MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

PERLREF

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlref - Perl references and nested data structures

DESCRIPTION

Before release 5 of Perl it was difficult to represent complex datastructures, because all references had to be symbolic, and even that wasdifficult to do when you wanted to refer to a variable rather than asymbol table entry. Perl not only makes it easier to use symbolicreferences to variables, but lets you have ``hard'' references to any pieceof data. Any scalar may hold a hard reference. Because arrays and hashescontain scalars, you can now easily build arrays of arrays, arrays ofhashes, 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 goesto zero. (Note: The reference counts for values in self-referential orcyclic data structures may not go to zero without a little help; seethe 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. Seethe perlobj manpage for more about objects. (In a sense, everything in Perl is anobject, but we usually reserve the word for references to objects thathave been officially ``blessed'' into a class package.)

Symbolic references are names of variables or other objects, just as asymbolic link in a Unix filesystem contains merely the name of a file.The *glob notation is a kind of symbolic reference. (Symbolicreferences are sometimes called ``soft references'', but please don't callthem that; references are confusing enough without useless synonyms.)

In contrast, hard references are more like hard links in a Unix filesystem: They are used to access an underlying object without concern forwhat its (other) name is. When the word ``reference'' is used without anadjective, like in the following paragraph, it usually is talking about ahard reference.

References are easy to use in Perl. There is just one overridingprinciple: Perl does no implicit referencing or dereferencing. When ascalar is holding a reference, it always behaves as a simple scalar. Itdoesn't magically start being an array or hash or subroutine; you have totell 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.) Notethat this typically creates ANOTHER reference to a variable, becausethere's already a reference to the variable in the symbol table. Butthe symbol table reference might go away, and you'll still have thereference 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 ordirhandle) using the backslash operator. See the explanation of the*foo{THING} syntax below. (However, you're apt to find Perl codeout there using globrefs as though they were IO handles, which isgrandfathered into continued functioning.)

2.

A reference to an anonymous array can be constructed using squarebrackets:

    $arrayref = [1, 2, ['a', 'b', 'c']];

Here we've constructed a reference to an anonymous array of three elementswhose final element is itself a reference to another anonymous array of threeelements. (The multidimensional syntax described later can be used toaccess this. For example, after the above, $arrayref->[2][1] would havethe value ``b''.)

Note that taking a reference to an enumerated list is not the sameas using square brackets---instead it's the same as creatinga 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 contentsof @foo, not a reference to @foo itself. Likewise for %foo.

3.

A reference to an anonymous hash can be constructed using curlybrackets:

    $hashref = {        'Adam'  => 'Eve',        'Clyde' => 'Bonnie',    };

Anonymous hash and array constructors can be intermixed freely toproduce as complicated a structure as you want. The multidimensionalsyntax described below works for these too. The values above areliterals, but variables and expressions would work just as well, becauseassignment operators in Perl (even within local() or my()) are executablestatements, not compile-time declarations.

Because curly brackets (braces) are used for several other thingsincluding BLOCKs, you may occasionally have to disambiguate braces at thebeginning of a statement by putting a + or a return in front sothat Perl realizes the opening brace isn't starting a BLOCK. The economy andmnemonic value of using curlies is deemed worth this occasional extrahassle.

For example, if you wanted a function to make a new hash and return areference 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 usingsub without a subname:

    $coderef = sub { print "Boink!\n" };

Note the presence of the semicolon. Except for the fact that the codeinside isn't executed immediately, a sub {} is not so much adeclaration as it is an operator, like do{} or eval{}. (However, nomatter how many times you execute that line (unless you're in aneval("...")), $coderef will still have a reference to the SAMEanonymous subroutine.)

Anonymous subroutines act as closures with respect to my() variables,that is, variables visible lexically within the current scope. Closureis a notion out of the Lisp world that says if you define an anonymousfunction in a particular lexical context, it pretends to run in thatcontext even when it's called outside of the context.

In human terms, it's a funny way of passing arguments to a subroutine whenyou define it as well as when you call it. It's useful for setting uplittle bits of code to run later, such as callbacks. You can evendo object-oriented stuff with it, though Perl already provides a differentmechanism to do that---see the perlobj manpage.

You can also think of closure as a way to write a subroutine template withoutusing eval. (In fact, in version 5.000, eval was the only way to getclosures. 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...

    &$h("world");    &$g("earthlings");

This prints

    Howdy, world!    Greetings, earthlings!

Note particularly that $x continues to refer to the value passed intonewprint() despite the fact that the ``my $x'' has seemingly gone out ofscope by the time the anonymous subroutine runs. That's what closureis all about.

This applies to only lexical variables, by the way. Dynamic variablescontinue to work as they have always worked. Closure is not somethingthat 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 knowwhich package it's associated with. Constructors are just specialsubroutines that know how to create that association. They do so bystarting with an ordinary reference, and it remains an ordinary referenceeven while it's also being an object. Constructors are customarilynamed 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 youdereference them in a context that assumes they exist. Because we haven'ttalked about dereferencing yet, we can't show you any examples yet.

7.

A reference can be created by using a special syntax, lovingly known asthe *foo{THING} syntax. *foo{THING} returns a reference to the THINGslot in *foo (which is the symbol table entry which holds everythingknown 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 theIO 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 previous versions ofPerl, *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 ananonymous scalar if $foo hasn't been used yet. This might change in afuture release.

