in reply to Blessing with unknown classnames

> What happens when a ref is blessed into a non-existent class. Afterall bless's 2nd argument is a string and not a package qualifier.

You don't reveal the answer on purpose?

The package/class is created.

you can check by inspecting the %main:: stash, where all packages live

  DB<30>  bless [1,2,3] => 'nonexistent'

  DB<31> p $main::{'nonexistent::'}
*main::nonexistent::
  DB<32>

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and I think ° it's not only the typeglob but also the stash %nonexistent:: is created, which will result in several hundreds of bytes of overhead

  DB<32> p *{$main::{'nonexistent::'}}{HASH}
HASH(0x2e7b830)
  DB<33>

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So better don't try to automatically bless into millions of pseudo packages ...

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

PS: According to some statistics this was my 10000st post here ... frightening.

update

°) I couldn't find a pure Perl way to check if the HASH slot of a package:: like entry is filled, without autovivifying that package. It's a very quantum-brainf*ck thing, the observation is changing the result.

Update

For clarification: using bless to solve the semipredicate problem is totally fine in my eyes.

Just be aware that the package will exist. And this globally.

So you should care about a naming convention in your own namespace.

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Re^2: Blessing with unknown classnames
by choroba (Cardinal) on Apr 01, 2021 at 23:44 UTC
    Run 
    perl -we 'bless {}, "A$_" for 1 .. 2000000'

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    and what your memory meter.

    map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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      Nice,

      on my windows 10 box each package seems to eat ~1k of memory: 

      perl -we "sub mem{system qq(tasklist /FI \"PID eq $$\"|findstr perl)};
+ mem();bless {}, qq(A$_) for 1 .. 1000000;mem()"

perl.exe                     29356 Console                    1      6
+.616 K
perl.exe                     29356 Console                    1  1.353
+.656 K

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      and quite the same on my Linux box:

      perl -we 'sub mem{system qq(cat /proc/$$/status | grep ^VmSize)}; mem(
+);bless {}, qq(A$_) for 1 .. 1000000;mem()'

VmSize:       23768 kB
VmSize:     1234168 kB

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      Obviously the package is empty.. 

      
perl -MDevel::Symdump -e "bless {},'notexisting';print Devel::Symdump-
+>new('noexisting')->as_string"

arrays

functions

hashes

ios

packages

scalars

unknowns

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      ..holding just the AUTOLOAD 

      use strict;
use warnings;

my %before = %main::;
bless {},'nonexistent';
my %after = %main::;

foreach my $symbol (sort keys %after) {
    next if exists $before{$symbol}; 

    local *myglob = $after{$symbol};
    if ( defined *myglob{HASH} ) {
        my %val = %{ *myglob{HASH} };
        print "HASH \%$symbol = ( ";
        while(  my ($key, $val) = each %val )  {
            print "$key=>'$val', ";
        }
        print ")\n" ;
    }
}

__END__

HASH %nonexistent:: = ( AUTOLOAD=>'*nonexistent::AUTOLOAD', )

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      ..but: For various obscure reasons, typeglobs are always created with a Null SV in the SCALAR slot.

      L*

      There are no rules, there are no thumbs..
      Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.
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        It doesn't matter if the package is empty, a STASH was created to hold the symbols.

        And the initial overhead for a hash is profound, even if it's empty. Like I said several hundreds of bytes.

        Furthermore do I remember someone explaining that packages which become classes receive further optimization. In order to speed up method lookup (IIRC), but I don't really know the details..

        I'm not aware of any application which tries to generate a massive amount of pseudo packages, I'm just saying it might not be the best idea to create many global data structures which are never destroyed.

        And if you only generate a small number, take care about controlling the namespace, because they are global.

        Fun fact: As a side effect of my experiments I was even able to block a package name in a way that any further package blocked_name; would globally fail.

        Like: "Hey I hate Moose, if you use my module in your project you won't be able to ever use Moose later!" ;-)

        Cheers Rolf
        (addicted to the Perl Programming Language :)
        Wikisyntax for the Monastery

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        > ..holding just the AUTOLOAD

        hmm interesting ... it's only the scalar $AUTOLOAD which is created and not any sub &AUTOLOAD

        And the value is undef

        I'd say that's only a random side-effect (probably bug), there is no use of $AUTOLOAD without &AUTOLOAD

        Cheers Rolf
        (addicted to the Perl Programming Language :)
        Wikisyntax for the Monastery

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