PERLREF(1) Perl Programmers Reference Guide PERLREF(1)NAMEperlref - Perl references and nested data structures
NOTE
This is complete documentation about all aspects of refer
ences. For a shorter, tutorial introduction to just the
essential features, see the perlreftut manpage.
DESCRIPTION
Before release 5 of Perl it was difficult to represent
complex data structures, because all references had to be
symbolic--and even then it was difficult to refer to a
variable instead of a symbol table entry. Perl now not
only makes it easier to use symbolic references to vari
ables, but also lets you have "hard" references to any
piece of data or code. Any scalar may hold a hard refer
ence. 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. (Reference
counts for values in self-referential or cyclic data
structures may not go to zero without a little help; see
the Two-Phased Garbage Collection entry in the perlobj
manpage for a detailed explanation.) If that thing hap
pens 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 offi
cially "blessed" into a class package.)
Symbolic references are names of variables or other
objects, just as a symbolic link in a Unix filesystem con
tains merely the name of a file. The "*glob" notation is
something of a of symbolic reference. (Symbolic refer
ences 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, as in
the following paragraph, it is usually 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.
Making References
References can be created in several ways.
1. By using the backslash operator on a variable, subrou
tine, or value. (This works much like the & (address-
of) operator in C.) 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. The most you can get is a reference to a
typeglob, which is actually a complete symbol table
entry. But see the explanation of the "*foo{THING}"
syntax below. However, you can still use type globs
and globrefs as though they were IO handles.
2. A reference to an anonymous array can be created using
square brackets:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've created a reference to an anonymous array
of three elements whose final element is itself a ref
erence to another anonymous array of three elements.
(The multidimensional syntax described later can be
used to access this. For example, after the above,
"$arrayref->[2][1]" would have the value "b".)
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 refer
ences to the contents of "@foo", not a reference to
"@foo" itself. Likewise for "%foo", except that the
key references are to copies (since the keys are just
strings rather than full-fledged scalars).
3. A reference to an anonymous hash can be created using
curly brackets:
$hashref = {
'Adam' => 'Eve',
'Clyde' => 'Bonnie',
};
Anonymous hash and array composers like these can be
intermixed freely to produce as complicated a struc
ture 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 state
ments, 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
On the other hand, if you want the other meaning, you
can do this:
sub showem { { @_ } } # ambiguous (currently ok, but may change)
sub showem { {; @_ } } # ok
sub showem { { return @_ } } # ok
The leading "+{" and "{;" always serve to disambiguate
the expression to mean either the HASH reference, or
the BLOCK.
4. A reference to an anonymous subroutine can be created
by using "sub" without a subname:
$coderef = sub { print "Boink!\n" };
Note the semicolon. Except for the code inside not
being immediately executed, a "sub {}" is not so much
a declaration as it is an operator, like "do{}" or
"eval{}". (However, no matter how many times you exe
cute that particular 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 lexically visible
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
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 might also think of closure as a way to write a
subroutine template without using eval(). Here's a
small example of how closures work:
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 "my $x" having
gone out of scope by the time the anonymous subroutine
runs. That's what a closure is all about.
This applies only to lexical variables, by the way.
Dynamic variables continue to work as they have always
worked. Closure is not something that most Perl pro
grammers 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 type of object that happens to know which
package it's associated with. Constructors are just
special subroutines that know how to create that asso
ciation. They do so by starting with an ordinary ref
erence, and it remains an ordinary reference even
while it's also being an object. Constructors are
often named new() and called indirectly:
$objref = new Doggie (Tail => 'short', Ears => 'long');
But don't have to be:
$objref = Doggie->new(Tail => 'short', Ears => 'long');
use Term::Cap;
$terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
use Tk;
$main = MainWindow->new();
$menubar = $main->Frame(-relief => "raised",
-borderwidth => 2)
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.
"*foo{IO}" is an alternative to the "*HANDLE" mecha
nism given in the Typeglobs and Filehandles entry in
the perldata manpage for passing filehandles into or
out of subroutines, or storing into larger data struc
tures. Its disadvantage is that it won't create a new
filehandle for you. Its advantage is that you have
less risk of clobbering more than you want to with a
typeglob assignment. (It still conflates file and
directory handles, though.) However, if you assign
the incoming value to a scalar instead of a typeglob
as we do in the examples below, there's no risk of
that happening.
splutter(*STDOUT); # pass the whole glob
splutter(*STDOUT{IO}); # pass both file and dir handles
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(*STDIN); # pass the whole glob
$rec = get_rec(*STDIN{IO}); # pass both file and dir handles
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
Using References
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 identi
fiers) as part of a variable or subroutine name, you
can replace the identifier with a simple scalar vari
able containing a reference of the correct type:
$bar = $$scalarref;
push(@$arrayref, $filename);
$$arrayref[0] = "January";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically
not dereferencing "$arrayref[0]" or "$hashref{"KEY"}"
there. The dereference of the scalar variable happens
before it does any key lookups. Anything more compli
cated than a simple scalar variable must use methods 2
or 3 below. However, a "simple scalar" includes an
identifier that itself uses method 1 recursively.
Therefore, the following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of identi
fiers) as part of a variable or subroutine name, you
can replace the identifier with a BLOCK returning a
reference of the correct type. In other words, the
previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in
this case, but the BLOCK can contain any arbitrary
expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the sim
ple case of "$$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 ele
ments arise often enough that it gets cumbersome to
use method 2. As a form of syntactic sugar, the exam
ples 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 derefer
ence. 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. This process is
called autovivification.
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.
