PERLREF(1)
NNAAMMEE
perlref - Perl references and nested data structures
DDEESSCCRRIIPPTTIIOONN
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 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. Because 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. (Note: The
reference counts for values in self-referential or cyclic
data structures may not go to zero without a little help;
see the section on Two-Phased Garbage Collection in the
perlobj manpage for a detailed explanation. 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 to objects that have been
officially "blessed" into a class package.)
Symbolic references are names of variables or other
objects, just as a symbolic link in a Unix filesystem
contains merely the name of a file. The *glob notation is
a kind of symbolic reference. (Symbolic references are
sometimes called "soft references", but please don't call
them that; references are confusing enough without useless
synonyms.)
In contrast, hard references are more like hard links in a
Unix file system: They are used to access an underlying
object without concern for what its (other) name is. When
the word "reference" is used without an adjective, like in
the following paragraph, it usually is talking about a
hard reference.
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 simple scalar. It doesn't magically
start being an array or hash or subroutine; you have to
tell it explicitly to do so, by dereferencing it.
References can be constructed in several ways.
1. By using the backslash operator on a variable,
subroutine, or value. (This works much like the &
(address-of) operator in C.) Note that this typically
creates ANOTHER reference to a variable, because
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 = \*foo;
It isn't possible to create a true reference to an IO
handle (filehandle or dirhandle) using the backslash
operator. See the explanation of the *foo{THING}
syntax below. (However, you're apt to find Perl code
out there using globrefs as though they were IO
handles, which is grandfathered into continued
functioning.)
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
a 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->[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!
As a special case, \(@foo) returns a list of
references to the contents of @foo, not a reference to
@foo itself. Likewise for %foo.
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 + or a 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
4. A reference to an anonymous subroutine can be
constructed by using 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 sub {} is not so much a declaration as it is an
operator, like do{} or eval{}. (However, no matter
how many times you execute that line (unless you're in
an eval("...")), $coderef will still have a reference
to the 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 already
provides a different mechanism to do that--see the
perlobj manpage.
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 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() 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 applies to only 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.
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');
6. References of the appropriate type can spring into
existence if you dereference them in a context that
assumes they exist. Because we haven't talked about
dereferencing yet, we can't show you any examples yet.
7. A reference can be created by using a special syntax,
lovingly known as the *foo{THING} syntax. *foo{THING}
returns a reference to the THING slot in *foo (which
is the symbol table entry which holds everything known
as foo).
$scalarref = *foo{SCALAR};
$arrayref = *ARGV{ARRAY};
$hashref = *ENV{HASH};
$coderef = *handler{CODE};
$ioref = *STDIN{IO};
$globref = *foo{GLOB};
All of these are self-explanatory except for *foo{IO}.
It returns the IO handle, used for file handles (the
open entry in the perlfunc manpage), sockets (the
socket entry in the perlfunc manpage and the
socketpair entry in the perlfunc manpage), and
directory handles (the opendir entry in the perlfunc
manpage). For compatibility with previous versions of
Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}.
*foo{THING} returns undef if that particular THING
hasn't been used yet, except in the case of scalars.
*foo{SCALAR} returns a reference to an anonymous
scalar if $foo hasn't been used yet. This might
change in a future release.
The use of *foo{IO} is the best way to pass bareword
filehandles into or out of subroutines, or to store
them in larger data structures.
splutter(*STDOUT{IO});
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(*STDIN{IO});
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
Beware, though, that you can't do this with a routine
which is going to open the filehandle for you, because
*HANDLE{IO} will be undef if HANDLE hasn't been used
yet. Use \*HANDLE for that sort of thing instead.
Using \*HANDLE (or *HANDLE) is another way to use and
store non-bareword filehandles (before perl version
5.002 it was the only way). The two methods are
largely interchangeable, you can do
splutter(\*STDOUT);
$rec = get_rec(\*STDIN);
with the above subroutine definitions.
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.
1. Anywhere you'd put an identifier (or chain of
identifiers) 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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically
NOT dereferencing $arrayref[0] or $hashref{"KEY"}
there. The dereference of the scalar variable happens
BEFORE it does any key lookups. Anything more
complicated than a simple scalar variable must use
methods 2 or 3 below. However, a "simple scalar"
includes an identifier that itself uses method 1
recursively. Therefore, the following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of
identifiers) as part of a variable or subroutine name,
you can replace the identifier with a BLOCK returning
a reference of the correct type. In other words, the
previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in
this case, but the BLOCK can contain any arbitrary
expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the
simple case of $$x, people often make the mistake of
viewing the dereferencing symbols as proper operators,
and wonder about their precedence. If they were,
though, you could use parentheses instead of braces.
