From: perl-win32-users-boun...@listserv.activestate.com
[mailto:perl-win32-users-boun...@listserv.activestate.com] On Behalf Of Greg
Aiken
Sent: 20 December 2012 22:31
To: Perl-Win32-Users@listserv.ActiveState.com
Subject: learning references/dereferencing (understand $,@,%, but trouble
understanding &)
> #if one desires to pass a scalar reference into a sub-routine,
> #the easiest way to assign a local scalar to the contents of the scalar
> reference is...
Depends. Sometimes it is easier store the reference locally, and dereference as
needed. See below.
> subroutine(\$scalar);
> sub subroutine {
> my $subroutine_scalar = ${$_[0]}; #note you need the {} brackets, or this
> doesn't work!
> print "$subroutine_scalar\n";
my $ref = shift;
print "$$ref\n";
> }
>
> #if one desires to pass an array reference into a sub-routine,
> #the easiest way to assign a local array to the contents of the array
> reference is...
> subroutine(\@array);
> sub subroutine {
> my @subroutine_array = @{$_[0]}; #note you need the {} brackets, or this
> doesn't work!
> print "in subroutine: " . join(' ', @subroutine_array) . "\n";
my $ref = shift;
print "in subroutine: @$ref\n";
print " first entry is $ref->[0]\n";
> }
>
> #if one desires to pass a hash reference into a sub-routine,
> #the easiest way to assign a local hash to the contents of the hash reference
> is...
> subroutine(\%hash);
> sub subroutine {
> my %subroutine_hash = %{$_[0]}; #note you need the {} brackets, or this
> doesn't work!
> print "in subroutine: " . join(' ', keys (%subroutine_hash)) . "\n";
my $ref = shift;
print "$_ = $ref->{$_}\n" foreach keys %$ref;
> }
>
> all above works fine and is easy for me to understand. its below that im
> having difficulty with...
>
> #seeing the 'pattern' of behavior for $, @, % variable types...
> #i, not knowing any better, assumed the same should also be able to be done
> for & (subroutines)
> #i therefore tried a test to see if i could assign a new subroutine to equal
> a de-referenced subroutine
> reference
> #i literally copied the same code as used above, but used the & operator
> instead of ($, @, %)
> #this did not give the expected result... perl reported:
> #hello CODE(0x237dbc)
> #Can't modify non-lvalue subroutine call at D:\_junk\TEST.PL line 6.
>
> sub subroutine {
> print "hello @_\n"
> }
> sub2(\&subroutine);
> sub sub2 {
> &sub3 = &{$_[0]}; #problem is obviously here with this line, seems its not
> being dereference
> sub3('world');
The only variable types are scalar array and hash. There is no subroutine
variable type, but you can store a reference to a subroutine, which is a
scalar, in a variable. You dereference it when you want to call it.
my $ref = shift;
$ref->('world');
> }
For more detail see 'perldoc perlref'
HTH
--
Brian Raven
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