Premise
The question is difficult to understand, due to a misunderstanding of the OO logic in perl by me, the OP. The comments can be useful to understand it.
Original question
The Class::Accessor module is extremely convenient for me, but I can't find any documentation about how to write a constructor such that I can, for instance, derive the values for a field out of some computation.
The closest thing I can think of, with the given documentation, is passing trough a "sort of" override:
package FooHack;
...
use Class::Accessor 'antlers';
has something => ( is => 'ro' );
# Methods here...
package Foo; # Foo is a plain module, not a class.
sub new {
my $macguffin = &crunch_chop_summon;
FooHack->new({something => $macguffin });
}
This kinda works, except my $f = Foo->new(); say ref $f will yield FooHack instead of Foo.
So my questions are:
Is my idea good enough or do you see some possible issues with it? Or maybe some improvements?
Is there a better way of doing the same thing?
Edit:
This is NOT an actual override. Foo is nowhere a class. It's just a plain module declaring a sub new. Plus, the module FooHack is not an external module. It is defined within the very same file.
The module Foo pretends to be a class in that it follows the convention of having a sub new, while new is actually just a function which calls the real constructor, FooHack->new and passes some initialization value for it.
TL;DR use Moose instead of use Class::Accessor will help a lot, and you shouldn't have to change your has definitions
As I wrote in my comment, you are asking Class::Accessor — a module that easily creates accessor methods — to provide the full quorum of object-oriented features
I also think your thoughts about object-oriented inheritance are confused. I don't see anything wrong in what you have written, but having Foo as a subclass of FooHack is wrong thinking, and confused me as well as probably many others
There should be a Foo base class and potentially multiple subclasses, like FooHack, FooPlus, FooOnHorseback etc.
Well, it's a little bit nasty to tamper with a module like that - if it's not doing what you want, then normally you'd just write a new one.
You can take a class and extend it to make a new class - that's what you'd normally do if the class in question doesn't do what you need. Applying a hack to override the constructor is subverting the expectations of the class maintainer, and the road to brittle code, which is why you can't do it easily.
That said - normally as part of a constructor you'll call bless to instantiate the object into a class. By convention, this is done into the current class, using the new method. But there's no real reason you can't:
my $self = {};
bless ( $self, 'Foo' );
Just bear in mind that if your constructor doesn't do things this object is expecting to have happened, then you might break things.
Related
Sorry to bother the community for this but I have unfortunately to code in Perl :'(. It is about an OO perl code I want to understand but I am failing to put all the pieces together.
The following is a template of code that represents somehow what I am currently looking at. The following is the class MyClass:
package Namespace::MyClass;
sub new($)
{
my ($class) = #_;
$self = { };
bless ($self, $class);
}
sub init($$)
{
my ($self, $param1) = #_;
$self->{whatever} = ($param1, $param1, $param1);
}
and then the following is a script.pl that supposedly uses the class:
#!/path/to/your/perl
require Namespace::MyClass;
my myClass = new Namespace::MyClass()
myClass->init("data_for_param1");
There may be error but I am interested more in having the following questions answered than having my possibly wrong code corrected:
Questions group 1 : "$" in a sub definition means I need to supply one parameter, right? If so, why does new ask for one and I do not supply it? Has this to do with the call in the script using () or something similar to how Python works (self is implied)?
Question group 2 : is for the same previous reason that the init subroutine (here a method) declares to expect two parameters? If so, is the blessing in some way implying a self is ever passed for all the function in the module?
I ask this because I saw that in non blessed modules one $ = one parameter.
Thank you for your time.
QG1:
Prototypes (like "$") mean exactly nothing in Method calls.
Method calls are not influenced by prototypes either, because the function to be called is indeterminate at compile time, since the exact code called depends on inheritance.
Most experienced Perl folk avoid prototypes entirely unless they are trying to imitate a built-in function. Some PHBs inexperienced in Perl mandate their use under the mistaken idea that they work like prototypes in other languages.
