I am writing a file manager for my Perl application. Information about each file is kept as an object. When I remove a file, I'd like to change the corresponding object's class to RemovedFile. For this class, call to any valid method of "File" would return a fatal error and a stack trace. This would be to catch cases where some stale reference to this object is kept (while it shouldn't).
I thought about two ways I could implement this:
"RemovedFile" inherits from "File" and redefines all its methods with a call to fatal error. Downside to this is that if I add a new method to "File" I need to add it to "RemovedFile" as well.
Adding a call to some empty method to every method of "File". "RemovedFile" would redefine this one method to report fatal error. (See code below for an example of what I mean). Downside to this is that every method of "File" would have to be bothered with calling the "remove_guard" in the beginning which IMO is not very clean.
# Inside File class:
sub any_method_of_file_class {
$self->_removed_guard();
#rest of code
}
sub _removed_guard {
#do nothing
}
# Inside RemovedFile class redefine only _removed_guard:
sub _removed_guard {
$self->{logger}->fatal_with_stack_trace();
}
I wanted to ask if there is any better way to implement this kind of behaviour in Perl?
For example, could I use some tricks to first list and then dynamically redefine all methods of a parent class without specifying their exact names?
You can define the RemovedFile class without any relation to the original File class. In RemovedFile, use AUTOLOAD to handle any method.
If you want to redefine (override) all the methods of the parent class, don't inherit from the parent class. You can use the same interface, but you don't need the connection to some module you are going to completely ignore.
I think I'd probably have a factory method in File that returns a new object for RemovedFile:
my $removed_file_obj = $file->remove;
That new class only knows what it needs to know about removed files. The remove can do whatever cleanup you require.
Then, when you are dealing with lists of objects, some of which may be File and some of which may be RemovedFile, filter the ones you want. This is outside of the class definitions because the class only defines the behavior of the objects and not how we employ the objects.
Here's one way to check by the object type, maybe even with the new isa feature:
use v5.32;
use experimental qw(isa);
foreach my $file ( #files ) {
# next if $file->isa( 'RemovedFile' );
next if $file isa 'RemovedFile';
...
}
But, you probably shouldn't check what something is. Check what it can do. Since you haven't inherited a bunch of methods that don't do anything, can should return false for that:
foreach my $file ( #files ) {
next if $file->can( 'some_method_not_in_RemovedFile' );
...
}
Related
I wish I knew the reference or document that told me that this is the best way to create a new object in Perl:
sub new {
my $package = shift;
my $class = ref($package) || $package
This is how I've been creating objects for years but now I'm wondering why go to the trouble? With the exception of some edge cases why not simply do the following:
sub new {
shift; # get rid of the object or package name
my $class = __PACKAGE__;
Are there any issues with simply doing using __PACKAGE__ when there's no special reason to try to detect what namespace the 'new' method is being called in?
Inheritance
# classA.pm
package ClassA;
sub new { $pkg = ref($_[0]) || $_[0] ; bless { foo => 42 }, $pkg }
# classB.pm
package ClassB;
use parent 'ClassA';
sub foo { ... }
# main.pl
use ClassA;
use ClassB;
$B = ClassB->new();
print $B->foo();
In this example, ClassB inherits methods from ClassA, including its constructor. But we still want to identify the object as belonging to ClassB, so the constructor in ClassA must respect the name of the reference passed to its constructor.
Easier and safer than
$B = bless { ClassA->new(), "ClassB" }; # or
$B = bless { ClassB->new(), "ClassA" };
or adding a pass-though constructor in ClassB.
package ClassB;
use parent 'ClassA';
sub new { bless { ClassA::new(#_), __PACAKGE__ } }
The class name is already the first argument to the constructor, so why not use that? You don't need to reach for anything and if you decide that the situation is more complex than you originally envisioned, you haven't artificially caused a speed bump. This code works with or without inheritance:
sub new {
my( $class, #args ) = #_;
...
bless {...}, $class;
}
Consider with anything that you program how much you'd have to change if the code situation changed. Maybe you add a couple of extra steps that you don't immediately need, but that keeps the code flexible for those times when you realize you actually needed those cases. In your case, you actually have to do extra work to ignore the invocant argument that Perl specifically provides to tell you which class is trying to mak a new object.
