Dallaylaen has asked for the wisdom of the Perl Monks concerning the following question:

Hello esteemed monks,

Say I have an object that has a construction phase that ends via a finalize() method. After that, no more modifications may be made.

Currently it is implemented via a lock within the object. However, I'm tempted to do a bless $self, "My::Module::Readonly"; and remove mutators from that class (or make them dying stubs).

This looks simple and efficient, but blessing an object into another package smells like a hack and/or abuse of Perl object system. What are the possible problems/pitfalls with such approach?

One thing that comes to mind is extending the initial class. Are there more?

Replies are listed 'Best First'.
Re: Problems and pitfalls with reblessing?
by choroba (Cardinal) on Nov 22, 2018 at 08:58 UTC
    If both the source and target classes are yours, I don't see a big problem in reblessing between them. Just make sure you don't carry any unwanted internal state from the previous class.

    On the other hand, if you're using Mo*, there might be cleaner ways how to achieve the same, e.g. applying roles to an instance at runtime: 

    'Moo::Role'->apply_roles_to_object($self, $read_only_class);

    [download]

    It reblesses the object behind the scenes, anyway. To disable the writers, you probably need to around them.

    ($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

    [download]
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[d/l]
[select]
      If you apply a role,doesnt that comes as an addition to the object's properties/methods ? Or do you mean that the role has properties with the same name as in the class so they overwrite them ?
[reply]
        Changing the properties of the attributes works in the role: 
        #!/usr/bin/perl
use warnings;
use strict;

{   package MyRole;
    use Moo::Role;
    has +x => (is => 'ro');
}

{   package MyObj;
    use Moo;
    has x => (is => 'rw');

    sub finalize {
        'Moo::Role'->apply_roles_to_object($_[0], 'MyRole');
    }
}

my $o = 'MyObj'->new;
$o->x(12);

$o->finalize;
$o->x(13);  # x is a read-only accessor

        [download]

        Another way is to wrap the accessor: 

        {   package MyRole;
    use Moo::Role;
    requires 'x';
    before x => sub { die "x is read only" if @_ > 1 };
}

        [download]

        ($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

        [download]
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Re: Problems and pitfalls with reblessing?
by Anonymous Monk on Nov 22, 2018 at 07:56 UTC
    why buy a shotgun when asking nicely is enough?
[reply]
      In most cases, yes. But, as a module user, I'd prefer it dying on erroneous operation rather than "asking politely".
[reply]

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