The 1st parameter of a Method call is the Object (Blessed Ref) or Class Name (String) that called the Method. In the case of your new Method that would be 'Namespace::MyClass'.
Word to the wise: Also avoid indirect Method calls. Rewrite your line using the direct Method call as follows: my $myClass = Namespace::MyClass->new;
QG2:
Your init method is getting $myClass as it's 1st parameter because it is what 'called' the method. The 2nd parameter is from the parameter list. Blessing binds the name of the Class to the Reference, so that when a method call is seen, It knows which class in which to start the search for the correct sub. If the correct sub is not immediately found, the search continues in the classes named in the class's #ISA array.
Don't use prototypes! They don't do what you think they do.
Prototypes in Perl are mainly used to allow functions to be defined without the use of parentheses or to allow for functions that take array references to use the array name like pop or push do. Otherwise, prototypes can cause more trouble and heartbreak than experienced by most soap opera characters.
is what you actually want to do validate parameters? if so then that is not the purpose of prototypes. you could try using signatures, but for some reason they are new and still experimental. some consider lack of a stable signatures feature to be a flaw of perl. the alternatives are CPAN and writing code in your subs/methods that explicitly validate the params.
In my current job I'm building a suite of Perl scripts that depend heavily on objects. (using Perl's bless() on a Hash to get as close to OO as possible)
Now, for lack of a better way of putting this, most programmers at my company aren't very smart. Worse, they don't like reading documentation and seem to have a problem understanding other people's code. Cowboy coding is the game here. Whenever they encounter a problem and try to fix it, they come up with a horrendous solution that actually solves nothing and usually makes it worse.
This results in me, frankly, not trusting them with code written in duck typed language. As an example, I see too many problems with them not getting an explicit error for misusing objects. For instance, if type A has member foo, and they do something like, instance->goo, they aren't going to see the problem immediately. It will return a null/undefined value, and they will probably waste an hour finding the cause. Then end up changing something else because they didn't properly identify the original problem.
So I'm brainstorming for a way to keep my scripting language (its rapid development is an advantage) but give an explicit error message when an object isn't used properly. I realize that since there isn't a compile stage or static typing, the error will have to be at run time. I'm fine with this, so long as the user gets a very explicit notice saying "this object doesn't have X"
As part of my solution, I don't want it to be required that they check if a method/variable exists before trying to use it.
Even though my work is in Perl, I think this can be language agnostic.
If you have any shot of adding modules to use, try Moose. It provides pretty much all the features you'd want in a modern programming environment, and more. It does type checking, excellent inheritance, has introspection capabilities, and with MooseX::Declare, one of the nicest interfaces for Perl classes out there. Take a look:
use MooseX::Declare;
class BankAccount {
has 'balance' => ( isa => 'Num', is => 'rw', default => 0 );
method deposit (Num $amount) {
$self->balance( $self->balance + $amount );
}
method withdraw (Num $amount) {
my $current_balance = $self->balance();
( $current_balance >= $amount )
|| confess "Account overdrawn";
$self->balance( $current_balance - $amount );
}
}
class CheckingAccount extends BankAccount {
has 'overdraft_account' => ( isa => 'BankAccount', is => 'rw' );
before withdraw (Num $amount) {
my $overdraft_amount = $amount - $self->balance();
if ( $self->overdraft_account && $overdraft_amount > 0 ) {
$self->overdraft_account->withdraw($overdraft_amount);
$self->deposit($overdraft_amount);
}
}
}
I think it's pretty cool, myself. :) It's a layer over Perl's object system, so it works with stuff you already have (basically.)
With Moose, you can create subtypes really easily, so you can make sure your input is valid. Lazy programmers agree: with so little that has to be done to make subtypes work in Moose, it's easier to do them than not! (from Cookbook 4)
subtype 'USState'
=> as Str
=> where {
( exists $STATES->{code2state}{ uc($_) }
|| exists $STATES->{state2code}{ uc($_) } );
};
And Tada, the USState is now a type you can use! No fuss, no muss, and just a small amount of code. It'll throw an error if it's not right, and all the consumers of your class have to do is pass a scalar with that string in it. If it's fine (which it should be...right? :) ) They use it like normal, and your class is protected from garbage. How nice is that!
