Recipe 11.5 Taking References to Scalars

11.5.1 Problem

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

11.5.2 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 to a dereferenced 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

11.5.3 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 this chapter's Introduction:

sub new_anon_scalar {
    my $temp;
    return \$temp;
}

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 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";
}

11.5.4 See Also

Chapters 8 and 9 of Programming Perl and perlref(1)