Hard references are smart--they keep track of reference counts for you, automatically freeing the thing referred to when the reference count goes to zero. If that thing happens to be an object, the object is destructed. See the perlobj manpage for more about objects. (In a sense, everything in Perl is an object, but we usually reserve the word for references that have been officially "blessed" into a class package.)
Some rather pathological examples of the use of references can be found in the t/op/ref.t regression test in the Perl source directory.
"Hard" references are easy to use in Perl. There is just one overriding principle: Perl does no implicit referencing or dereferencing. When a scalar is holding a reference, it always behaves as a scalar. It doesn't magically start being an array or a hash unless you tell it to explicitly.
References can be constructed in one of several ways.
$scalarref = \$foo; $arrayref = \@ARGV; $hashref = \%ENV; $coderef = \&handler;
$arrayref = [1,2,['a','b','c']];Note that we've constructed a reference to an anonymous array, the final element of which is a reference to another anonymous array. (The multidimensional syntax described later can be used to access this. For example, after the above, $arrayref->[2][1] would have the value "b".)
$hashref = { 'Adam' => 'Eve', 'Clyde' => 'Bonnie', };Anonymous hash and array constructors can be intermixed freely to produce as complicated a structure as you want. The multidimensional syntax described below works for these too. The values above are literals, but variables and expressions would work just as well, since these are executable operators, not declarations.
Because curly brackets are used for several other things including BLOCKs, you may occasionally have to disambiguate brackets at the beginning of a statement by putting a + or a return in front, so that Perl knows the opening bracket isn't starting a BLOCK. The economy and mnemonic value of using curlies is deemed worth this occasional extra hassle.
$coderef = sub { print "Boink!\n" };Note the presence of the semicolon. Except for the fact that the code inside isn't executed immediately, a sub {} is not so much a declaration as it is an operator, like do {} or eval {}.
For those who worry about these things, the current implementation uses shallow binding of local() variables; my() variables are not accessible. This precludes true closures. However, you can work around this with a run-time eval():
{ my $x = time; $coderef = eval "sub { \$x }"; }This will generate a unique subroutine each time called, one which accesses the my() variable above it, even when it would otherwise go out of scope.
$objref = new Doggie (Tail => 'short', Ears => 'long');
$bar = $$scalarref; push(@$arrayref, $filename); $$arrayref[0] = "January"; $$hashref{"KEY"} = "VALUE"; &$coderef(1,2,3);Note that we are specifically not referencing $arrayref[0] or $hashref{"KEY"} there. The dereference of the scalar variable happens before it does any key lookups. Anything more complicated than a simple scalar variable must use methods 2 or 3 below. Note, however, that a simple scalar includes an identifier that use method 1 recursively. The following prints "howdy".
$refrefref = \\\"howdy"; print $$$$refrefref;
$bar = ${$scalarref}; push(@{$arrayref}, $filename); ${$arrayref}[0] = "January"; ${$hashref}{"KEY"} = "VALUE"; &{$coderef}(1,2,3);Admittedly, it's a little silly to use the curlies in this case, but the BLOCK can contain any arbitrary expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
$arrayref->[0] = "January"; $hashref->{"KEY} = "VALUE";The left side of the array can be any expression returning a reference, including a previous dereference. Note that $array[$x] is not the same thing as $array-[$x]> here:
$array[$x]->{"foo"}->[0] = "January";This is one of the cases we mentioned earlier in which references could spring into existence when in an lvalue context. Before this statement, $array[$x] may have been undefined. If so, it's automatically defined with a hash reference so that we can look up {"foo"} in it. Likewise $array[$x]->{"foo"} will automatically get defined with an array reference so that we can look up [0] in it.
One more thing here. The arrow is optional between subscripts, so you can shrink the above down to
$array[$x]{"foo"}[0] = "January";Which, in the degenerate case of using only ordinary arrays, gives you multidimensional arrays just like C's:
$score[$x][$y][$z] += 42;Well, okay, not entirely like C's arrays, actually. C doesn't know how to grow its arrays on demand. Perl does.
The bless() operator may be used to associate a reference with a package functioning as an object class. See the perlobj manpage .
A type glob may be dereferenced the same way a reference can, since the dereference syntax always indicates the kind of reference desired. So ${*foo} and ${\$foo} both indicate the same scalar variable.
Here's a trick for interpolating a subroutine call into a string:
print "My sub returned ${\mysub(1,2,3)}\n";
People frequently expect it to work like this. So it does.
$name = "foo"; $$name = 1; # Sets $foo. ${$name} = 2; # Sets $foo. ${$name x 2} = 3; # Sets $foofoo. $name->[0] = 4; # Sets $foo[0]. @$name = (); # Clears @foo. &$name; # Calls &foo (as in Perl 4) $pack = "THAT"; ${"${pack}::$name"} = 5; # Sets $THAT::foo without evalThis is very powerful, and slightly dangerous, in that it's possible to intend (with the utmost sincerity) to use a hard reference, and accidentally use a symbolic reference instead. To protect against that, you can say
use strict 'refs';and then only hard references will be allowed for the rest of the enclosing block. An inner block may countermand that with
no strict 'refs';Only package variables are visible to symbolic references. Lexical variables (declared with my()) aren't in a symbol table, and thus are invisible to this mechanism. For example:
local($value) = 10; $ref = \$value; { my $value = 20; print $$ref; }This will still print 10, not 20. Remember that local() affects package variables, which are all "global" to the package.