Moose has tons of awesome stuff like this.
Trust me. Check it out. :)
In Perl,
make it required that use strict and use warnings are on in 100% of the code
You can try to make an almost private member variables by creating closures. A very good example is "Private Member Variables, Sort of " section in http://www.usenix.org/publications/login/1998-10/perl.html . They are not 100% private but fairly un-obvious how to access unless you really know what you're doing (and require them to read your code and do research to find out how).
If you don't want to use closures, the following approach works somewhat well:
Make all of your object member variables (aka object hash keys in Perl) wrapped in accessors. There are ways to do this efficiently from coding standards POV. One of the least safe is Class::Accessor::Fast. I'm sure Moose has better ways but I'm not that familiar with Moose.
Make sure to "hide" actual member variables in private-convention names, e.g. $object->{'__private__var1'} would be the member variable, and $object->var1() would be a getter/setter accessor.
NOTE: For the last, Class::Accessor::Fast is bad since its member variables share names with accessors. But you can have very easy builders that work just like Class::Accessor::Fast and create key values such as $obj->{'__private__foo'} for "foo".
This won't prevent them shooting themselves in the foot, but WILL make it a lot harder to do so.
In your case, if they use $obj->goo or $obj->goo(), they WOULD get a runtime error, at least in Perl.
They could of course go out of their way to do $obj->{'__private__goo'}, but if they do the gonzo cowboy crap due to sheer laziness, the latter is a lot more work than doing the correct $obj->foo().
You can also have a scan of code-base which detects $object->{"_ type strings, though from your description that might not work as a deterrent that much.
You can use Class::InsideOut or Object::InsideOut which give you true data privacy. Rather than storing data in a blessed hash reference, a blessed scalar reference is used as a key to lexical data hashes. Long story short, if your co-workers try $obj->{member} they'll get a run time error. There's nothing in $obj for them to grab at and no easy way to get at the data except through accessors.
Here is a discussion of the inside-out technique and various implementations.
This may turn out to be an embarrassingly stupid question, but better than potentially creating embarrassingly stupid code. :-) This is an OO design question, really.
Let's say I have an object class 'Foos' that represents a set of dynamic configuration elements, which are obtained by querying a command on disk, 'mycrazyfoos -getconfig'. Let's say that there are two categories of behavior that I want 'Foos' objects to have:
Existing ones: one is, query ones that exist in the command output I just mentioned (/usr/bin/mycrazyfoos -getconfig`. Make modifications to existing ones via shelling out commands.
Create new ones that don't exist; new 'crazyfoos', using a complex set of /usr/bin/mycrazyfoos commands and parameters. Here I'm not really just querying, but actually running a bunch of system() commands. Affecting changes.
Here's my class structure:
Foos.pm
package Foos, which has a new($hashref->{name => 'myfooname',) constructor that takes a 'crazyfoo NAME' and then queries the existence of that NAME to see if it already exists (by shelling out and running the mycrazyfoos command above). If that crazyfoo already exists, return a Foos::Existing object. Any changes to this object requires shelling out, running commands and getting confirmation that everything ran okay.
If this is the way to go, then the new() constructor needs to have a test to see which subclass constructor to use (if that even makes sense in this context). Here are the subclasses:
Foos/Existing.pm
As mentioned above, this is for when a Foos object already exists.
Foos/Pending.pm
This is an object that will be created if, in the above, the 'crazyfoo NAME' doesn't actually exist. In this case, the new() constructor above will be checked for additional parameters, and it will go ahead and, when called using ->create() shell out using system() and create a new object... possibly returning an 'Existing' one...
OR
As I type this out, I am realizing it is perhaps it's better to have a single:
(an alternative arrangement)
Foos class, that has a
->new() that takes just a name
->create() that takes additional creation parameters
->delete(), ->change() and other params that affect ones that exist; that will have to just be checked dynamically.
