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

Somewhere in Main application call the dirtree_sub below to choose a working area. Once user select the work area through dirtree, it will set the path in $area entry box. But when I add $dirtree_frame->destroy in dirtree_sub below, it print warning: 

Useless use of reference constructor in void context at geometry_place
+.pl line 564.
Useless use of reference constructor in void context at geometry_place
+.pl line 570.

[download]

Any idea how to solve this?

Some code in main application: 

my $area = $mw->Entry(-background => "white", -foreground => "black", 
+-width => 51)->place(-x=>100, -y=>$ystart + 60);
$area->insert('end', "$workarea");


sub dirtree_sub {
    
    if ($area->get() ne "") {
        $CWD = $area->get();
    } else {
        $CWD = Cwd::cwd();
    };

    my $dirtree_frame = $mw->Toplevel;
    $dirtree_frame->title("Select work area");

    my $DIR_TREE = $dirtree_frame->Scrolled('DirTree',
                                 -scrollbars => "osoe",
                                 -width => 50,
                                 -height => 25,
                                 -exportselection => 1,
                                 -browsecmd => sub {$CWD = shift},
                                 -command   => sub {\&show_cwd(); $dir
+tree_frame->destroy})->pack(-fill => "both",
                                                                 -expa
+nd => 1);

    $DIR_TREE->chdir($CWD);

    my $button_frame = $dirtree_frame->Frame()->pack(-side => "bottom"
+);
    $button_frame->Button(-text => "Ok",
                          -command => sub {\&show_cwd(); $dirtree_fram
+e->destroy})->pack(-side => "left");
    $button_frame->Button(-text => "Exit",
                          -command => [$dirtree_frame=>'destroy'])->pa
+ck(-side => "left");

};

sub show_cwd {


  $area->delete('0.0', 'end');
  $area->insert('end', $CWD);


}

[download]

Replies are listed 'Best First'.
Re: Useless use of reference constructor in void context
by Athanasius (Archbishop) on Jul 29, 2012 at 15:12 UTC

    Hello myfamilygeneral, and welcome to the Monastery!

    The problem is not with $dirtree_frame->destroy, but with the immediately preceding term \&show_cwd(). Consider the following:

    #! perl
use strict;
use warnings;

sub foo
{
    print "in foo()\n";
}

sub bar
{
    \&foo();
    print "in bar()\n";
}

bar();

    [download]

    Output:

    Useless use of reference constructor in void context at test.pl line 1
+2.
in foo()
in bar()

    [download]

    But remove the backslash (which does nothing useful anyway) from before &foo(), and the warning goes away.

    Update: The & sigil probably does nothing useful either. See What's the difference between calling a function as &foo and foo()?

    HTH,

    Athanasius <°(((>< contra mundum

[reply]
[d/l]
[select]
      s/The & sigil probably does nothing useful either./The & sigil does nothing useful here, and, generally, may do something you won't like./;
[reply]
[d/l]
      Thanks a lot this was very useful.
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Re: Useless use of reference constructor in void context
by NetWallah (Canon) on Jul 29, 2012 at 14:33 UTC
    Please use "code" tags to preserve code formatting.

    In this snippet: 

     sub {\&show_cwd(); $dirtree_frame->destroy})->pack(-fill => "both",
                                                          -expand => 1
+);

    [download]
    The "sub" returns a code-reference
    The "->pack" is a dereference followed by a call to the "pack" method.

    Since the subref does not de-reference to something that allows a method call, perl complains.

    Update 1: Sorry - this is rubbish. I just noticed the "), which means you are calling "pack" on whatever is returned by "$dirtree_frame->Scrolled".

    Update 2: New theory :

    $dirtree_frame is not created properly.
    Evidence is the TWO messages complaining about the object - one for each method call (Scrolled and destroy).
    Update 3: Athanasius (++) , below is right. And thanks for fixing the formatting, and Welcome to the Monastery!

                 I hope life isn't a big joke, because I don't get it.
                       -SNL

[reply]
[d/l]
Re: Useless use of reference constructor in void context
by zentara (Cardinal) on Jul 29, 2012 at 15:10 UTC
    If you havn't figured it out yet, please post a complete running example so we can run the code.
    I'm not really a human, but I play one on earth.
    Old Perl Programmer Haiku ................... flash japh
[reply]

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