Using a string or number as a reference produces a sym
bolic reference, as explained above. Using a reference as
a number produces an integer representing its storage
location in memory. The only useful thing to be done with
this is to compare two references numerically to see
whether they refer to the same location.
if ($ref1 == $ref2) { # cheap numeric compare of references
print "refs 1 and 2 refer to the same thing\n";
}
Using a reference as a string produces both its referent's
type, including any package blessing as described in the
perlobj manpage, as well as the numeric address expressed
in hex. The ref() operator returns just the type of thing
the reference is pointing to, without the address. See
the ref entry in the perlfunc manpage for details and
examples of its use.
The bless() operator may be used to associate the object a
reference points to 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
type 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";
Symbolic references
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, 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 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 (globals, even if localized) 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.
Not-so-symbolic references
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", even though 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.) 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 "use warnings" pragma or the -w 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.
Pseudo-hashes: Using an array as a hash
WARNING: This section describes an experimental feature.
Details may change without notice in future versions.
Beginning with release 5.005 of Perl, you may use an array
reference in some contexts that would normally require a
hash reference. This allows you to access array elements
using symbolic names, as if they were fields in a struc
ture.
For this to work, the array must contain extra informa
tion. The first element of the array has to be a hash
reference that maps field names to array indices. Here is
an example:
$struct = [{foo => 1, bar => 2}, "FOO", "BAR"];
$struct->{foo}; # same as $struct->[1], i.e. "FOO"
$struct->{bar}; # same as $struct->[2], i.e. "BAR"
keys %$struct; # will return ("foo", "bar") in some order
values %$struct; # will return ("FOO", "BAR") in same some order
while (my($k,$v) = each %$struct) {
print "$k => $v\n";
}
Perl will raise an exception if you try to access nonexis
tent fields. To avoid inconsistencies, always use the
fields::phash() function provided by the "fields" pragma.
use fields;
$pseudohash = fields::phash(foo => "FOO", bar => "BAR");
For better performance, Perl can also do the translation
from field names to array indices at compile time for
typed object references. See the fields manpage.
There are two ways to check for the existence of a key in
a pseudo-hash. The first is to use exists(). This checks
to see if the given field has ever been set. It acts this
way to match the behavior of a regular hash. For
instance:
use fields;
$phash = fields::phash([qw(foo bar pants)], ['FOO']);
$phash->{pants} = undef;
print exists $phash->{foo}; # true, 'foo' was set in the declaration
print exists $phash->{bar}; # false, 'bar' has not been used.
print exists $phash->{pants}; # true, your 'pants' have been touched
The second is to use exists() on the hash reference sit
ting in the first array element. This checks to see if
the given key is a valid field in the pseudo-hash.
print exists $phash->[0]{bar}; # true, 'bar' is a valid field
print exists $phash->[0]{shoes};# false, 'shoes' can't be used
delete() on a pseudo-hash element only deletes the value
corresponding to the key, not the key itself. To delete
the key, you'll have to explicitly delete it from the
first hash element.
print delete $phash->{foo}; # prints $phash->[1], "FOO"
print exists $phash->{foo}; # false
print exists $phash->[0]{foo}; # true, key still exists
print delete $phash->[0]{foo}; # now key is gone
print $phash->{foo}; # runtime exception
Function Templates
As explained above, a closure is an anonymous function
with access to the lexical variables visible when that
function was compiled. It retains access to those vari
ables even though it doesn't get run until later, such as
in a signal handler or a Tk callback.
Using a closure as a function template allows us to
generate many functions that act similarly. Suppose you
wanted functions named after the colors that generated
HTML font changes for the various colors:
print "Be ", red("careful"), "with that ", green("light");
The red() and green() functions would be similar. To cre
ate these, we'll assign a closure to a typeglob of the
name of the function we're trying to build.
@colors = qw(red blue green yellow orange purple violet);
for my $name (@colors) {
no strict 'refs'; # allow symbol table manipulation
*$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
}
Now all those different functions appear to exist indepen
dently. You can call red(), RED(), blue(), BLUE(),
green(), etc. This technique saves on both compile time
and memory use, and is less error-prone as well, since
syntax checks happen at compile time. It's critical that
any variables in the anonymous subroutine be lexicals in
order to create a proper closure. That's the reasons for
the "my" on the loop iteration variable.
This is one of the only places where giving a prototype to
a closure makes much sense. If you wanted to impose
scalar context on the arguments of these functions (proba
bly not a wise idea for this particular example), you
could have written it this way instead:
*$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
However, since prototype checking happens at compile time,
the assignment above happens too late to be of much use.
You could address this by putting the whole loop of
assignments within a BEGIN block, forcing it to occur dur
ing compilation.
Access to lexicals that change over type--like those in
the "for" loop above--only works with closures, not gen
eral subroutines. In the general case, then, named sub
routines do not nest properly, although anonymous ones do.
If you are accustomed to using nested subroutines in other
programming languages with their own private variables,
you'll have to work at it a bit in Perl. The intuitive
coding of this type of thing incurs mysterious warnings
about ``will not stay shared''. For example, this won't
work:
sub outer {
my $x = $_[0] + 35;
sub inner { return $x * 19 } # WRONG
return $x + inner();
}
A work-around is the following:
sub outer {
my $x = $_[0] + 35;
local *inner = sub { return $x * 19 };
return $x + inner();
}
Now inner() can only be called from within outer(),
because of the temporary assignments of the closure
(anonymous subroutine). But when it does, it has normal
access to the lexical variable $x from the scope of
outer().
This has the interesting effect of creating a function
local to another function, something not normally sup
ported in Perl.
WARNING
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 attempt
ing. 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.
The standard Tie::RefHash module provides a convenient
workaround to this.
SEE ALSO
Besides the obvious documents, source code can be instruc
tive. Some 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 perltoot manpage, the perlobj manpage, and the
perlbot manpage for how to use them to create objects.
2001-03-18 perl v5.6.1 PERLREF(1)
