=head1 NAME
+X<reference> X<pointer> X<data structure> X<structure> X<struct>
perlref - Perl references and nested data structures
+=head1 NOTE
+
+This is complete documentation about all aspects of references.
+For a shorter, tutorial introduction to just the essential features,
+see L<perlreftut>.
+
=head1 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.
+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. (Note: the reference counts for values in self-referential or
+to zero. (Reference counts for values in self-referential or
cyclic data structures may not go to zero without a little help; see
-L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
+L</"Circular References"> for a detailed explanation.)
If that thing happens to be an object, the object is destructed. See
L<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
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 C<*glob> notation is a kind of symbolic reference. (Symbolic
+The C<*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.)
+X<reference, symbolic> X<reference, soft>
+X<symbolic reference> X<soft reference>
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
+adjective, as in the following paragraph, it is usually talking about a
hard reference.
+X<reference, hard> X<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
+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.
-References can be constructed in several ways.
+=head2 Making References
+X<reference, creation> X<referencing>
+
+References can be created in several ways.
=over 4
=item 1.
+X<\> X<backslash>
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 I<ANOTHER> reference to a variable, because
+(This works much like the & (address-of) operator in C.)
+This typically creates I<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:
$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.)
+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 C<*foo{THING}> syntax below. However,
+you can still use type globs and globrefs as though they were IO handles.
=item 2.
+X<array, anonymous> X<[> X<[]> X<square bracket>
+X<bracket, square> X<arrayref> X<array reference> X<reference, array>
-A reference to an anonymous array can be constructed using square
+A reference to an anonymous array can be created using square
brackets:
$arrayref = [1, 2, ['a', 'b', 'c']];
-Here we've constructed a reference to an anonymous array of three elements
+Here we've created a reference to an anonymous array of three elements
whose final element is itself a reference to another anonymous array of three
elements. (The multidimensional syntax described later can be used to
-access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
+access this. For example, after the above, C<< $arrayref->[2][1] >> would have
the value "b".)
-Note that taking a reference to an enumerated list is not the same
+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!
+ @list = \($a, @b, %c); # same thing!
As a special case, C<\(@foo)> returns a list of references to the contents
-of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>.
+of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
+except that the key references are to copies (since the keys are just
+strings rather than full-fledged scalars).
=item 3.
+X<hash, anonymous> X<{> X<{}> X<curly bracket>
+X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash>
-A reference to an anonymous hash can be constructed using curly
+A reference to an anonymous hash can be created using curly
brackets:
$hashref = {
- 'Adam' => 'Eve',
- 'Clyde' => 'Bonnie',
+ 'Adam' => 'Eve',
+ 'Clyde' => 'Bonnie',
};
-Anonymous hash and array constructors can be intermixed freely to
+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
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 C<+{> and C<{;> always serve to disambiguate
+the expression to mean either the HASH reference, or the BLOCK.
+
=item 4.
+X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine>
+X<scope, lexical> X<closure> X<lexical> X<lexical scope>
-A reference to an anonymous subroutine can be constructed by using
+A reference to an anonymous subroutine can be created by using
C<sub> without a subname:
$coderef = sub { print "Boink!\n" };
-Note the presence of the semicolon. Except for the fact that the code
-inside isn't executed immediately, a C<sub {}> is not so much a
+Note the semicolon. Except for the code
+inside not being immediately executed, a C<sub {}> is not so much a
declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
-matter how many times you execute that line (unless you're in an
-C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
+matter how many times you execute that particular line (unless you're in an
+C<eval("...")>), $coderef will still have a reference to the I<same>
anonymous subroutine.)
Anonymous subroutines act as closures with respect to my() variables,
-that is, variables visible lexically within the current scope. Closure
+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 of the context.
+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
do object-oriented stuff with it, though Perl already provides a different
mechanism to do that--see L<perlobj>.
-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 I<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:
+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"; };
+ my $x = shift;
+ return sub { my $y = shift; print "$x, $y!\n"; };
}
$h = newprint("Howdy");
$g = newprint("Greetings");
Howdy, world!
Greetings, earthlings!
-Note particularly that $x continues to refer to the value passed into
-newprint() I<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.
+Note particularly that $x continues to refer to the value passed
+into newprint() I<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 to only lexical variables, by the way. Dynamic variables
+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.
=item 5.
+X<constructor> X<new>
+
+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 C<new()>. You I<can> call them
+indirectly:
+
+ $objref = new Doggie( Tail => 'short', Ears => 'long' );
-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:
+But that can produce ambiguous syntax in certain cases, so it's often
+better to use the direct method invocation approach:
- $objref = new Doggie (Tail => 'short', Ears => 'long');
+ $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)
=item 6.
