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

Dear Monks

Recently, I have done some reading regarding "Tie" in perl. And I have an question:

package Tie::Constvar;
use Carp;
sub TIESCALAR {
    my ($class, $initval) = @_;
    my $var = $initval;
    return bless \$var => $class;
}
sub FETCH {
    my $selfref = shift;
    return $$selfref;
}
sub STORE {
    confess "cannot assign value to a constant!";
}

package main;
tie my $AVO, Tie::Constvar, 6;
print $AVO,"\n";

tie  $AVO, Tie::Constvar, 3;  #this works
print $AVO,"\n";

$AVO = 3;  #this will throw an exception!
print $AVO,"\n";

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As you can see that we cannot assign a value to "$AVO" as "$AVO = 3". This is expected. However, after I "Tied" $AVO to 6, I can "Tie" it again with 3 which didn't generate any error.

This made me wonder if Tying a variable is not considered "assignment".

So, how does the tied variable associate with its underlying returned object by "TIESCALAR"? It seems a hash like association. Is this right?

Does anyone have any insight on this? Thank you

Replies are listed 'Best First'.
Re: question regarding "Tie" in Perl
by LanX (Saint) on Nov 04, 2013 at 20:05 UTC
    You have proven that tie doesn't suit here. =)

    I hope this code is only a sample for demonstration and not meant for production.

    IIRC do Perl variables have flags telling if they are blessed or tied and slots holding the bound class-names. (there is also untie, but no unbless)

    You might wanna read perlguts for details, I also recommend the (old) Panther book for internals.

    Cheers Rolf

    ( addicted to the Perl Programming Language)

[reply]
[d/l]

      Rolf

      Thank you for your answer.

      By the way, what is IIRC?

[reply]
        IIRC: If I Recall Correctly
[reply]
Re: question regarding "Tie" in Perl
by BrowserUk (Patriarch) on Nov 04, 2013 at 20:03 UTC

    That's a horribly inefficient way of creating & using constants.

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

      This is just for question regarding "Tie" behavior.

      However, this implementation has advantage of using the tied variable as constant in a string which can be interpolated.

      "use constant AVO => 6" can not interpolate it in a string.

[reply]
        However, this implementation has advantage of using the tied variable as constant in a string which can be interpolated.

        And the benefit of interpolating a compile-time constant into a string at runtime is? 

        use constant AVO => 6;

...

print "The constant is " . AVO . "\n";

        [download]

        Or

        printf "The constant is %d\n", AVO;

print "The constant is ${ \AVO }";;

        [download]
        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
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[d/l]
[select]

        That's no excuse for implementing a constant using tie.

        use strict;
use warnings;

Internals::SvREADONLY(my $answer = 42, 1);

print "This interpolates: $answer\n";
$answer++;  # dies

        [download]
        use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name
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[d/l]
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