PERLREF(1) PERLREF(1)
NAME
perlref - Perl references and nested data structures
DESCRIPTION
Before release 5 of Perl it was difficult to represent complex data
structures, because all references had to be symbolic, and even that was
difficult to do when you wanted to refer to a variable rather than a
symbol table entry. Perl not only makes it easier to use symbolic
references to variables, but lets you have "hard" references to any piece
of data. Any scalar may hold a hard reference. Because arrays and
hashes contain scalars, you can now easily build arrays of arrays, arrays
of hashes, hashes of arrays, arrays of hashes of functions, and so on.
Hard references are smart--they keep track of reference counts for you,
automatically freeing the thing referred to when its reference count goes
to zero. (Note: The reference counts for values in self-referential or
cyclic data structures may not go to zero without a little help; see the
section on Two-Phased Garbage Collection in the perlobj manpage for a
detailed explanation. If that thing happens to be an object, the object
is destructed. See the perlobj manpage for more about objects. (In a
sense, everything in Perl is an object, but we usually reserve the word
for references to objects that have been officially "blessed" into a
class package.)
Symbolic references are names of variables or other objects, just as a
symbolic link in a Unix filesystem contains merely the name of a file.
The *glob notation is a kind of symbolic reference. (Symbolic references
are sometimes called "soft references", but please don't call them that;
references are confusing enough without useless synonyms.)
In contrast, hard references are more like hard links in a Unix file
system: They are used to access an underlying object without concern for
what its (other) name is. When the word "reference" is used without an
adjective, like in the following paragraph, it usually is talking about a
hard reference.
References are easy to use in Perl. There is just one overriding
principle: Perl does no implicit referencing or dereferencing. When a
scalar is holding a reference, it always behaves as a simple scalar. It
doesn't magically start being an array or hash or subroutine; you have to
tell it explicitly to do so, by dereferencing it.
References can be constructed in several ways.
1. By using the backslash operator on a variable, subroutine, or value.
(This works much like the & (address-of) operator in C.) Note that
this typically creates ANOTHER reference to a variable, because
there's already a reference to the variable in the symbol table. But
the symbol table reference might go away, and you'll still have the
reference that the backslash returned. Here are some examples:
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$scalarref = \$foo;
$arrayref = \@ARGV;
$hashref = \%ENV;
$coderef = \&handler;
$globref = \*foo;
It isn't possible to create a true reference to an IO handle
(filehandle or dirhandle) using the backslash operator. See the
explanation of the *foo{THING} syntax below. (However, you're apt to
find Perl code out there using globrefs as though they were IO
handles, which is grandfathered into continued functioning.)
2. A reference to an anonymous array can be constructed using square
brackets:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've constructed a reference to an anonymous array of three
elements whose final element is itself a reference to another
anonymous array of three elements. (The multidimensional syntax
described later can be used to access this. For example, after the
above, $arrayref->[2][1] would have the value "b".)
Note that taking a reference to an enumerated list is not the same as
using square brackets--instead it's the same as creating a list of
references!
@list = (\$a, \@b, \%c);
@list = \($a, @b, %c); # same thing!
As a special case, \(@foo) returns a list of references to the
contents of @foo, not a reference to @foo itself. Likewise for %foo.
3. A reference to an anonymous hash can be constructed using curly
brackets:
$hashref = {
'Adam' => 'Eve',
'Clyde' => 'Bonnie',
};
Anonymous hash and array constructors can be intermixed freely to
produce as complicated a structure as you want. The multidimensional
syntax described below works for these too. The values above are
literals, but variables and expressions would work just as well,
because assignment operators in Perl (even within local() or my())
are executable statements, not compile-time declarations.
Because curly brackets (braces) are used for several other things
including BLOCKs, you may occasionally have to disambiguate braces at
the beginning of a statement by putting a + or a return in front so
that Perl realizes the opening brace isn't starting a BLOCK. The
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economy and mnemonic value of using curlies is deemed worth this
occasional extra hassle.
For example, if you wanted a function to make a new hash and return a
reference to it, you have these options:
sub hashem { { @_ } } # silently wrong
sub hashem { +{ @_ } } # ok
sub hashem { return { @_ } } # ok
4. A reference to an anonymous subroutine can be constructed by using
sub without a subname:
$coderef = sub { print "Boink!\n" };
Note the presence of the semicolon. Except for the fact that the
code inside isn't executed immediately, a sub {} is not so much a
declaration as it is an operator, like do{} or eval{}. (However, no
matter how many times you execute that line (unless you're in an
eval("...")), $coderef will still have a reference to the SAME
anonymous subroutine.)
