deref question

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

Hi Monks,

In a sub, when I do:
    my $rs_en1  =   shift();
I can set the value of en1:
    $$rs_en1 = "new"

But, if I do:
    my $en1  =   ${shift()};
Setting $en1 = "new" doesn't work
[download]

(I am setting some values in a Tk::Entry widget... and I wonder why both are not equiv...)
thanks

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Re: deref question by graff (Chancellor) on Apr 10, 2003 at 03:33 UTC
The two are not equivalent. Given that the item being shifted is a reference to a scalar, the first case makes a copy of the reference, in the sense that "$rs_en1" is also a reference to the same scalar -- i.e. it points to the same location in memory, and using it as a reference affects that location. Meanwhile, the second case de-references the item being shifted, and so assigns the value of the scalar to $en1 -- as a result, $en1 contains a copy of the value and this copy has its own distinct location in memory, separate from the location pointed to by the reference that was shifted.	[reply]
Re: deref question by pg (Canon) on Apr 10, 2003 at 05:57 UTC
Not sure how you used Tk::Entry. If you use the textvariable option of Tk::Entry, it becomes much more simple for you to set/get the content of Tk::Entry (avoid passing params to your callback function, and do all those handling by yourself, that is more complex): `use Tk; use strict; my $entry_val = "initial value"; my $mw = new MainWindow(title => "demo"); $mw->Entry(width => 20, textvariable => \$entry_val)->pack; $mw->Button(text => "click", command => sub {print $entry_val, "\n";}) +->pack; MainLoop;` [download]	[reply] [d/l]