For example, you create your class, it works for you and does its job well. It works so well that you share it and someone else uses it and they are mostly happy with it for awhile until they need this one little modification. They should be able to subclass your module (so, no changes to it or for you) to extend or override a method. But, your code doesn't allow that because you blessed the object in a more restrictive way that didn't add any benefit.
That first bit of code in your question is using the first argument too, but it's not really a prescription for constructors. It's doing an extra thing by allowing an already existing object create a new one. The ref extracts the blessed package name from the object and uses that for the new object.
I don't particularly like that way of creating new objects and I think the interface will likely confuse people. What should happen when you call new on an existing object? Do you clone the existing one? If so, there are better names, such as clone (Ruby uses dup). Do you get a completely fresh object? If so, why do you go through an existing object to get one completely unrelated to it?
There was a time that many examples of OO Perl showed that same sort of constructor, and it was copy and pasted into many places. But we've learned a lot since then. Maybe you have a good answer why you'd allow that, but so far I haven't heard a compelling one.
I would like you to share your experience on the matter of using some common object by different components of a library (or an application). I'd like to understand, which practice is better, which way is more "ecological".
Let's imagine that we have some complex library that consists of a bunch of classes: Library, Library::Foo, Library::Foo::Bar and many other ones. An application that uses the library initializes a Library-classed object, at some stage this object initializes some quantity of Library::Foo-classed objects, they initialize Library::Foo::Bar-classed objects and so on.
And now let's imagine that there is the Library::Logger class that is being used by all components. It's being initialized only once by the constructor of the Library class and all other sub-classes need it for their logging purposed.
The question is: how to let them all have the reference to this object?
I see the following ways to acheive it:
Passing it to the constructor
When a Library-based object initializes a Library::Foo-classed object, it passes the reference to the logger to the newly-created object as a parameter:
$self->_set_logger(Library::Logger->new());
my $logger = $self->_get_logger;
my $foo = Library::Foo->new(logger => $logger);
So the Library::Foo-classed object could get the logger and even pass it to a Library::Foo::Bar-classed object when it's needed:
my $logger = $self->_get_logger;
my $bar = Library::Foo::Bar(logger => $logger);
Accessing it as a method
When a Library-based object initializes the Library::Foo-based object, it passes the reference to itself to the newly-created object:
$self->_set_logger(Library::Logger->new());
my $foo = Library::Foo->new(papa => $self);
So the Library::Foo-based object could access the logger this way:
$my logger = $self->_get_papa->_get_logger;
my $bar = Library::Foo::Bar(papa => $self);
It breaks encapsulation, but it also lets the Library::Foo-based object to get other data from its "papa".
Using a global variable
Also we can store the reference to the logger in some global variable, so any other object could access it this way:
$logger = $Library::_LOGGER;
Well, it really sucks, I know... :)
...
So, what do you think, which way seems to be better? What alternative way would you propose? How do you solve this problem for yourself? Would you pass the data to the "child" manually or would you let it to get the data from the "parent"?
Many thanks in advance!
There is no answer to your problem. It realy depends on your exact use-case. If (just guessing) you only want to have exactly one Logger in your Library, I think using the software pattern of a "singleton" might exactly fit. E.g.
package Library::Logger;
use strict;
my $logger;
sub new {
unless ($logger) {
.... initializing $logger ...
}
return $logger;
}
and in each of your sublclasses you can access it by
Library::Logger->new()->...
HTH
Georg
I have a base class, named Foo::Base, I need to inherit its methods, like 'new' and to import some subroutines names in a scope:
package Foo::Base;
sub new { ... }
sub import {
no strict 'refs';
my $caller = caller;
*{"${caller}::my_sub"} = sub { 1 };
}
1;
So, I need to use this base class in my second class, Foo::Child:
use base 'Foo::Base';
... and it works for inheritance, but it doesn't import 'my_sub' in a scope. I can add string
use Foo::Base;
for it and it helps, but I don't want to write something like this:
use base 'Foo::Base';
use Foo::Base;
This is looks kind of wierd... Is there any suggestions for this problem?
There's two reasons one might want to do what you're doing, both of them are bad.
First is you're trying to import methods from your parent class... for some reason. Maybe you misunderstand how OO works. You don't need to do that. Just call inherited methods as methods and unless those methods are doing something wacky it will work fine.
More likely is this a mixed-use module where some of it is methods and some of it is imported functions. And for that you can do...
use base 'Foo::Base';
use Foo::Base;
And you rightly observed that it looks kind of weird... because it is kind of weird. A class that also exports is mixing idioms, and that's going to result in weird usage patterns.