Anonymous subroutines act as closures with respect to my() variables,
that is, variables visible lexically within the current scope.
Closure is a notion out of the Lisp world that says if you define an
anonymous function in a particular lexical context, it pretends to
run in that context even when it's called outside of the context.
In human terms, it's a funny way of passing arguments to a subroutine
when you define it as well as when you call it. It's useful for
setting up little bits of code to run later, such as callbacks. You
can even do object-oriented stuff with it, though Perl already
provides a different mechanism to do that--see the perlobj manpage.
You can also think of closure as a way to write a subroutine template
without using eval. (In fact, in version 5.000, eval was the only
way to get closures. You may wish to use "require 5.001" if you use
closures.)
Here's a small example of how closures works:
sub newprint {
my $x = shift;
return sub { my $y = shift; print "$x, $y!\n"; };
}
$h = newprint("Howdy");
$g = newprint("Greetings");
# Time passes...
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&$h("world");
&$g("earthlings");
This prints
Howdy, world!
Greetings, earthlings!
Note particularly that $x continues to refer to the value passed into
newprint() despite the fact that the "my $x" has seemingly gone out
of scope by the time the anonymous subroutine runs. That's what
closure is all about.
This applies to only lexical variables, by the way. Dynamic
variables continue to work as they have always worked. Closure is
not something that most Perl programmers need trouble themselves
about to begin with.
5. References are often returned by special subroutines called
constructors. Perl objects are just references to a special kind of
object that happens to know which package it's associated with.
Constructors are just special subroutines that know how to create
that association. They do so by starting with an ordinary reference,
and it remains an ordinary reference even while it's also being an
object. Constructors are customarily named new(), but don't have to
be:
$objref = new Doggie (Tail => 'short', Ears => 'long');
6. References of the appropriate type can spring into existence if you
dereference them in a context that assumes they exist. Because we
haven't talked about dereferencing yet, we can't show you any
examples yet.
7. A reference can be created by using a special syntax, lovingly known
as the *foo{THING} syntax. *foo{THING} returns a reference to the
THING slot in *foo (which is the symbol table entry which holds
everything known as foo).
$scalarref = *foo{SCALAR};
$arrayref = *ARGV{ARRAY};
$hashref = *ENV{HASH};
$coderef = *handler{CODE};
$ioref = *STDIN{IO};
$globref = *foo{GLOB};
All of these are self-explanatory except for *foo{IO}. It returns
the IO handle, used for file handles (the open entry in the perlfunc
manpage), sockets (the socket entry in the perlfunc manpage and the
socketpair entry in the perlfunc manpage), and directory handles (the
opendir entry in the perlfunc manpage). For compatibility with
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previous versions of Perl, *foo{FILEHANDLE} is a synonym for
*foo{IO}.
*foo{THING} returns undef if that particular THING hasn't been used
yet, except in the case of scalars. *foo{SCALAR} returns a reference
to an anonymous scalar if $foo hasn't been used yet. This might
change in a future release.
The use of *foo{IO} is the best way to pass bareword filehandles into
or out of subroutines, or to store them in larger data structures.
splutter(*STDOUT{IO});
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(*STDIN{IO});
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
Beware, though, that you can't do this with a routine which is going
to open the filehandle for you, because *HANDLE{IO} will be undef if
HANDLE hasn't been used yet. Use \*HANDLE for that sort of thing
instead.
Using \*HANDLE (or *HANDLE) is another way to use and store non-
bareword filehandles (before perl version 5.002 it was the only way).
The two methods are largely interchangeable, you can do
splutter(\*STDOUT);
$rec = get_rec(\*STDIN);
with the above subroutine definitions.
That's it for creating references. By now you're probably dying to know
how to use references to get back to your long-lost data. There are
several basic methods.
1. Anywhere you'd put an identifier (or chain of identifiers) as part of
a variable or subroutine name, you can replace the identifier with a
simple scalar variable containing a reference of the correct type:
$bar = $$scalarref;
push(@$arrayref, $filename);
$$arrayref[0] = "January";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
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It's important to understand that we are specifically NOT
dereferencing $arrayref[0] or $hashref{"KEY"} there. The dereference
of the scalar variable happens BEFORE it does any key lookups.
