Can't understand one line

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

Replies are listed 'Best First'.

Re: Can't understand one line
by BrowserUk (Patriarch) on Jan 07, 2013 at 07:16 UTC

$registry->$_( shift(@_) )
^^^^^^^^^^^  ^           ^............invoke a method
           ^^             ............the name of which is stored in $
+_
              ^^^^^^^^^^  ............passing the next value from @_ a
+s an argument.
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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

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Re^2: Can't understand one line

by anaconda_wly (Scribe) on Jan 07, 2013 at 07:34 UTC

$registry->$_( shift(@_) )
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Re^3: Can't understand one line

by AnomalousMonk (Archbishop) on Jan 07, 2013 at 07:43 UTC

... shift(@_) must be first ...

Quite right.

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Re: Can't understand one line
by Athanasius (Archbishop) on Jan 07, 2013 at 08:09 UTC

shift(@_) is a current $_

No. Here is the skeleton of the opening of the import subroutine in Win32::TieRegistry:

sub import
{
    my $pkg = shift(@_);
    ...
    local( $_ );
    while(  @_  ) {
    $_= shift(@_);
    if( ... ) {
            ...
    } elsif( ... ) {
            ...
            ...
    } elsif(  exists $_opt_subs{$_}  ) {
            ...
        $registry->$_( shift(@_) );
    } elsif( ... ) {
            ...
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Each call to shift removes another element from the head of the @_ array, i.e., another argument from the list of those passed into sub import. So in the line:

$registry->$_( shift(@_) );
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the variable $_ holds the last argument to have been removed from @_; but the expression shift(@_) removes the next argument from @_. $_ and shift(@_) will (most likely) have different values.

shift(@_) does not reset the value of $_:

17:43 >perl -wE "f(1, 2, 3); sub f { local($_) = shift(@_); my $c = sh
+ift(@_); say '$_ = ', $_; say '$c = ', $c; say '@_ = ', @_; }"
$_ = 1
$c = 2
@_ = 3

18:07 >
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Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^2: Can't understand one line

by anaconda_wly (Scribe) on Jan 07, 2013 at 08:44 UTC

That' great!Thanks!

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Re: Can't understand one line
by CountZero (Bishop) on Jan 07, 2013 at 09:06 UTC

shift(@_

is a current

$_

They are totally separate variables.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

My blog: Imperial Deltronics

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Re: Can't understand one line
by 7stud (Deacon) on Jan 07, 2013 at 09:43 UTC

use strict; 
use warnings; 
use 5.012;

package Person;
sub new
{
    my $class = shift;
    my $self = {};
    bless $self, $class;
    return $self;
}

sub do_this {
    my($self, $greetee) = @_;
    say "hello $greetee";
}

sub do_that {
    my($self, $greetee) = @_;
    say "goodbye $greetee";
}

1;
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use strict; 
use warnings; 
use 5.012;

use Person;

my $obj = Person->new;
$obj->do_this("Sam");
$obj->do_that("Cathy");

say '*' x 20;

my @func_names = ('do_this', 'do_that');
my @names = ("Sam", "Cathy");

call_funcs(@names);

sub call_funcs {
    for (@func_names) {
        $obj->$_(shift @_);
    }
}

--output:--
hello Sam
goodbye Cathy
********************
hello Sam
goodbye Cathy
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