PERLREF(1) Perl Programmers Reference Guide PERLREF(1)NAME
perlref - 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 perlreftut.
DESCRIPTION
Before release 5 of Perl it was difficult to represent com-
plex data structures, because all references had to be
symbolic--and even then it was difficult to refer to a vari-
able instead of a symbol table entry. Perl now not only
makes it easier to use symbolic references to variables, 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 Gar-
bage Collection" in perlobj for a detailed explanation.) If
that thing happens to be an object, the object is des-
tructed. 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 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 over-
riding principle: Perl does no implicit referencing or dere-
ferencing. 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.
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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 glo-
brefs as though they were IO handles.
2. A reference to an anonymous array can be created using
square brackets:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've created a reference to an anonymous array of
three elements whose final element is itself a reference
to another anonymous array of three elements. (The mul-
tidimensional syntax described later can be used to
access this. For example, after the above,
"$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 refer-
ences are to copies (since the keys are just strings
rather than full-fledged scalars).
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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 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
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
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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() 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 perlobj.
You might also think of closure as a way to write a sub-
routine 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
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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 associ-
ation. 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 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};
$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 "socket-
pair" 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.
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*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 mechanism
given in "Typeglobs and Filehandles" in perldata for
passing filehandles into or out of subroutines, or stor-
ing 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 identif-
iers) 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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically
not dereferencing $arrayref[0] or $hashref{"KEY"} there.
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The dereference of the scalar variable happens before it
does any key lookups. Anything more complicated than a
simple scalar variable must use methods 2 or 3 below.
However, a "simple scalar" includes an identifier that
itself uses method 1 recursively. Therefore, the fol-
lowing prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of identif-
iers) as part of a variable or subroutine name, you can
replace the identifier with a BLOCK returning a refer-
ence of the correct type. In other words, the previous
examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in
this case, but the BLOCK can contain any arbitrary
expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple
case of $$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 ele-
ments 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:
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$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 return-
ing 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. 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 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.
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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 per-
lobj, 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 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.
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$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 pos-
sible 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
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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 space-
less form.) This construct is not considered to be a sym-
bolic 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 bare-
word 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.
NOTE: The current user-visible implementation of pseudo-
hashes (the weird use of the first array element) is depre-
cated starting from Perl 5.8.0 and will be removed in Perl
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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
available.
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 nonex-
istent 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 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
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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 functions that act similarly. Suppose you wanted func-
tions 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 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.
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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 (probably not a
wise idea for this particular example), you could have writ-
ten 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 assign-
ments within a BEGIN block, forcing it to occur during com-
pilation.
Access to lexicals that change over type--like those in the
"for" loop above--only works with closures, not general sub-
routines. In the general 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 anonymous sub-
routines get to capture each time you execute the 'sub'
operator. If you are accustomed to using nested subroutines
in other programming languages with their own private vari-
ables, you'll have to work at it a bit in Perl. The intui-
tive 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 sub-
routine). 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.
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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 wor-
karound 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 perldsc and perllol for how to use references to
create complex data structures, and perltoot, perlobj, and
perlbot for how to use them to create objects.
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