Taking References to Scalars

Problem

You want to create and manipulate a reference to a scalar value.

Solution

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

Discussion

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"; }