perlref(1)
NAME
perlref - Perl references and nested data structures
NOTE
This is complete documentation about all aspects of references. For a
shorter, tutorial introduction to just the essential features, see
perlreftut.
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 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. (Reference counts for values in self-referential or
cyclic data structures may not go to zero without a little help; see
"Two-Phased Garbage Collection" in perlobj for a detailed explanation.)
If that thing happens to be an object, the object is destructed. See
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.)
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 something of a symbolic reference. (Symbolic
references are sometimes called "soft references", but please don't
call them that; references are confusing enough without useless syn-
onyms.)
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 with-
out 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 prin-
ciple: 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, subroutine, 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 (file-
handle 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:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've created a reference to an anonymous array of three ele-
ments whose final element is itself a reference to another anony-
mous array of three elements. (The multidimensional syntax
described later can be used to access this. For example, after the
above, "$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 refer-
ences!
@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, 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 brack-
ets:
$hashref = {
'Adam' => 'Eve',
'Clyde' => 'Bonnie',
};
Anonymous hash and array composers like these can be intermixed
freely to produce as complicated a structure as you want. The mul-
tidimensional syntax described below works for these too. The val-
ues 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 decla-
rations.
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 expres-
sion 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 immedi-
ately 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 execute 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() vari-
ables, 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 subrou-
tine 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 perlobj.
You might also think of closure as a way to write a subroutine tem-
plate 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 vari-
ables 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 con-
structors. 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 association. They do so by starting with an ordinary refer-
ence, 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 exam-
ples 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};
$formatref = *foo{FORMAT};
All of these are self-explanatory except for *foo{IO}. It returns
the IO handle, used for file handles ("open" in perlfunc), sockets
("socket" in perlfunc and "socketpair" in perlfunc), and directory
handles ("opendir" in perlfunc). For compatibility with previous
versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO},
though it is deprecated as of 5.8.0. If deprecation warnings are
in effect, it will warn of its use.
*foo{THING} returns undef if that particular THING hasn't been used
yet, except in the case of scalars. *foo{SCALAR} returns a refer-
ence 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 mechanism given in "Type-
globs and Filehandles" in perldata for passing filehandles into or
out of subroutines, or storing into larger data structures. 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 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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically not derefer-
encing $arrayref[0] or $hashref{"KEY"} there. The dereference of
the scalar variable happens before it does any key lookups. Any-
thing more complicated than a simple scalar variable must use meth-
ods 2 or 3 below. However, a "simple scalar" includes an identi-
fier that itself uses method 1 recursively. Therefore, the follow-
ing prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of identifiers) as part
of a variable or subroutine name, you can replace the identifier
with a BLOCK returning a reference of the correct type. In other
words, the previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in this case,
but the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of
$$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 pre-
sumably 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 refer-
ence, 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 auto-
matically 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 sub-
scripts, 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 symbolic 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 perlobj, 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
"ref" in perlfunc 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 perlobj.
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 refer-
ence 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 acci-
dentally 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 sym-
bolic 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 pack-
age 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 con-
struct 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 inter-
prets 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.
NOTE: The current user-visible implementation of pseudo-hashes (the
weird use of the first array element) is deprecated starting from Perl
5.8.0 and will be removed in Perl 5.10.0, and the feature will be
implemented differently. Not only is the current interface rather
ugly, but the current implementation slows down normal array and hash
use quite noticeably. The 'fields' pragma interface will remain avail-
able.
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 structure.
For this to work, the array must contain extra information. 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 nonexistent fields.
To avoid inconsistencies, always use the fields::phash() function pro-
vided 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 fields.
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 sitting 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, an anonymous function with access to the lexical
variables visible when that function was compiled, creates a closure.
It retains access to those variables 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 func-
tions 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 create 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 independently. 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 cre-
ate 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 argu-
ments of these functions (probably 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 assign-
ment 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 during compilation.
Access to lexicals that change over type--like those in the "for" loop
above--only works with closures, not general subroutines. In the gen-
eral case, then, named subroutines do not nest properly, although
anonymous ones do. Thus is because named subroutines are created (and
capture any outer lexicals) only once at compile time, whereas anony-
mous subroutines get to capture each time you execute the 'sub' opera-
tor. If you are accustomed to using nested subroutines in other pro-
gramming 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 tem-
porary 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 supported 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 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.
The standard Tie::RefHash module provides a convenient workaround to
this.
SEE ALSO
Besides the obvious documents, source code can be instructive. 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 perldsc and perllol for how to use references to create com-
plex data structures, and perltoot, perlobj, and perlbot for how to use
them to create objects.
perl v5.8.8 2006-06-14 PERLREF(1)
Man(1) output converted with
man2html