invoking object is a reference to an instantiated object, the &spawn constructor uses that existing object as a template.

    $ob4 = $ob3->spawn();       # $ob3 now template, not %Vermin

    print $ob4->color();        # prints "chartreuse"

Any actual values set on the template object will be copied to the new object. But attributes undefined in the template object, being translucent, will remain undefined and consequently translucent in the new one as well. Now let's change the color attribute on the entire class:

    Vermin->color("azure");

    print $ob1->color();        # prints "azure"

    print $ob2->color();        # prints "azure"

    print $ob3->color();        # prints "chartreuse"

    print $ob4->color();        # prints "chartreuse"

That color change took effect only in the first pair of objects, which were still translucently accessing the meta-object's values. The second pair had per-object initialized colors, and so didn't change. One important question remains. Changes to the meta-object are reflected in translucent attributes in the entire class, but what about changes to discrete objects? If you change the color of $ob3, does the value of $ob4 see that change? Or vice-versa. If you change the color of $ob4, does then the value of $ob3 shift?

    $ob3->color("amethyst");

    print $ob3->color();        # prints "amethyst"

    print $ob4->color();        # hmm: "chartreuse" or "amethyst"?

While one could argue that in certain rare cases it should, let's not do that. Good taste aside, we want the answer to the question posed in the comment above to be ``chartreuse'', not ``amethyst''. So we'll treat these attributes similar to the way process attributes like environment variables, user and group IDs, or the current working directory are treated across a fork(). You can change only yourself, but you will see those changes reflected in your unspawned children. Changes to one object will propagate neither up to the parent nor down to any existing child objects. Those objects made later, however, will see the changes. If you have an object with an actual attribute value, and you want to make that object's attribute value translucent again, what do you do? Let's design the class so that when you invoke an accessor method with undef as its argument, that attribute returns to translucency.

    $ob4->color(undef);         # back to "azure"

Here's a complete implementation of Vermin as described above.

    package Vermin;

    # here's the class meta-object, eponymously named.

    # it holds all class attributes, and also all instance attributes

    # so the latter can be used for both initialization

    # and translucency.

    our %Vermin = (             # our() is new to perl5.6

        PopCount => 0,          # capital for class attributes

        color    => "beige",    # small for instance attributes

    );

    # constructor method

    # invoked as class method or object method

    sub spawn {

        my $obclass = shift;

        my $class   = ref($obclass) || $obclass;

        my $self = {};

        bless($self, $class);

        $class->{PopCount}++;

        # init fields from invoking object, or omit if

        # invoking object is the class to provide translucency

        %$self = %$obclass if ref $obclass;

        return $self;

    }

    # translucent accessor for "color" attribute

    # invoked as class method or object method

    sub color {

        my $self  = shift;

        my $class = ref($self) || $self;

        # handle class invocation

        unless (ref $self) {

            $class->{color} = shift if @_;

            return $class->{color}

        }

        # handle object invocation

        $self->{color} = shift if @_;

        if (defined $self->{color}) {  # not exists!

            return $self->{color};

        } else {

            return $class->{color};

        }

    }

    # accessor for "PopCount" class attribute

    # invoked as class method or object method

    # but uses object solely to locate meta-object

    sub population {

        my $obclass = shift;

        my $class   = ref($obclass) || $obclass;

        return $class->{PopCount};

    }

    # instance destructor

    # invoked only as object method

    sub DESTROY {

        my $self  = shift;

        my $class = ref $self;

        $class->{PopCount}--;

    }

Here are a couple of helper methods that might be convenient. They aren't accessor methods at all. They're used to detect accessibility of data attributes. The &is_translucent method determines whether a particular object attribute is coming from the meta-object. The &has_attribute method detects whether a class implements a particular property at all. It could also be used to distinguish undefined properties from non-existent ones.