So here we are, two main directions to go with this. I'm curious which would be the more intelligent way to go.
In general it's a mistake (design-wise, not syntax-wise) for the new method to return anything but a new object. If you want to sometimes return an existing object, call that method something else, e.g. new_from_cache().
I also find it odd that you're splitting up this functionality (constructing a new object, and returning an existing one) not just into separate namespaces, but also different objects. So in general, you're closer with your second approach, but you can still have the main constructor (new) handle a variety of arguments:
package Foos;
use strict;
use warnings;
sub new
{
my ($class, %args) = #_;
if ($args{name})
{
# handle the name => value option
}
if ($args{some_other_option})
{
# ...
}
my $this = {
# fill in any fields you need...
};
return bless $this, $class;
}
sub new_from_cache
{
my ($class, %args) = #_;
# check if the object already exists...
# if not, create a new object
return $class->new(%args);
}
Note: I don't want to complicate things while you're still learning, but you may also want to look at Moose, which takes care of a lot of the gory details of construction for you, and the definition of attributes and their accessors.
It is generally speaking a bad idea for a superclass to know about its subclasses, a principle which extends to construction.[1] If you need to decide at runtime what kind of object to create (and you do), create a fourth class to have just that job. This is one kind of "factory".
Having said that in answer to your nominal question, your problem as described does not seem to call for subclassing. In particular, you apparently are going to be treating the different classes of Foos differently depending on which concrete class they belong to. All you're really asking for is a unified way to instantiate two separate classes of objects.
So how's this suggestion[3]: Make Foos::Exists and Foos::Pending two separate and unrelated classes and provide (in Foos) a method that returns the appropriate one. Don't call it new; you're not making a new Foos.
If you want to unify the interfaces so that clients don't have to know which kind they're talking about, then we can talk subclassing (or better yet, delegation to a lazily-created and -updated Foos::Handle).
[1]: Explaining why this is true is a subject hefty enough for a book[2], but the short answer is that it creates a dependency cycle between the subclass (which depends on its superclass by definition) and the superclass (which is being made to depend on its subclass by a poor design decision).
[2]: Lakos, John. (1996). Large-scale C++ Software Design. Addison-Wesley.
[3]: Not a recommendation, since I can't get a good enough handle on your requirements to be sure I'm not shooting fish in a dark ocean.
It is also a factory pattern (bad in Perl) if the object's constructor will return an instance blessed into more than one package.
I would create something like this. If the names exists than is_created is set to 1, otherwise it is set to 0.. I would merge the ::Pending, and ::Existing together, and if the object isn't created just put that into the default for the _object, the check happens lazily. Also, Foo->delete() and Foo->change() will defer to the instance in _object.
package Foo;
use Moose;
has 'name' => ( is => 'ro', isa => 'Str', required => 1 );
has 'is_created' => (
is => 'ro'
, isa => 'Bool'
, init_arg => undef
, default => sub {
stuff_if_exists ? 1 : 0
}
);
has '_object' => (
isa => 'Object'
, is => 'ro'
, lazy => 1
, init_arg => undef
, default => sub {
my $self = shift;
$self->is_created
? Foo->new
: Bar->new
}
, handles => [qw/delete change/]
);
Interesting answers! I am digesting it as I try out different things in code.
Well, I have another variation of the same question -- the same question, mind you, just a different problem to the same class:subclass creation issue!
This time:
This code is an interface to a command line that has a number of different complex options. I told you about /usr/bin/mycrazyfoos before, right? Well, what if I told you that that binary changes based on versions, and sometimes it completely changes its underlying options. And that this class we're writing, it has to be able to account for all of these things. The goal (or perhaps idea) is to do: (perhaps called FROM the Foos class we were discussing above):
Foos::Commandline, which has as subclasses different versions of the underlying '/usr/bin/mycrazyfoos' command.
