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 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
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
"Circular References" 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
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.
References are easy to use in Perl. There is just one overriding
principle: in general, 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.
That said, be aware that Perl version 5.14 introduces an exception to
the rule, for syntactic convenience. Experimental array and hash
container function behavior allows array and hash references to be
handled by Perl as if they had been explicitly syntactically
dereferenced. See "Syntactical Enhancements" in perl5140delta and
perlfunc for details.
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
(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:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've created a reference to an anonymous array of three
elements whose final element is itself a reference to another
anonymous array of three elements. (The multidimensional syntax
described later can be used to access this. For example, after the
above, "$arrayref->[2][1]" would have the value "b".)
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, 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 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 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 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 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 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
reference, and it remains an ordinary reference even while it's
also being an object. Constructors are often named "new()". You
can call them indirectly:
$objref = new Doggie( Tail => 'short', Ears => 'long' );
But that can produce ambiguous syntax in certain cases, so it's
often better to use the direct method invocation approach:
$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 "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
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 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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically not
dereferencing $arrayref[0] or $hashref{"KEY"} there. The
dereference of the scalar variable happens before it does any key
lookups. Anything more complicated than a simple scalar variable
must use methods 2 or 3 below. However, a "simple scalar" includes
an identifier that itself uses method 1 recursively. Therefore,
the following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of identifiers) as part
of a variable or subroutine name, you can replace the identifier
with a BLOCK returning a reference of the correct type. In other
words, the previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in this case,
but the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of
$$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. 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.
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";
Similarly, an expression that returns a reference to a scalar can be
dereferenced via "${...}". Thus, the above expression may be written
as:
print "That yields ${\($n + 5)} widgets\n";
Circular References
It is possible to create a "circular reference" in Perl, which can lead
to memory leaks. A circular reference occurs when two references
contain a reference to each other, like this:
my $foo = {};
my $bar = { foo => $foo };
$foo->{bar} = $bar;
You can also create a circular reference with a single variable:
my $foo;
$foo = \$foo;
In this case, the reference count for the variables will never reach 0,
and the references will never be garbage-collected. This can lead to
memory leaks.
Because objects in Perl are implemented as references, it's possible to
have circular references with objects as well. Imagine a TreeNode class
where each node references its parent and child nodes. Any node with a
parent will be part of a circular reference.
You can break circular references by creating a "weak reference". A
weak reference does not increment the reference count for a variable,
which means that the object can go out of scope and be destroyed. You
can weaken a reference with the "weaken" function exported by the
Scalar::Util module.
Here's how we can make the first example safer:
use Scalar::Util 'weaken';
my $foo = {};
my $bar = { foo => $foo };
$foo->{bar} = $bar;
weaken $foo->{bar};
The reference from $foo to $bar has been weakened. When the $bar
variable goes out of scope, it will be garbage-collected. The next time
you look at the value of the "$foo->{bar}" key, it will be "undef".
This action at a distance can be confusing, so you should be careful
with your use of weaken. You should weaken the reference in the
variable that will go out of scope first. That way, the longer-lived
variable will contain the expected reference until it goes out of
scope.
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
Since Perl verion 5.001, brackets around a symbolic reference can
simply serve to isolate an identifier or variable name from the rest of
an expression, just as they always have within a string. For example,
$push = "pop on ";
print "${push}over";
has always meant to print "pop on over", even though push is a reserved
word. In 5.001, this was generalized to work the same without the
enclosing double 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, the same rule applies 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
Pseudo-hashes have been removed from Perl. The 'fields' pragma remains
available.
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 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
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 (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
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 during compilation.
Access to lexicals that change over time--like those in the "for" loop
above, basically aliases to elements from the surrounding lexical
scopes-- only works with anonymous subs, not with named subroutines.
Generally said, named subroutines do not nest properly and should only
be declared in the main package scope.
This is because named subroutines are created at compile time so their
lexical variables get assigned to the parent lexicals from the first
execution of the parent block. If a parent scope is entered a second
time, its lexicals are created again, while the nested subs still
reference the old ones.
Anonymous subroutines get to capture each time you execute the "sub"
operator, as they are created on the fly. 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" due to the reasons explained above. 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 anonymous subroutine. But when it does, it
has normal access to the lexical variable $x from the scope of outer()
at the time outer is invoked.
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
complex data structures, and perlootut and perlobj for how to use them
to create objects.
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