    # detect whether an object attribute is translucent

    # (typically?) invoked only as object method

    sub is_translucent {

        my($self, $attr)  = @_;

        return !defined $self->{$attr};

    }

    # test for presence of attribute in class

    # invoked as class method or object method

    sub has_attribute {

        my($self, $attr)  = @_;

        my $class = ref($self) || $self;

        return exists $class->{$attr};

    }

If you prefer to install your accessors more generically, you can make use of the upper-case versus lower-case convention to register into the package appropriate methods cloned from generic closures.

    for my $datum (keys %{ +__PACKAGE__ }) {

        *$datum = ($datum =~ /^[A-Z]/)

            ? sub {  # install class accessor

                    my $obclass = shift;

                    my $class   = ref($obclass) || $obclass;

                    return $class->{$datum};

                  }

            : sub { # install translucent accessor

                    my $self  = shift;

                    my $class = ref($self) || $self;

                    unless (ref $self) {

                        $class->{$datum} = shift if @_;

                        return $class->{$datum}

                    }

                    $self->{$datum} = shift if @_;

                    return defined $self->{$datum}

                        ? $self  -> {$datum}

                        : $class -> {$datum}

                  }

    }

Translations of this closure-based approach into C++, Java, and Python have been left as exercises for the reader. Be sure to send us mail as soon as you're done.

voltar para o topo

---

Class Data as Lexical Variables

Privacy and Responsibility

Unlike conventions used by some Perl programmers, in the previous examples, we didn't prefix the package variables used for class attributes with an underscore, nor did we do so for the names of the hash keys used for instance attributes. You don't need little markers on data names to suggest nominal privacy on attribute variables or hash keys, because these are already notionally private! Outsiders have no business whatsoever playing with anything within a class save through the mediated access of its documented interface; in other words, through method invocations. And not even through just any method, either. Methods that begin with an underscore are traditionally considered off-limits outside the class. If outsiders skip the documented method interface to poke around the internals of your class and end up breaking something, that's not your fault--it's theirs. Perl believes in individual responsibility rather than mandated control. Perl respects you enough to let you choose your own preferred level of pain, or of pleasure. Perl believes that you are creative, intelligent, and capable of making your own decisions--and fully expects you to take complete responsibility for your own actions. In a perfect world, these admonitions alone would suffice, and everyone would be intelligent, responsible, happy, and creative. And careful. One probably shouldn't forget careful, and that's a good bit harder to expect. Even Einstein would take wrong turns by accident and end up lost in the wrong part of town. Some folks get the heebie-jeebies when they see package variables hanging out there for anyone to reach over and alter them. Some folks live in constant fear that someone somewhere might do something wicked. The solution to that problem is simply to fire the wicked, of course. But unfortunately, it's not as simple as all that. These cautious types are also afraid that they or others will do something not so much wicked as careless, whether by accident or out of desperation. If we fire everyone who ever gets careless, pretty soon there won't be anybody left to get any work done. Whether it's needless paranoia or sensible caution, this uneasiness can be a problem for some people. We can take the edge off their discomfort by providing the option of storing class attributes as lexical variables instead of as package variables. The my() operator is the source of all privacy in Perl, and it is a powerful form of privacy indeed. It is widely perceived, and indeed has often been written, that Perl provides no data hiding, that it affords the class designer no privacy nor isolation, merely a rag-tag assortment of weak and unenforceable social conventions instead. This perception is demonstrably false and easily disproven. In the next section, we show how to implement forms of privacy that are far stronger than those provided in nearly any other object-oriented language.