Example:
my $fcommandobj = new Foos::Commandline;
my #raw_output_list = $fcommandobj->getlist();
my $result_dance = $fcommandobj->dance();
where 'getlist' and 'dance' are version-dependent. I thought about doing this:
package Foos::Commandline;
new (
#Figure out some clever way to decide what version user has
# (automagically)
# And call appropriate subclass? Wait, you all are telling me this is bad OO:
# if v1.0.1 (new Foos::Commandline::v1.0.1.....
# else if v1.2 (new Foos::Commandline::v1.2....
#etc
}
then
package Foos::Commandline::v1.0.1;
sub getlist ( eval... system ("/usr/bin/mycrazyfoos", "-getlistbaby"
# etc etc
and (different .pm files, in subdir of Foos/Commandline)
package Foos::Commandline::v1.2;
sub getlist ( eval... system ("/usr/bin/mycrazyfoos", "-getlistohyeahrightheh"
#etc
Make sense? I expressed in code what I'd like to do, but it just doesn't feel right, particularly in light of what was discussed in the above responses. What DOES feel right is that there should be a generic interface / superclass to Commandline... and that different versions should be able to override it. Right? Would appreciate a suggestion or two on that. Gracias.
Many years ago I remember a fellow programmer counselling this:
new Some::Class; # bad! (but why?)
Some::Class->new(); # good!
Sadly now I cannot remember the/his reason why. :( Both forms will work correctly even if the constructor does not actually exist in the Some::Class module but instead is inherited from a parent somewhere.
Neither of these forms are the same as Some::Class::new(), which will not pass the name of the class as the first parameter to the constructor -- so this form is always incorrect.
Even if the two forms are equivalent, I find Some::Class->new() to be much more clear, as it follows the standard convention for calling a method on a module, and in perl, the 'new' method is not special - a constructor could be called anything, and new() could do anything (although of course we generally expect it to be a constructor).
Using new Some::Class is called "indirect" method invocation, and it's bad because it introduces some ambiguity into the syntax.
One reason it can fail is if you have an array or hash of objects. You might expect
dosomethingwith $hashref->{obj}
to be equal to
$hashref->{obj}->dosomethingwith();
but it actually parses as:
$hashref->dosomethingwith->{obj}
which probably isn't what you wanted.
Another problem is if there happens to be a function in your package with the same name as a method you're trying to call. For example, what if some module that you use'd exported a function called dosomethingwith? In that case, dosomethingwith $object is ambiguous, and can result in puzzling bugs.
Using the -> syntax exclusively eliminates these problems, because the method and what you want the method to operate upon are always clear to the compiler.
See Indirect Object Syntax in the perlobj documentation for an explanation of its pitfalls. freido's answer covers one of them (although I tend to avoid that with explicit parens around my function calls).
Larry once joked that it was there to make the C++ feel happy about new, and although people will tell you not to ever use it, you're probably doing it all the time. Consider this:
print FH "Some message";
Have you ever wondered my there was no comma after the filehandle? And there's no comma after the class name in the indirect object notation? That's what's going on here. You could rewrite that as a method call on print:
FH->print( "Some message" );
You may have experienced some weirdness in print if you do it wrong. Putting a comma after the explicit file handle turns it into an argument:
print FH, "some message"; # GLOB(0xDEADBEEF)some message
Sadly, we have this goofiness in Perl. Not everything that got into the syntax was the best idea, but that's what happens when you pull from so many sources for inspiration. Some of the ideas have to be the bad ones.
The indirect object syntax is frowned upon, for good reasons, but that's got nothing to do with constructors. You're almost never going to have a new() function in the calling package. Rather, you should use Package->new() for two other (better?) reasons:
As you said, all other class methods take the form Package->method(), so consistency is a Good Thing
If you're supplying arguments to the constructor, or you're taking the result of the constructor and immediately calling methods on it (if e.g. you don't care about keeping the object around), it's simpler to say e.g.
