Dereference, my dear

Return with us to our simple example:

$name="Moe";
$name_reference=\$name;

Merrily we stroll along, and attempt to print out the value that this reference points to:

print $name_reference;

Yields:

SCALAR(0xba5e84)

Ack! What is that? Gibberish, but let's look at it from Perl's perspective: $name_reference is a pointer to what? It's a pointer to a scalar variable, which is exactly what Perl reports when we try to output the value of $name_reference. As humans, we don't want to know this, we want to know the value of the variable which $name_reference points to. A-ha! That my dear is called dereferencing. Whereas creating a reference builds the pointer, dereferencing follows the pointer to access its end value.

The simplest way to dereference a reference, especially a reference that points to a scalar variable, is to precede the reference with a dollar sign. Yes, that means dual dollar signs:

print $$name_reference;

Yields:

Moe

Looks funny, but it works. Dereferencing is very simple when your reference points to a scalar variable. Things get a little more complicated when the references leads to a list or a hash. Let's consider. You can dereference a list for two different yields: you may wish to access a single element of the list, or you may wish to access the list as a whole. First, let's create a list and a reference to it:

@wingsauces=("mild","medium","hot");
$wingsauce=\@wingsauces;

We can use the reference to pluck the first item from the list:

print ${$wingsauce}[0];

print $wingsauce->[0];

The above syntaxes are equivalent and yield the same result, in this case, "mild".

We can also use the reference to grab the whole list and work with it in a list context:

print join(",",@{$wingsauce});

Here, the join() function is used to demonstrate the list context; this function takes a list and joins the elements using the given character (here, a comma); you can see that we dereference the whole list using the @{ } construction, placing the dereference variable $wingsauce within the curly braces.

When dereferencing a hash, the syntax is a little different but no more complicated.

%mandms=("red"=>25,"brown"=>15,"blue"=>40);
$mmhash=\%mandms;

print $mmhash->{"blue"};

Yields:

40

The arrow constructor is used to dereference a particular key of a hash. Here, we dereference the key "blue". For some heartier acrobatics, let's use a dereference to pull out the keys of the hash %mandms:

print keys %{$mmhash}

Not bad at all! The keys function requires a hash, which we provide with the % prefix, within whose curly braces we specify the reference scalar, $mmhash. Works like a charm indeed.

The Perl You Need to Know Part 11: A Reference of References
The Perl You Need to Know
Shall we dance, sweet reference?