File-Scoped Lexicals

A lexical variable is visible only through the end of its static scope. That means that the only code able to access that variable is code residing textually below the my() operator through the end of its block if it has one, or through the end of the current file if it doesn't. Starting again with our simplest example given at the start of this document, we replace our() variables with my() versions.

    package Some_Class;

    my($CData1, $CData2);   # file scope, not in any package

    sub CData1 {

        shift;  # XXX: ignore calling class/object

        $CData1 = shift if @_;

        return $CData1;

    }

    sub CData2 {

        shift;  # XXX: ignore calling class/object

        $CData2 = shift if @_;

        return $CData2;

    }

So much for that old $Some_Class::CData1 package variable and its brethren! Those are gone now, replaced with lexicals. No one outside the scope can reach in and alter the class state without resorting to the documented interface. Not even subclasses or superclasses of this one have unmediated access to $CData1. They have to invoke the &CData1 method against Some_Class or an instance thereof, just like anybody else. To be scrupulously honest, that last statement assumes you haven't packed several classes together into the same file scope, nor strewn your class implementation across several different files. Accessibility of those variables is based uniquely on the static file scope. It has nothing to do with the package. That means that code in a different file but the same package (class) could not access those variables, yet code in the same file but a different package (class) could. There are sound reasons why we usually suggest a one-to-one mapping between files and packages and modules and classes. You don't have to stick to this suggestion if you really know what you're doing, but you're apt to confuse yourself otherwise, especially at first. If you'd like to aggregate your class attributes into one lexically scoped, composite structure, you're perfectly free to do so.

    package Some_Class;

    my %ClassData = (

        CData1 => "",

        CData2 => "",

    );

    sub CData1 {

        shift;  # XXX: ignore calling class/object

        $ClassData{CData1} = shift if @_;

        return $ClassData{CData1};

    }

    sub CData2 {

        shift;  # XXX: ignore calling class/object

        $ClassData{CData2} = shift if @_;

        return $ClassData{CData2};

    }

To make this more scalable as other class attributes are added, we can again register closures into the package symbol table to create accessor methods for them.

    package Some_Class;

    my %ClassData = (

        CData1 => "",

        CData2 => "",

    );

    for my $datum (keys %ClassData) {

        no strict "refs";

        *$datum = sub {

            shift;      # XXX: ignore calling class/object

            $ClassData{$datum} = shift if @_;

            return $ClassData{$datum};

        };

    }

Requiring even your own class to use accessor methods like anybody else is probably a good thing. But demanding and expecting that everyone else, be they subclass or superclass, friend or foe, will all come to your object through mediation is more than just a good idea. It's absolutely critical to the model. Let there be in your mind no such thing as ``public'' data, nor even ``protected'' data, which is a seductive but ultimately destructive notion. Both will come back to bite at you. That's because as soon as you take that first step out of the solid position in which all state is considered completely private, save from the perspective of its own accessor methods, you have violated the envelope. And, having pierced that encapsulating envelope, you shall doubtless someday pay the price when future changes in the implementation break unrelated code. Considering that avoiding this infelicitous outcome was precisely why you consented to suffer the slings and arrows of obsequious abstraction by turning to object orientation in the first place, such breakage seems unfortunate in the extreme.

More Inheritance Concerns

Suppose that Some_Class were used as a base class from which to derive Another_Class. If you invoke a &CData method on the derived class or on an object of that class, what do you get? Would the derived class have its own state, or would it piggyback on its base class's versions of the class attributes? The answer is that under the scheme outlined above, the derived class would not have its own state data. As before, whether you consider this a good thing or a bad one depends on the semantics of the classes involved. The cleanest, sanest, simplest way to address per-class state in a lexical is for the derived class to override its base class's version of the method that accesses the class attributes. Since the actual method called is the one in the object's derived class if this exists, you automatically get per-class state this way. Any urge to provide an unadvertised method to sneak out a reference to the %ClassData hash should be strenuously resisted. As with any other overridden method, the implementation in the derived class always has the option of invoking its base class's version of the method in addition to its own. Here's an example:

    package Another_Class;

    @ISA = qw(Some_Class);

    my %ClassData = (

        CData1 => "",

    );

    sub CData1 {

        my($self, $newvalue) = @_;

        if (@_ > 1) {

            # set locally first

            $ClassData{CData1} = $newvalue;