Best thing to do is to redesign the class to instead of exporting functions, either split the functions out into their own module, or make them class methods. If the functions really don't have much to do with the class, then its best to spin them off. If they do relate to the class, then make them class methods.
use base 'Foo::Base';
Foo::Base->some_function_that_used_to_be_exported;
This eliminates the interface mismatch, and as a bonus, subclasses can override class method behavior just like any other method.
package Bar;
use base 'Foo::Base';
# override
sub some_function_that_used_to_be_exported {
my($class, #args) = #_;
...do something extra maybe...
$class->SUPER::some_function_that_used_to_be_exported(#args);
...and maybe something else...
}
If you don't have control over the base class, you can still make the interface sane by writing a subclass which turns the exported functions into methods.
package SaneFoo;
use base 'Foo::Base';
# For each function exported by Foo::Base, create a wrapper class
# method which throws away the first argument (the class name) and
# calls the function.
for my $name (#Foo::Base::EXPORT, #Foo::Base::EXPORT_OK) {
my $function = Foo::Base->can($name);
*{$name} = sub {
my $class = shift;
return $function->(#_);
};
}
When you write use base, you're using the facilities of the base module. And you're passing it the parameter of of the module which you want to be your base class.
In an OO IS-A relationship, no import needed. You call the methods with the OO-pattern: $object_or_class->method_name( #args ). Sometimes that means that you don't care who the invocant is, like so:
sub inherited_util {
my ( undef, #args ) = #_;
...
}
or
sub inherited2 {
shift;
...
}
However, if you want to use utilities defined in the base module and inherit from the class behavior defined in that module, then that's exactly what the two use statements indicate.
Still, if you have two different types of behavior you want to use in modules, it's probably better to split the utility type things off into their own module, and use it from both modules. Either way, explicit behavior is often better than implicit.
However, I have used this pattern before:
sub import {
shift;
my ( $inherit_flag ) = #_;
my $inherit
= lc( $inherit_flag ) ne 'inherit' ? 0
: shift() && !!shift() ? 1
: 0
;
if ( $inherit ) {
no strict 'refs';
push #{caller().'::ISA'}, __PACKAGE__;
...
}
...
}
that way, I make one call with an explicit bundling of usages.
use UtilityParent inherit => 1, qw<normal args>;
To clarify a little more the question of "but why doesn't the import in Foo::Child work?", here's a program that will spell out what's actually happening:
use Foo::Child;
print my_sub(); # Really? Yes, really!
print Foo::Child::my_sub()," done\n";
and added this to the Foo::Base::import() routine:
print "Caller is $caller\n";
When you run this, you see this as the output:
Caller is main
1
Undefined subroutine &Foo::Child::my_sub called at foo_user.pl line 4.
We see Foo::Base reporting in, letting us know who the caller is: it's the main program! We see the '1', which proves that yes, main::my_sub now exists, and then the failure because the import went to the wrong namespace.
Why is this? Because the import process is all being handled by the main program. Foo::Base's import doesn't get called by anything in Foo::Child; it's getting called by the main program in the process of loading the modules. If you really, really want to force subs, as opposed to methods, to import into Foo::Child, you need to explicitly make it happen in Foo::Child itself. The 'use Foo::Base' will do it, as that makes the import execute with Foo::Child as the caller; if you insist on the import, but the double use squicks you out too much, you can call Foo::Base::import() right after the 'use base'. This is exactly what the double 'use' does anyway.
Nevertheless, I like Schwern's class methods better, and I recommend that alternative.
I want to create a generic class, whose builder would not return an instance of this generic class, but an instance of a dedicated child class.
As Moose does automatic object building, I do not get to understand if this something possible, and how to create a Moose class with Moose syntax and having this behaviour.
e.g.:
The user asks: $file = Repository->new(uri=>'sftp://blabla') .... and is returned an `Repository::_Sftp`` instance
User would use $file as if it is a Repository instance, without the need to know the real subclass (polymorphism)
Note:
As requested, maybe i should have been more clear about what i was trying to achieve:
The purpose of my class is to be able to add new Repository schemes (eg over sftp), by simply creating an "hidden" Repository::_Stfp class, and adding a case in the Repository constructor to factory the correct specialized object depending of url. Repository would be like a virtual base class, providing an interface that specialized objects would implement.