+X<autovivification>
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.
=item 7.
+X<*foo{THING}> X<*>
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
$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 (L<perlfunc/open>), sockets
-(L<perlfunc/socket> and L<perlfunc/socketpair>), and directory handles
-(L<perlfunc/opendir>). For compatibility with 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
+ $formatref = *foo{FORMAT};
+ $globname = *foo{NAME}; # "foo"
+ $pkgname = *foo{PACKAGE}; # "main"
+
+Most of these are self-explanatory, but C<*foo{IO}>
+deserves special attention. It returns
+the IO handle, used for file handles (L<perlfunc/open>), sockets
+(L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
+handles (L<perlfunc/opendir>). For compatibility with previous
+versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
+is discouraged, to encourage a consistent use of one name: IO. On perls
+between v5.8 and v5.22, it will issue a deprecation warning, but this
+deprecation has since been rescinded.
+
+C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
+except in the case of scalars. C<*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.
+C<*foo{NAME}> and C<*foo{PACKAGE}> are the exception, in that they return
+strings, rather than references. These return the package and name of the
+typeglob itself, rather than one that has been assigned to it. So, after
+C<*foo=*Foo::bar>, C<*foo> will become "*Foo::bar" when used as a string,
+but C<*foo{PACKAGE}> and C<*foo{NAME}> will continue to produce "main" and
+"foo", respectively.
+
+C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
+L<perldata/"Typeglobs and Filehandles"> 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
- splutter(*STDOUT{IO});
sub splutter {
- my $fh = shift;
- print $fh "her um well a hmmm\n";
+ my $fh = shift;
+ print $fh "her um well a hmmm\n";
}
- $rec = get_rec(*STDIN{IO});
+ $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>;
+ 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);
- $rec = get_rec(\*STDIN);
-
-with the above subroutine definitions.
-
=back
+=head2 Using References
+X<reference, use> X<dereferencing> X<dereference>
+
That's it for creating references. By now you're probably dying to
know how to use references to get back to your long-lost data. There
are several basic methods.
&$coderef(1,2,3);
print $globref "output\n";
-It's important to understand that we are specifically I<NOT> dereferencing
+It's important to understand that we are specifically I<not> dereferencing
C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
-scalar variable happens I<BEFORE> it does any key lookups. Anything more
+scalar variable happens I<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".
the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:
- &{ $dispatch{$index} }(1,2,3); # call correct routine
+ &{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of C<$$x>,
people often make the mistake of viewing the dereferencing symbols as
proper operators, and wonder about their precedence. If they were,
though, you could use parentheses instead of braces. That's not the case.
Consider the difference below; case 0 is a short-hand version of case 1,
-I<NOT> case 2:
+I<not> case 2:
- $$hashref{"KEY"} = "VALUE"; # CASE 0
- ${$hashref}{"KEY"} = "VALUE"; # CASE 1
- ${$hashref{"KEY"}} = "VALUE"; # CASE 2
- ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
+ $$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
$coderef->(1,2,3); # Subroutine call
The left side of the arrow can be any expression returning a reference,
-including a previous dereference. Note that C<$array[$x]> is I<NOT> the
-same thing as C<$array-E<gt>[$x]> here:
+including a previous dereference. Note that C<$array[$x]> is I<not> the
+same thing as C<< $array->[$x] >> here:
$array[$x]->{"foo"}->[0] = "January";
spring into existence when in an lvalue context. Before this
statement, C<$array[$x]> may have been undefined. If so, it's
automatically defined with a hash reference so that we can look up
-C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get
+C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
defined with an array reference so that we can look up C<[0]> in it.
+This process is called I<autovivification>.
-One more thing here. The arrow is optional I<BETWEEN> brackets
+One more thing here. The arrow is optional I<between> brackets
subscripts, so you can shrink the above down to
$array[$x]{"foo"}[0] = "January";
=back
-The ref() operator may be used to determine what type of thing the
-reference is pointing to. See L<perlfunc>.
+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.
+X<reference, numeric context>
+
+ if ($ref1 == $ref2) { # cheap numeric compare of references
+ print "refs 1 and 2 refer to the same thing\n";
+ }
-The bless() operator may be used to associate a reference with a package
-functioning as an object class. See L<perlobj>.
+Using a reference as a string produces both its referent's type,
+including any package blessing as described in L<perlobj>, as well
+as the numeric address expressed in hex. The ref() operator returns
+just the type of thing the reference is pointing to, without the
+address. See L<perlfunc/ref> for details and examples of its use.