            # then pass the buck up to the first

            # overridden version, if there is one

            if ($self->can("SUPER::CData1")) {

                $self->SUPER::CData1($newvalue);

            }

        }

        return $ClassData{CData1};

    }

Those dabbling in multiple inheritance might be concerned about there being more than one override.

    for my $parent (@ISA) {

        my $methname = $parent . "::CData1";

        if ($self->can($methname)) {

            $self->$methname($newvalue);

        }

    }

Because the &UNIVERSAL::can method returns a reference to the function directly, you can use this directly for a significant performance improvement:

    for my $parent (@ISA) {

        if (my $coderef = $self->can($parent . "::CData1")) {

            $self->$coderef($newvalue);

        }

    }

If you override UNIVERSAL::can in your own classes, be sure to return the reference appropriately.

Locking the Door and Throwing Away the Key

As currently implemented, any code within the same scope as the file-scoped lexical %ClassData can alter that hash directly. Is that ok? Is it acceptable or even desirable to allow other parts of the implementation of this class to access class attributes directly? That depends on how careful you want to be. Think back to the Cosmos class. If the &supernova method had directly altered $Cosmos::Stars or $Cosmos::Cosmos{stars}, then we wouldn't have been able to reuse the class when it came to inventing a Multiverse. So letting even the class itself access its own class attributes without the mediating intervention of properly designed accessor methods is probably not a good idea after all. Restricting access to class attributes from the class itself is usually not enforceable even in strongly object-oriented languages. But in Perl, you can. Here's one way:

    package Some_Class;

    {  # scope for hiding $CData1

        my $CData1;

        sub CData1 {

            shift;      # XXX: unused

            $CData1 = shift if @_;

            return $CData1;

        }

    }

    {  # scope for hiding $CData2

        my $CData2;

        sub CData2 {

            shift;      # XXX: unused

            $CData2 = shift if @_;

            return $CData2;

        }

    }

No one--absolutely no one--is allowed to read or write the class attributes without the mediation of the managing accessor method, since only that method has access to the lexical variable it's managing. This use of mediated access to class attributes is a form of privacy far stronger than most OO languages provide. The repetition of code used to create per-datum accessor methods chafes at our Laziness, so we'll again use closures to create similar methods.

    package Some_Class;

    {  # scope for ultra-private meta-object for class attributes

        my %ClassData = (

            CData1 => "",

            CData2 => "",

        );

        for my $datum (keys %ClassData ) {

            no strict "refs";

            *$datum = sub {

                use strict "refs";

                my ($self, $newvalue) = @_;

                $ClassData{$datum} = $newvalue if @_ > 1;

                return $ClassData{$datum};

            }

        }

    }

The closure above can be modified to take inheritance into account using the &UNIVERSAL::can method and SUPER as shown previously.

Translucency Revisited

The Vermin class demonstrates translucency using a package variable, eponymously named %Vermin, as its meta-object. If you prefer to use absolutely no package variables beyond those necessary to appease inheritance or possibly the Exporter, this strategy is closed to you. That's too bad, because translucent attributes are an appealing technique, so it would be valuable to devise an implementation using only lexicals. There's a second reason why you might wish to avoid the eponymous package hash. If you use class names with double-colons in them, you would end up poking around somewhere you might not have meant to poke.

    package Vermin;

    $class = "Vermin";

    $class->{PopCount}++;

    # accesses $Vermin::Vermin{PopCount}

    package Vermin::Noxious;

    $class = "Vermin::Noxious";

    $class->{PopCount}++;