All of this is for adding new repository schemes without having the rest of the program to be modified: it would unknowingly deal with the specialized instance as if it is a Repository instance.
new builds the builder. You want some other method to actually return the built object.
Here's an example:
class RepositoryBuilder {
has 'allow_network_repositories' => (
is => 'ro',
isa => 'Bool',
required => 1,
);
method build_repository(Uri $url) {
confess 'network access is not allowed'
if $url->is_network_url && !$self->allow_network_repositories;
my $class = $self->determine_class_for($url); # Repository::Whatever
return $class->new( url => $url );
}
}
role Repository { <whatever }
class Repository::File with Repository {}
class Repository::HTTP with Repository {}
Here, the builder and the built object are distinct. The builder is a
real object, complete with parameters, that can be customized to build
objects as the situation demands. Then, the "built" objects are
merely return values of a method. This allows you to build other
builders depending on the situation. (A problem with builder
functions is that they are very inflexible -- it's hard to teach them
a new special case. This problem still exists with a builder object,
but at least your application can create a subclass, instantiate it,
and pass this object to anything that needs to create objects. But
dependency injection is a better approach in this case.)
Also, there is no need for the repositories you build to inherit from
anything, they just need a tag indicating that they are repositories.
And that's what our Repository role does. (You will want to add the
API code here, and any methods that should be reused. But be careful
about forcing reuse -- are you sure everything tagged with the
Repository role will want that code? If not, just put the code in
another role and apply that one to the classes that require that
functionality.)
Here's how we use the builder we created. If, say, you don't want to
touch the network:
my $b = RepositoryBuilder->new( allow_network_repositories => 0 );
$b->build_repository( 'http://google.com/' ); # error
$b->build_repository( 'file:///home/whatever' ); # returns a Repository::Foo
But if you do:
my $b = RepositoryBuilder->new( allow_network_repositories => 1 );
$b->build_repository( 'http://google.com/' ); # Repository::HTTP
Now that you have a builder that builds the objects the way you like,
you just need to use these objects in other code. So the last piece
in the puzzle is referring to "any" type of Repository object in other
code. That's simple, you use does instead of isa:
class SomethingThatHasARepository {
has 'repository' => (
is => 'ro',
does => 'Repository',
required => 1,
);
}
And you're done.
No (not directly). In general in Moose, calling CLASS->new where CLASS isa Moose::Object will return an instance of CLASS.
Can you describe in more detail what you are trying to achieve, and why you think this is what you want? You probably want to build a factory class -- when you call a method on it, it will call the appropriate class's constructor and return that object to you, without you having to be concerned with the particular type you get back:
package MyApp::Factory::Repository;
sub getFactory
{
my ($class, %attrs);
# figure out what the caller wants, and decide what type to return
$class ||= 'Repository::_Sftp';
return $class->new(attr1 => 'foo', attr2 => 'bar', %attrs);
}
my $file = MyApp::Factory::Repository->getFactory(uri=>'sftp://blabla');
What's the best practice for implementing Singletons in Perl?
You can use the Class::Singleton module.
A "Singleton" class can also be easily implemented using either my or state variable (the latter is available since Perl 5.10). But see the #Michael's comment below.
package MySingletonClass;
use strict;
use warnings;
use feature 'state';
sub new {
my ($class) = #_;
state $instance;
if (! defined $instance) {
$instance = bless {}, $class;
}
return $instance;
}
If you're using Moose, then MooseX::Singleton. Its interface is compatible with Class::Singleton.
Singleton Summary:
Most of the time a normal object will work.
Be careful with singletons.
Localize interaction as much as possible
While singletons are a nice idea, I tend to just implement a normal object and use it. If it is critical that I only have one such object, I'll modify the constructor to throw a fatal exception when the second object is created. The various singleton modules don't seem to do much besides add a dependency.
I do this because it is easy, it works, and when in some weird future I need to work with a second object in my app, the changes are minimized.
I also like to localize interaction with my 'singleton' objects--keep interaction in as few places as possible. So instead of every object having direct access to the singleton, I mediate all interaction through my "Application" object. Whenever possible, the application object gets data from the 'singleton', and passes it as a parameter to the method in the other objects. Responses from other objects may also be munged and passed to the 'singleton'. All this effort helps when I need to make changes in the 'singleton' object, and when I want to reuse other objects in another app that may not need or be able to use the original 'singleton' object.