+X<reference, string context>
+
+The bless() operator may be used to associate the object a reference
+points to with a package functioning as an object class. See L<perlobj>.
A typeglob may be dereferenced the same way a reference can, because
-the dereference syntax always indicates the kind of reference desired.
+the dereference syntax always indicates the type of reference desired.
So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
Here's a trick for interpolating a subroutine call into a string:
print "That yields @{[$n + 5]} widgets\n";
+Similarly, an expression that returns a reference to a scalar can be
+dereferenced via C<${...}>. Thus, the above expression may be written
+as:
+
+ print "That yields ${\($n + 5)} widgets\n";
+
+=head2 Circular References
+X<circular reference> X<reference, circular>
+
+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 C<weaken> function exported by the
+L<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 $foo->{bar};
+
+The reference from C<$foo> to C<$bar> has been weakened. When the
+C<$bar> variable goes out of scope, it will be garbage-collected. The
+next time you look at the value of the C<< $foo->{bar} >> key, it will
+be C<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 I<first>. That way, the longer-lived
+variable will contain the expected reference until it goes out of
+scope.
+
=head2 Symbolic references
+X<reference, symbolic> X<reference, soft>
+X<symbolic reference> X<soft reference>
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 I<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 I<NAME>
+reference is already defined, but I<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 I<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";
- $$name = 1; # Sets $foo
- ${$name} = 2; # Sets $foo
- ${$name x 2} = 3; # Sets $foofoo
- $name->[0] = 4; # Sets $foo[0]
- @$name = (); # Clears @foo
- &$name(); # Calls &foo() (as in Perl 4)
+ $$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()
$pack = "THAT";
- ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
+ ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
-This is very powerful, and slightly dangerous, in that it's possible
+This is powerful, and slightly dangerous, in that it's possible
to intend (with the utmost sincerity) to use a hard reference, and
accidentally use a symbolic reference instead. To protect against
that, you can say
no strict 'refs';
-Only package variables are visible to symbolic references. Lexical
-variables (declared with my()) aren't in a symbol table, and thus are
-invisible to this mechanism. For example:
+Only package variables (globals, even if localized) are visible to
+symbolic references. Lexical variables (declared with my()) aren't in
+a symbol table, and thus are invisible to this mechanism. For example:
- local($value) = 10;
- $ref = \$value;
+ local $value = 10;
+ $ref = "value";
{
- my $value = 20;
- print $$ref;
+ my $value = 20;
+ print $$ref;
}
This will still print 10, not 20. Remember that local() affects package
=head2 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,
+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", despite the fact that push is
-a reserved word. This has been generalized to work the same outside
-of quotes, so that
+has always meant to print "pop on over", even though push is
+a reserved word. This is generalized to work the same
+without the enclosing double quotes, so that
print ${push} . "over";
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
+will have the same effect. This
construct is I<not> considered to be a symbolic reference when you're
using strict refs:
use strict 'refs';
- ${ bareword }; # Okay, means $bareword.
- ${ "bareword" }; # Error, symbolic reference.
+ ${ 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
+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" }
$array{ +shift }
$array{ shift @_ }
-The B<-w> switch will warn you if it interprets a reserved word as a string.
+The C<use warnings> pragma or the B<-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.
+=head2 Pseudo-hashes: Using an array as a hash
+X<pseudo-hash> X<pseudo hash> X<pseudohash>
+
+Pseudo-hashes have been removed from Perl. The 'fields' pragma
+remains available.
+
+=head2 Function Templates
+X<scope, lexical> X<closure> X<lexical> X<lexical scope>
+X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local>
+
+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 C<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 C<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 C<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.
+
=head1 WARNING
+X<reference, string context> X<reference, use as hash key>
You may not (usefully) use a reference as the key to a hash. It will be
converted into a string:
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.
+
+=head1 Postfix Dereference Syntax
+
+Beginning in v5.20.0, a postfix syntax for using references is
+available. It behaves as described in L</Using References>, but instead
+of a prefixed sigil, a postfixed sigil-and-star is used.
+
+For example:
+
+ $r = \@a;
+ @b = $r->@*; # equivalent to @$r or @{ $r }
+
+ $r = [ 1, [ 2, 3 ], 4 ];
+ $r->[1]->@*; # equivalent to @{ $r->[1] }
+
+In Perl 5.20 and 5.22, this syntax must be enabled with C<use feature
+'postderef'>. As of Perl 5.24, no feature declarations are required to make
+it available.
