Perl references - $$var and $var notations
Archive - Originally posted on "The Horse's Mouth" - 2009-06-15 14:00:42 - Graham Ellis
In Perl, if I write:$stuff = "Porridge";then I'm setting up a variable to contain the value "Porridge".
If I write:
$stuffat = $stuff;then I'm setting up a variable to contain the ADDRESS OF the variable $stuff.
So if $stuff was in memory at (hex)fe80 ... then $stuffat will be assigned that value 0xfe80. And we can look use it by using an extra $ character :
print $$stuffat;which literally means "the contents of the contents of stuffat".
What use is this?
Lots ... not only for scalars, but for the addresses of lists too. If I write
$holder = \@biglist;I have assigned a single scalar variable to point to what is potentially a huge dataset and (for starters) I can easily pass it around to various subs without the need to replicate the data.
Other languages have this concept - "pointer" - too. See & and * in C, and the use of & in PHP.