You want to create and manipulate a reference to a scalar value.
To create a reference to a scalar variable, use the backslash operator:
$scalar_ref = \$scalar; # get reference to named scalar
To create a reference to an anonymous scalar value (a value that isn't in a variable), assign through a dereference of an undefined variable:
undef $anon_scalar_ref; $$anon_scalar_ref = 15;
This creates a reference to a constant scalar:
$anon_scalar_ref = \15;
Use ${...}
to dereference:
print ${ $scalar_ref }; # dereference it ${ $scalar_ref } .= "string"; # alter referent's value
If you want to create many new anonymous scalars, use a subroutine that returns a reference to a lexical variable out of scope, as explained in the Introduction:
sub new_anon_scalar { my $temp; return \$temp; }
Perl almost never implicitly dereferences for you. Exceptions include references to filehandles, code references to sort
, and the reference argument to bless
. Because of this, you can only dereference a scalar reference by prefacing it with $
to get at its contents:
$sref = new_anon_scalar(); $$sref = 3; print "Three = $$sref\n"; @array_of_srefs = ( new_anon_scalar(), new_anon_scalar() ); ${ $array[0] } = 6.02e23; ${ $array[1] } = "avocado"; print "\@array contains: ", join(", ", map { $$_ } @array ), "\n";
Notice we have to put braces around $array[0]
and $array[1]
. If we tried to say $$array[0]
, the tight binding of dereferencing would turn it into $array->[0]
. It would treat $array
as an array reference and return the element at index zero.
Here are other examples where it is safe to omit the braces:
$var = `uptime`; # $var holds text $vref = \$var; # $vref "points to" $var if ($$vref =~ /load/) {} # look at $var, indirectly chomp $$vref; # alter $var, indirectly
As mentioned in the introduction, you may use the ref
built-in to inspect a reference for its referent's type. Calling ref
on a scalar reference returns the string "SCALAR
":
# check whether $someref contains a simple scalar reference if (ref($someref) ne 'SCALAR') { die "Expected a scalar reference, not $someref\n"; }
Chapter 4 of Programming Perl and perlref (1)