The use of *foo{IO} is the best way to pass bareword filehandles into orout 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 toopen the filehandle for you, because *HANDLE{IO} will be undef if HANDLEhasn't been used yet. Use \*HANDLE for that sort of thing instead.

Using \*HANDLE (or *HANDLE) is another way to use and store non-barewordfilehandles (before perl version 5.002 it was the only way). The twomethods 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 toknow how to use references to get back to your long-lost data. Thereare several basic methods.

1.

Anywhere you'd put an identifier (or chain of identifiers) as partof a variable or subroutine name, you can replace the identifier witha 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 thescalar variable happens BEFORE it does any key lookups. Anything morecomplicated than a simple scalar variable must use methods 2 or 3 below.However, a ``simple scalar'' includes an identifier that itself uses method1 recursively. Therefore, the following prints ``howdy''.

    $refrefref = \\\"howdy";    print $$$$refrefref;

2.

Anywhere you'd put an identifier (or chain of identifiers) as part of avariable or subroutine name, you can replace the identifier with aBLOCK returning a reference of the correct type. In other words, theprevious 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, butthe 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 asproper 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 variablecalled %hashref, not dereferencing through $hashref to the hashit's presumably referencing. That would be case 3.

3.

Subroutine calls and lookups of individual array elements arise oftenenough that it gets cumbersome to use method 2. As a form ofsyntactic 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 thesame thing as $array->[$x] here:

    $array[$x]->{"foo"}->[0] = "January";

This is one of the cases we mentioned earlier in which references couldspring into existence when in an lvalue context. Before thisstatement, $array[$x] may have been undefined. If so, it'sautomatically defined with a hash reference so that we can look up{"foo"} in it. Likewise $array[$x]->{"foo"} will automatically getdefined with an array reference so that we can look up [0] in it.

One more thing here. The arrow is optional BETWEEN bracketssubscripts, so you can shrink the above down to

    $array[$x]{"foo"}[0] = "January";

Which, in the degenerate case of using only ordinary arrays, gives youmultidimensional arrays just like C's:

    $score[$x][$y][$z] += 42;

Well, okay, not entirely like C's arrays, actually. C doesn't know howto grow its arrays on demand. Perl does.

4.

If a reference happens to be a reference to an object, then there areprobably methods to access the things referred to, and you should probablystick to those methods unless you're in the class package that defines theobject's methods. In other words, be nice, and don't violate the object'sencapsulation without a very good reason. Perl does not enforceencapsulation. We are not totalitarians here. We do expect some basiccivility though.

The ref() operator may be used to determine what type of thing thereference is pointing to. See the perlfunc manpage.

The bless() operator may be used to associate a reference with a packagefunctioning as an object class. See the perlobj manpage.

A typeglob may be dereferenced the same way a reference can, becausethe 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:

    print "My sub returned @{[mysub(1,2,3)]} that time.\n";

The way it works is that when the @{...} is seen in the double-quotedstring, it's evaluated as a block. The block creates a reference to ananonymous array containing the results of the call to mysub(1,2,3). Sothe whole block returns a reference to an array, which is thendereferenced by @{...} and stuck into the double-quoted string. Thischicanery 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 areundefined, but we didn't say what happens if a value used as areference is already defined, but ISN'T a hard reference. If youuse it as a reference in this case, it'll be treated as a symbolicreference. That is, the value of the scalar is taken to be the NAMEof a variable, rather than a direct link to a (possibly) anonymousvalue.

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 possibleto intend (with the utmost sincerity) to use a hard reference, andaccidentally use a symbolic reference instead. To protect againstthat, you can say

    use strict 'refs';

and then only hard references will be allowed for the rest of the enclosingblock. An inner block may countermand that with

    no strict 'refs';

Only package variables are visible to symbolic references. Lexicalvariables (declared with my()) aren't in a symbol table, and thus areinvisible 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 packagevariables, which are all ``global'' to the package.

Not-so-symbolic references

A new feature contributing to readability in perl version 5.001 is that thebrackets around a symbolic reference behave more like quotes, just as theyalways 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 isa reserved word. This has been generalized to work the same outsideof quotes, so that

    print ${push} . "over";

and even

    print ${ push } . "over";

will have the same effect. (This would have been a syntax error inPerl 5.000, though Perl 4 allowed it in the spaceless form.) Note that thisconstruct is not considered to be a symbolic reference when you'reusing strict refs:

    use strict 'refs';    ${ bareword };      # Okay, means $bareword.    ${ "bareword" };    # Error, symbolic reference.

Similarly, because of all the subscripting that is done using singlewords, we've applied the same rule to any bareword that is used forsubscripting 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 therare event that you do wish to do something like

    $array{ shift }

you can force interpretation as a reserved word by adding anything thatmakes 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 thestring is effectively quoted.

WARNING

You may not (usefully) use a reference as the key to a hash. It will beconverted into a string:

    $x{ \$a } = $a;

If you try to dereference the key, it won't do a hard dereference, andyou won't accomplish what you're attempting. You might want to do somethingmore like

    $r = \@a;    $x{ $r } = $r;

And then at least you can use the values(), which will bereal refs, instead of the keys(), which won't.

This document was created byman2html,using the manual pages.