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 symbolicand 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 smartthey 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
*globnotation is something of a symbolic reference. (Symbolic references are sometimes called soft references, but please dont 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 doesnt 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
theres already a reference to the variable in the symbol table. But
the symbol table reference might go away, and youll still have the
reference that the backslash returned. Here are some examples:
$scalarref = \$foo;
$arrayref = \@ARGV;
$hashref = \%ENV;
$coderef = \&handler;
$globref = \*foo;
It isnt 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 weve 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 bracketsinstead its 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 @fooitself. 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 returnin front so that Perl realizes the opening brace isnt 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
subwithout 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 youre in an eval("...")), $coderefwill 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 its called outside the context. In human terms, its a funny way of passing arguments to a subroutine when you define it as well as when you call it. Its 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 thatsee perlobj. You might also think of closure as a way to write a subroutine template without using eval(). Heres 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 $xcontinues to refer to the value passed into newprint() despite my $xhaving gone out of scope by the time the anonymous subroutine runs. Thats 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 its 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 its 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 its 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 havent talked about dereferencing yet, we cant 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 hasnt been used yet, except in the case of scalars. *foo{SCALAR}returns a reference to an anonymous scalar if $foohasnt been used yet. This might change in a future release.
*foo{IO}is an alternative to the *HANDLEmechanism 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 wont 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, theres 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
Thats it for creating references. By now youre probably dying to know how to use references to get back to your long-lost data. There are several basic methods.
1. |
Anywhere youd 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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
Its important to understand that we are specifically not dereferencing $arrayref[0]or $hashref{"KEY"}there. The dereference of the scalar variable happens before it does any key lookups. Anything more complicated than a simple scalar variable must use methods 2 or 3 below. However, a simple scalar includes an identifier that itself uses method 1 recursively. Therefore, the following prints howdy.
$refrefref = \\\"howdy";
print $$$$refrefref;
|
2. |
Anywhere youd put an identifier (or chain of identifiers) as part of a
variable or subroutine name, you can replace the identifier with a
BLOCK returning a reference of the correct type. In other words, the
previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, its a little silly to use the curlies in this case, but the BLOCK can contain any arbitrary expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of $$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. Thats 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 youre accessing a variable called %hashref, not dereferencing through $hashrefto the hash its 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, its 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 Cs:
$score[$x][$y][$z] += 42;
Well, okay, not entirely like Cs arrays, actually. C doesnt 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 youre in the class package that defines the objects methods. In other words, be nice, and dont violate the objects encapsulation without a very good reason. Perl does not enforce encapsulation. We are not totalitarians here. We do expect some basic civility though. |
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 referents 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
${\$foo}both indicate the same scalar variable.
Heres a trick for interpolating a subroutine call into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
The way it works is that when the
@{...}is seen in the double-quoted string, its evaluated as a block. The block creates a reference to an anonymous array containing the results of the call to
mysub(1,2,3). So the whole block returns a reference to an array, which is then dereferenced by
@{...}and stuck into the double-quoted string. This chicanery is also useful for arbitrary expressions:
print "That yields @{[$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";
Symbolic references
We said that references spring into existence as necessary if they are undefined, but we didnt say what happens if a value used as a reference is already defined, but isnt a hard reference. If you use it as a reference, itll 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";
$$name = 1; # Sets $foo
${$name} = 2; # Sets $foo
${$name x 2} = 3; # Sets $foofoo
$name->[0] = 4; # Sets $foo[0]
@$name = (); # Clears @foo
&$name(); # Calls &foo() (as in Perl 4)
$pack = "THAT";
${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
This is powerful, and slightly dangerous, in that its possible to intend (with the utmost sincerity) to use a hard reference, and accidentally use a symbolic reference instead. To protect against that, you can say
use strict refs;
and then only hard references will be allowed for the rest of the enclosing block. An inner block may countermand that with
no strict refs;
Only package variables (globals, even if localized) are visible to symbolic references. Lexical variables (declared with my()) arent in a symbol table, and thus are invisible to this mechanism. For example:
local $value = 10;
$ref = "value";
{
my $value = 20;
print $$ref;
}
This will still print 10, not 20. Remember that local() affects package variables, which are all global to the package.
Not-so-symbolic references
A new feature contributing to readability in perl version 5.001 is that the brackets around a symbolic reference behave more like quotes, just as they always have within a string. That is,
$push = "pop on ";
print "${push}over";
has always meant to print pop on over, even though push is a reserved word. This has been generalized to work the same outside of quotes, so that
print ${push} . "over";
and even
print ${ push } . "over";
will have the same effect. (This would have been a syntax error in Perl 5.000, though Perl 4 allowed it in the spaceless form.) This construct is not considered to be a symbolic reference when youre 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, weve 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 warningspragma 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 doesnt 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, well assign a closure to a typeglob of the name of the function were 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. Its critical that any variables in the anonymous subroutine be lexicals in order to create a proper closure. Thats the reasons for the
myon 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 timelike those in the
forloop 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
suboperator, as they are created on the fly. If you are accustomed to using nested subroutines in other programming languages with their own private variables, youll 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 wont 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
$xfrom 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 wont do a hard dereference, and you wont accomplish what youre 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 wont.
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 perltoot, perlobj, and perlbot for how to use them to create objects.