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1996-06-28
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=head1 NAME
perlref - Perl references and nested data structures
=head1 DESCRIPTION
Before release 5 of Perl it was difficult to represent complex data
structures, because all references had to be symbolic, and even that was
difficult to do when you wanted to refer to a variable rather than a
symbol table entry. Perl 5 not only makes it easier to use symbolic
references to variables, but lets you have "hard" references to any piece
of data. Any scalar may hold a hard reference. Since arrays and hashes
contain scalars, you can now easily build arrays of arrays, arrays of
hashes, hashes of arrays, arrays of hashes of functions, and so on.
Hard references are smart--they keep track of reference counts for you,
automatically freeing the thing referred to when its reference count
goes to zero. If that thing happens to be an object, the object is
destructed. See L<perlobj> for more about objects. (In a sense,
everything in Perl is an object, but we usually reserve the word for
references to objects that have been officially "blessed" into a class package.)
A symbolic reference contains the name of a variable, just as a
symbolic link in the filesystem merely contains the name of a file.
The C<*glob> notation is a kind of symbolic reference. Hard references
are more like hard links in the file system: merely another way
at getting at the same underlying object, irrespective of its name.
"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 so explicitly by dereferencing it.
References can be constructed several ways.
=over 4
=item 1.
By using the backslash operator on a variable, subroutine, or value.
(This works much like the & (address-of) operator works in C.) Note
that this typically creates I<ANOTHER> reference to a variable, since
there's already a reference to the variable in the symbol table. But
the symbol table reference might go away, and you'll still have the
reference that the backslash returned. Here are some examples:
$scalarref = \$foo;
$arrayref = \@ARGV;
$hashref = \%ENV;
$coderef = \&handler;
$globref = \*STDOUT;
=item 2.
A reference to an anonymous array can be constructed using square
brackets:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've constructed a reference to an anonymous array of three elements
whose final element is itself reference to another anonymous array of three
elements. (The multidimensional syntax described later can be used to
access this. For example, after the above, $arrayref-E<gt>[2][1] would have
the value "b".)
Note that taking a reference to an enumerated list is not the same
as using square brackets--instead it's the same as creating
a list of references!
@list = (\$a, \$b, \$c);
@list = \($a, $b, $c); # same thing!
=item 3.
A reference to an anonymous hash can be constructed using curly
brackets:
$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, because
assignment operators in Perl (even within local() or my()) are executable
statements, not compile-time declarations.
Because curly brackets (braces) are used for several other things
including BLOCKs, you may occasionally have to disambiguate braces at the
beginning of a statement by putting a C<+> or a C<return> in front so
that Perl realizes the opening brace isn't starting a BLOCK. The economy and
mnemonic value of using curlies is deemed worth this occasional extra
hassle.
For example, if you wanted a function to make a new hash and return a
reference to it, you have these options:
sub hashem { { @_ } } # silently wrong
sub hashem { +{ @_ } } # ok
sub hashem { return { @_ } } # ok
=item 4.
A reference to an anonymous subroutine can be constructed by using
C<sub> without a subname:
$coderef = sub { print "Boink!\n" };
Note the presence of the semicolon. Except for the fact that the code
inside isn't executed immediately, a C<sub {}> is not so much a
declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
matter how many times you execute that line (unless you're in an
C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
anonymous subroutine.)
Anonymous subroutines act as closures with respect to my() variables,
that is, variables visible lexically within the current scope. Closure
is a notion out of the Lisp world that says if you define an anonymous
function in a particular lexical context, it pretends to run in that
context even when it's called outside of the context.
In human terms, it's a funny way of passing arguments to a subroutine when
you define it as well as when you call it. It's useful for setting up
little bits of code to run later, such as callbacks. You can even
do object-oriented stuff with it, though Perl provides a different
mechanism to do that already--see L<perlobj>.
You can also think of closure as a way to write a subroutine template without
using eval. (In fact, in version 5.000, eval was the I<only> way to get
closures. You may wish to use "require 5.001" if you use closures.)
Here's a small example of how closures works:
sub newprint {
my $x = shift;
return sub { my $y = shift; print "$x, $y!\n"; };
}
$h = newprint("Howdy");
$g = newprint("Greetings");
# Time passes...
&$h("world");
&$g("earthlings");
This prints
Howdy, world!
Greetings, earthlings!
Note particularly that $x continues to refer to the value passed into
newprint() I<despite> the fact that the "my $x" has seemingly gone out of
scope by the time the anonymous subroutine runs. That's what closure
is all about.
This only applies to lexical variables, by the way. Dynamic variables
continue to work as they have always worked. Closure is not something
that most Perl programmers need trouble themselves about to begin with.
=item 5.
References are often returned by special subroutines called constructors.
Perl objects are just references to a special kind of object that happens to know
which package it's associated with. Constructors are just special
subroutines that know how to create that association. They do so by
starting with an ordinary reference, and it remains an ordinary reference
even while it's also being an object. Constructors are customarily
named new(), but don't have to be:
$objref = new Doggie (Tail => 'short', Ears => 'long');
=item 6.
References of the appropriate type can spring into existence if you
dereference them in a context that assumes they exist. Since we haven't
talked about dereferencing yet, we can't show you any examples yet.
=item 7.
References to filehandles can be created by taking a reference to
a typeglob. This is currently the best way to pass filehandles into or
out of subroutines, or to store them in larger data structures.
splutter(\*STDOUT);
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(\*STDIN);
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
=back
That's it for creating references. By now you're probably dying to
know how to use references to get back to your long-lost data. There
are several basic methods.
=over 4
=item 1.
Anywhere you'd put an identifier as part of a variable or subroutine
name, you can replace the identifier with a simple scalar variable
containing a reference of the correct type:
$bar = $$scalarref;
push(@$arrayref, $filename);
$$arrayref[0] = "January";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically I<NOT> dereferencing
C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
s