    # accesses $Vermin::Noxious{PopCount}

In the first case, because the class name had no double-colons, we got the hash in the current package. But in the second case, instead of getting some hash in the current package, we got the hash %Noxious in the Vermin package. (The noxious vermin just invaded another package and sprayed their data around it. Perl doesn't support relative packages in its naming conventions, so any double-colons trigger a fully-qualified lookup instead of just looking in the current package. In practice, it is unlikely that the Vermin class had an existing package variable named %Noxious that you just blew away. If you're still mistrustful, you could always stake out your own territory where you know the rules, such as using Eponymous::Vermin::Noxious or Hieronymus::Vermin::Boschious or Leave_Me_Alone::Vermin::Noxious as class names instead. Sure, it's in theory possible that someone else has a class named Eponymous::Vermin with its own %Noxious hash, but this kind of thing is always true. There's no arbiter of package names. It's always the case that globals like @Cwd::ISA would collide if more than one class uses the same Cwd package. If this still leaves you with an uncomfortable twinge of paranoia, we have another solution for you. There's nothing that says that you have to have a package variable to hold a class meta-object, either for monadic classes or for translucent attributes. Just code up the methods so that they access a lexical instead. Here's another implementation of the Vermin class with semantics identical to those given previously, but this time using no package variables.

    package Vermin;

    # Here's the class meta-object, eponymously named.

    # It holds all class data, and also all instance data

    # so the latter can be used for both initialization

    # and translucency.  it's a template.

    my %ClassData = (

        PopCount => 0,          # capital for class attributes

        color    => "beige",    # small for instance attributes

    );

    # constructor method

    # invoked as class method or object method

    sub spawn {

        my $obclass = shift;

        my $class   = ref($obclass) || $obclass;

        my $self = {};

        bless($self, $class);

        $ClassData{PopCount}++;

        # init fields from invoking object, or omit if

        # invoking object is the class to provide translucency

        %$self = %$obclass if ref $obclass;

        return $self;

    }

    # translucent accessor for "color" attribute

    # invoked as class method or object method

    sub color {

        my $self  = shift;

        # handle class invocation

        unless (ref $self) {

            $ClassData{color} = shift if @_;

            return $ClassData{color}

        }

        # handle object invocation

        $self->{color} = shift if @_;

        if (defined $self->{color}) {  # not exists!

            return $self->{color};

        } else {

            return $ClassData{color};

        }

    }

    # class attribute accessor for "PopCount" attribute

    # invoked as class method or object method

    sub population {

        return $ClassData{PopCount};

    }

    # instance destructor; invoked only as object method

    sub DESTROY {

        $ClassData{PopCount}--;

    }

    # detect whether an object attribute is translucent

    # (typically?) invoked only as object method

    sub is_translucent {

        my($self, $attr)  = @_;

        $self = \%ClassData if !ref $self;

        return !defined $self->{$attr};

    }

    # test for presence of attribute in class

    # invoked as class method or object method

    sub has_attribute {

        my($self, $attr)  = @_;

        return exists $ClassData{$attr};

    }

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NOTES

Inheritance is a powerful but subtle device, best used only after careful forethought and design. Aggregation instead of inheritance is often a better approach. You can't use file-scoped lexicals in conjunction with the SelfLoader? or the AutoLoader? , because they alter the lexical scope in which the module's methods wind up getting compiled. The usual mealy-mouthed package-munging doubtless applies to setting up names of object attributes. For example, $self->{ObData1} should probably be $self->{ PACKAGE . "_ObData1" }, but that would just confuse the examples.

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SEE ALSO

perltoot, perlobj, perlmod, and perlbot. The Tie::SecureHash and Class::Data::Inheritable modules from CPAN are worth checking out.

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AUTHOR AND COPYRIGHT

Copyright (c) 1999 Tom Christiansen. All rights reserved. This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself. Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.

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ACKNOWLEDGEMENTS

Russ Allbery, Jon Orwant, Randy Ray, Larry Rosler, Nat Torkington, and Stephen Warren all contributed suggestions and corrections to this piece. Thanks especially to Damian Conway for his ideas and feedback, and without whose indirect prodding I might never have taken the time to show others how much Perl has to offer in the way of objects once you start thinking outside the tiny little box that today's ``popular'' object-oriented languages enforce.

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HISTORY

Last edit: Sun Feb 4 20:50:28 EST 2001

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