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Re^10: Getting for() to accept a tied array in one statement

by hdb (Monsignor)
on Apr 18, 2019 at 16:34 UTC ( #1232767=note: print w/replies, xml ) Need Help??


in reply to Re^9: Getting for() to accept a tied array in one statement
in thread Getting for() to accept a tied array in one statement

The magic of tie seems to be strong. Even when using a reference to an element of a tied array one cannot bypass the associated class. And the access through the reference provides the correct index. So is this what happens when Perl aliases elements in a loop?

use strict; use warnings; package MyClass; use Tie::Array; our @ISA = ('Tie::Array'); our @data; # mandatory methods sub TIEARRAY { my $class = shift; bless \@data, $class; @data = @_ +; return \@data } sub FETCH { my ($self, $index ) = @_; print "FETCH($index)\n"; ret +urn $data[$index] } sub STORE { my ($self, $index, $value) = @_; print "STORE($index)\ +n"; $data[$index] = $value } sub FETCHSIZE { print "<FETCHSIZE> "; return scalar @data } package main; my @x; tie @x, "MyClass", 0, 0, 0; my $x = \$x[2]; $$x++; print "@x\n";

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Re^11: Getting for() to accept a tied array in one statement [TANGENT: untie attempted while # references still exist]
by pryrt (Monsignor) on Apr 18, 2019 at 17:09 UTC

    I'm not good with tie, but thought I'd take this as a learning exercise. I was surprised by using our @data instead of a fresh anonymous aref for each instance, so I tied @y to the same class to confirm it (confirmed). I wanted to untie those variables and re-use them to tie to a second class that used separate data structures in TIEARRAY (in spoiler; the mods worked as I expected, with separate data structures for @x and @y)... but was surprised at the "untie attempted while 2 inner references still exist" warning when I did the untie. I thought the $x reference was probably the culprit, so undefined that.... but there was still one remaining inner reference. What is the second inner reference, and where did it come from?

    use strict; use warnings; package MyClass; { use Tie::Array; our @ISA = ('Tie::Array'); our @data; #mandatory methods sub TIEARRAY { my $class = shift; bless \@data, $class; @data = @_ +; return \@data } sub FETCH { my ($self, $index ) = @_; print "FETCH($index)\n"; ret +urn $data[$index] } sub STORE { my ($self, $index, $value) = @_; print "STORE($index)\ +n"; $data[$index] = $value } sub FETCHSIZE { print "<FETCHSIZE> "; return scalar @data } }; package main; $|++; local $" = ", "; my @x; tie @x, "MyClass", 0, 0, 0; my $x = \$x[2]; $$x++; print "x = (@x)\n"; =begin comment When I first saw the above, it looked like no matter how many items we +re tied, they would all refer to the same @MyClass::data internal arr +ay. The next few lines showed that's true: when I tied @y to the same clas +s, @x lost its data; and when an element of @y was changed, the same +happened to @x. =cut my @y; tie @y, "MyClass", 0, 0, 0; print "y = (@y)\n"; print "x = (@x)\n"; $y[1] = 3.14; print "y = (@y)\n"; print "x = (@x)\n"; untie @y; # gives a warning: untie attempted while 2 inner refer +ences still exist undef $x; # uncomment to reduce next warning to 1 instead of 2; +comment to keep next warning at 2 untie @x; # warning, but with only 1 warning if previous line un +commented print "y = (@y)\n"; print "x = (@x)\n"; # so what's the second inner reference?

      Thanks for pointing to this! So it turns out that I am not an expert on classes either...

Re^11: Getting for() to accept a tied array in one statement
by LanX (Cardinal) on Apr 19, 2019 at 12:37 UTC
    > So is this what happens when Perl aliases elements in a loop?

    if you are interested to find out using B::Concise might help.

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

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