That's not the case. Consider the difference below;
case 0 is a short-hand version of case 1, NOT case 2:
$$hashref{"KEY"} = "VALUE"; # CASE 0
${$hashref}{"KEY"} = "VALUE"; # CASE 1
${$hashref{"KEY"}} = "VALUE"; # CASE 2
${$hashref->{"KEY"}} = "VALUE"; # CASE 3
Case 2 is also deceptive in that you're accessing a
variable called %hashref, not dereferencing through
$hashref to the hash it's presumably referencing.
That would be case 3.
3. Subroutine calls and lookups of individual array
elements arise often enough that it gets cumbersome to
use method 2. As a form of syntactic sugar, the
examples for method 2 may be written:
$arrayref->[0] = "January"; # Array element
$hashref->{"KEY"} = "VALUE"; # Hash element
$coderef->(1,2,3); # Subroutine call
The left side of the arrow 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
brackets 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.
4. If a reference happens to be a reference to an object,
then there are probably methods to access the things
referred to, and you should probably stick to those
methods unless you're in the class package that
defines the object's methods. In other words, be
nice, and don't violate the object's encapsulation
without a very good reason. Perl does not enforce
encapsulation. We are not totalitarians here. We do
expect some basic civility though.
The ref() operator may be used to determine what type of
thing the reference is pointing to. See the perlfunc
manpage.
The bless() operator may be used to associate a reference
with a package functioning as an object class. See the
perlobj manpage.
A typeglob may be dereferenced the same way a reference
can, because 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)]} that time.\n";
The way it works is that when the @{...} is seen in the
double-quoted string, it's evaluated as a block. The
block creates a reference to an anonymous array containing
the results of the call to mysub(1,2,3). So the whole
block returns a reference to an array, which is then
dereferenced by @{...} and stuck into the double-quoted
string. This chicanery is also useful for arbitrary
expressions:
print "That yields @{[$n + 5]} widgets\n";
SSyymmbboolliicc rreeffeerreenncceess
We said that references spring into existence as necessary
if they are undefined, but we didn't say what happens if a
value used as a reference is already defined, but ISN'T a
hard reference. If you use it as a reference in this
case, it'll be treated as a symbolic reference. That is,
the value of the scalar is taken to be the NAME of a
variable, rather than a direct link to a (possibly)
anonymous value.
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 $THAT::foo without eval
This is very powerful, and slightly dangerous, in that
it's possible to intend (with the utmost sincerity) to use
a hard reference, and accidentally use a symbolic
reference instead. To protect against that, you can say
use strict 'refs';
and then only hard references will be allowed for the rest
of the enclosing block. An inner block may countermand
that with
no strict 'refs';
Only package variables are visible to symbolic references.
Lexical variables (declared with my()) aren't in a symbol
table, and thus are invisible to this mechanism. For
example:
local($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.
NNoott--ssoo--ssyymmbboolliicc rreeffeerreenncceess
A new feature contributing to readability in perl version
5.001 is that the brackets around a symbolic reference
behave more like quotes, just as they always have within a
string. That is,
$push = "pop on ";
print "${push}over";
has always meant to print "pop on over", despite the fact
that push is a reserved word. This has been generalized
to work the same outside of quotes, so that
print ${push} . "over";
and even
print ${ push } . "over";
will have the same effect. (This would have been a syntax
error in Perl 5.000, though Perl 4 allowed it in the
spaceless form.) Note that this construct is not
considered to be a symbolic reference when you're using
strict refs:
use strict 'refs';
${ bareword }; # Okay, means $bareword.
${ "bareword" }; # Error, symbolic reference.
Similarly, because of all the subscripting that is done
using single words, we've applied the same rule to any
bareword that is used for subscripting a hash. So now,
instead of writing
$array{ "aaa" }{ "bbb" }{ "ccc" }
you can write just
$array{ aaa }{ bbb }{ ccc }
and not worry about whether the subscripts are reserved
words. In the rare event that you do wish to do something
like
$array{ shift }
you can force interpretation as a reserved word by adding
anything that makes it more than a bareword:
$array{ shift() }
$array{ +shift }
$array{ shift @_ }
The --ww switch will warn you if it interprets a reserved
word as a string. But it will no longer warn you about
using lowercase words, because the string is effectively
quoted.
WWAARRNNIINNGG
You may not (usefully) use a reference as the key to a
hash. It will be converted into a string:
$x{ \$a } = $a;
If you try to dereference the key, it won't do a hard
dereference, and you won't accomplish what you're
attempting. You might want to do something more like
$r = \@a;
$x{ $r } = $r;
And then at least you can use the values(), which will be
real refs, instead of the keys(), which won't.
SSEEEE AALLSSOO
Besides the obvious documents, source code can be
instructive. 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.
See also the perldsc manpage and the perllol manpage for
how to use references to create complex data structures,
and the perlobj manpage for how to use them to create
objects.