Anything more complicated than a simple scalar variable must use
methods 2 or 3 below. However, a "simple scalar" includes an
identifier that itself uses method 1 recursively. Therefore, the
following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of identifiers) as part of
a variable or subroutine name, you can replace the identifier with a
BLOCK returning a reference of the correct type. In other words, the
previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in this case, but
the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of $$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:
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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 returning a
reference, including a previous dereference. Note that $array[$x] is
NOT the same thing as $array->[$x] here:
$array[$x]->{"foo"}->[0] = "January";
This is one of the cases we mentioned earlier in which references
could spring into existence when in an lvalue context. Before this
statement, $array[$x] may have been undefined. If so, it's
automatically defined with a hash reference so that we can look up
{"foo"} in it. Likewise $array[$x]->{"foo"} will automatically get
defined with an array reference so that we can look up [0] in it.
One more thing here. The arrow is optional BETWEEN brackets
subscripts, so you can shrink the above down to
$array[$x]{"foo"}[0] = "January";
Which, in the degenerate case of using only ordinary arrays, gives
you multidimensional arrays just like C's:
$score[$x][$y][$z] += 42;
Well, okay, not entirely like C's arrays, actually. C doesn't know
how to grow its arrays on demand. Perl does.
4. If a reference happens to be a reference to an object, then there are
probably methods to access the things referred to, and you should
probably stick to those methods unless you're in the class package
that defines the object's methods. In other words, be nice, and
don't violate the object's encapsulation without a very good reason.
Perl does not enforce encapsulation. We are not totalitarians here.
We do expect some basic civility though.
The ref() operator may be used to determine what type of thing the
reference is pointing to. See the perlfunc manpage.
The bless() operator may be used to associate a reference with a package
functioning as an object class. See the perlobj manpage.
A typeglob may be dereferenced the same way a reference can, because the
dereference syntax always indicates the kind of reference desired. So
${*foo} and ${\$foo} both indicate the same scalar variable.
Here's a trick for interpolating a subroutine call into a string:
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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 in this case, it'll be treated as a symbolic reference. That
is, the value of the scalar is taken to be the NAME of a variable, rather
than a direct link to a (possibly) anonymous value.
People frequently expect it to work like this. So it does.
$name = "foo";
$$name = 1; # Sets $foo
${$name} = 2; # Sets $foo
${$name x 2} = 3; # Sets $foofoo
$name->[0] = 4; # Sets $foo[0]
@$name = (); # Clears @foo
&$name(); # Calls &foo() (as in Perl 4)
$pack = "THAT";
${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
This is very powerful, and slightly dangerous, in that it's possible to
intend (with the utmost sincerity) to use a hard reference, and
accidentally use a symbolic reference instead. To protect against that,
you can say
use strict 'refs';
and then only hard references will be allowed for the rest of the
enclosing block. An inner block may countermand that with
no strict 'refs';
Only package variables are visible to symbolic references. Lexical
variables (declared with my()) aren't in a symbol table, and thus are
invisible to this mechanism. For example:
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local($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", despite the fact that push is a
reserved word. This has been generalized to work the same outside of
quotes, so that
print ${push} . "over";
and even
print ${ push } . "over";
will have the same effect. (This would have been a syntax error in Perl
5.000, though Perl 4 allowed it in the spaceless form.) Note that this
construct is not considered to be a symbolic reference when you're using
strict refs:
use strict 'refs';
${ bareword }; # Okay, means $bareword.
${ "bareword" }; # Error, symbolic reference.
Similarly, because of all the subscripting that is done using single
words, we've applied the same rule to any bareword that is used for
subscripting a hash. So now, instead of writing
$array{ "aaa" }{ "bbb" }{ "ccc" }
you can write just
$array{ aaa }{ bbb }{ ccc }
and not worry about whether the subscripts are reserved words. In the
rare event that you do wish to do something like
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$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 -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.
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.
SEE ALSO
Besides the obvious documents, source code can be instructive. Some
rather pathological examples of the use of references can be found in the
t/op/ref.t regression test in the Perl source directory.
See also the perldsc manpage and the perllol manpage for how to use
references to create complex data structures, and the perlobj manpage for
how to use them to create objects.
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