$foo = Foo->new(type => 'bar', style => 'baz');
Bar->new->do_stuff;
than
$foo = new Foo(type => 'bar', style => 'baz');
(new Bar)->do_stuff;
Another problem is that new Some::Class happens at run time. If there is an error and you testing never branches to this statement, you never know it until it happens in production. It is better to use Some::Class->new unless you are doing dynamic programing.
Perl has OOP features, but they are somewhat rarely used. How do you create and use Perl objects with methods and properties?
You should definitely take a look at Moose.
package Point;
use Moose; # automatically turns on strict and warnings
has 'x' => (is => 'rw', isa => 'Int');
has 'y' => (is => 'rw', isa => 'Int');
sub clear {
my $self = shift;
$self->x(0);
$self->y(0);
}
Moose gives you (among other things) a constructor, accessor methods, and type checking for free!
So in your code you can:
my $p = Point->new({x=>10 , y=>20}); # Free constructor
$p->x(15); # Free setter
print $p->x(); # Free getter
$p->clear();
$p->x(15.5); # FAILS! Free type check.
A good starting point is Moose::Manual and Moose::Cookbook
If you just need the basic stuff you can also use Mouse which is not as complete, but without most of the compile time penalty.
Moose, definitely.
package Person;
use Moose;
has age => ( isa => Int, is => 'rw');
has name => ( isa => Str, is => 'rw');
1;
Immediately, you have for free a new() method, and accessor methods for the attributes you just defined with 'has'. So, you can say:
my $person = Person->new();
$person->age(34);
$person->name('Mike');
print $person->name, "\n";
and so on. Not only that, but your accessor methods come type-checked for free (and you can define your own types as well as the standard ones). Plus you get 'extends' for subclassing, 'with' for roles/traits, and all manner of other great stuff that allows you to get on with the real job of writing good robust maintainable OO code.
TMTOWTDI, but this one works.
Currently I use Object::InsideOut whenever I want objects, its quite nice and will give you a lot of features over standard blessed hash objects. Having said that, if I was starting a new project I would seriously look at Moose.
While it is good to read the official PERL documentation, I would NOT recommend trying to role your own object framework, or building objects using hashes, its far to tempting to take the easy road and "peak" directly into the objects "private" variables completely breaking encapsulation, this will come back to bite you when you want to refactor the object.
Perl objects are NOT just blessed hashes. They are blessed REFERENCES. They can be (and most often are) blessed hash references, but they could just as easily be blessed scalar or array references.
The official tutorial on the CPAN site is good.
There's also a good article called Camel POOP at CodeProject.
I highly recommend taking a look at Moose if you want to do OO in Perl. However, it's not very useful if you don't understand what OO in Perl means. To better understand how Perl OO works under the hood, I wrote an overview on my blog: http://augustinalareina.wordpress.com/2010/06/06/an-introduction-to-object-oriented-perl/
From a data structure point of view, an Object is reference with a few extra features. The interpreter knows to treat these special references as Objects because they have been "blessed" with the keyword "bless". Blessed references contain a flag indicating they are an Object. Essentially this means you can define and call methods on them.
For instance if you created a basic hashref, this wouldn't work:
$hashref->foo();
But if you create a blessed hashref (aka an Object) this does work:
$blessed_hashref->foo();
Moose is an excellent module for OOP in Perl because it creates an enforceable OO layer AND automagically handles accessor methods so you don't have to define a bunch of getters and setters. If you're interested in using Devel::Peak to see how the Perl interpreter stores objects, follow the link to the blog entry I posted above.
On one foot, each class is a package; you establish (multiple, if desired) inheritance by setting the package variable #ISA (preferably at compile time); you create an object from an existing piece of data (often, but not always, an anonymous hash used to store instance variables) with bless(REFERENCE [, CLASSNAME]); you call object methods like $obj->methodname(#ARGS) and class methods like "CLASSNAME"->methodname(#ARGS).
Multiple inheritance method resolution order can be altered using mro.
Because this is somewhat minimalistic and doesn't force encapsulation, there are many different modules that provide more or different functionality.