+
+Postfix dereference should work in all circumstances where block
+(circumfix) dereference worked, and should be entirely equivalent. This
+syntax allows dereferencing to be written and read entirely
+left-to-right. The following equivalencies are defined:
+
+ $sref->$*; # same as ${ $sref }
+ $aref->@*; # same as @{ $aref }
+ $aref->$#*; # same as $#{ $aref }
+ $href->%*; # same as %{ $href }
+ $cref->&*; # same as &{ $cref }
+ $gref->**; # same as *{ $gref }
+
+Note especially that C<< $cref->&* >> is I<not> equivalent to C<<
+$cref->() >>, and can serve different purposes.
+
+Glob elements can be extracted through the postfix dereferencing feature:
+
+ $gref->*{SCALAR}; # same as *{ $gref }{SCALAR}
+
+Postfix array and scalar dereferencing I<can> be used in interpolating
+strings (double quotes or the C<qq> operator), but only if the
+C<postderef_qq> feature is enabled.
+
+=head2 Postfix Reference Slicing
+
+Value slices of arrays and hashes may also be taken with postfix
+dereferencing notation, with the following equivalencies:
+
+ $aref->@[ ... ]; # same as @$aref[ ... ]
+ $href->@{ ... }; # same as @$href{ ... }
+
+Postfix key/value pair slicing, added in 5.20.0 and documented in
+L<the KeyE<sol>Value Hash Slices section of perldata|perldata/"Key/Value Hash
+Slices">, also behaves as expected:
+
+ $aref->%[ ... ]; # same as %$aref[ ... ]
+ $href->%{ ... }; # same as %$href{ ... }
+
+As with postfix array, postfix value slice dereferencing I<can> be used
+in interpolating strings (double quotes or the C<qq> operator), but only
+if the C<postderef_qq> L<feature> is enabled.
+
+=head1 Assigning to References
+
+Beginning in v5.22.0, the referencing operator can be assigned to. It
+performs an aliasing operation, so that the variable name referenced on the
+left-hand side becomes an alias for the thing referenced on the right-hand
+side:
+
+ \$a = \$b; # $a and $b now point to the same scalar
+ \&foo = \&bar; # foo() now means bar()
+
+This syntax must be enabled with C<use feature 'refaliasing'>. It is
+experimental, and will warn by default unless C<no warnings
+'experimental::refaliasing'> is in effect.
+
+These forms may be assigned to, and cause the right-hand side to be
+evaluated in scalar context:
+
+ \$scalar
+ \@array
+ \%hash
+ \&sub
+ \my $scalar
+ \my @array
+ \my %hash
+ \state $scalar # or @array, etc.
+ \our $scalar # etc.
+ \local $scalar # etc.
+ \local our $scalar # etc.
+ \$some_array[$index]
+ \$some_hash{$key}
+ \local $some_array[$index]
+ \local $some_hash{$key}
+ condition ? \$this : \$that[0] # etc.
+
+Slicing operations and parentheses cause
+the right-hand side to be evaluated in
+list context:
+
+ \@array[5..7]
+ (\@array[5..7])
+ \(@array[5..7])
+ \@hash{'foo','bar'}
+ (\@hash{'foo','bar'})
+ \(@hash{'foo','bar'})
+ (\$scalar)
+ \($scalar)
+ \(my $scalar)
+ \my($scalar)
+ (\@array)
+ (\%hash)
+ (\&sub)
+ \(&sub)
+ \($foo, @bar, %baz)
+ (\$foo, \@bar, \%baz)
+
+Each element on the right-hand side must be a reference to a datum of the
+right type. Parentheses immediately surrounding an array (and possibly
+also C<my>/C<state>/C<our>/C<local>) will make each element of the array an
+alias to the corresponding scalar referenced on the right-hand side:
+
+ \(@a) = \(@b); # @a and @b now have the same elements
+ \my(@a) = \(@b); # likewise
+ \(my @a) = \(@b); # likewise
+ push @a, 3; # but now @a has an extra element that @b lacks
+ \(@a) = (\$a, \$b, \$c); # @a now contains $a, $b, and $c
+
+Combining that form with C<local> and putting parentheses immediately
+around a hash are forbidden (because it is not clear what they should do):
+
+ \local(@array) = foo(); # WRONG
+ \(%hash) = bar(); # wRONG
+
+Assignment to references and non-references may be combined in lists and
+conditional ternary expressions, as long as the values on the right-hand
+side are the right type for each element on the left, though this may make
+for obfuscated code:
+
+ (my $tom, \my $dick, \my @harry) = (\1, \2, [1..3]);
+ # $tom is now \1
+ # $dick is now 2 (read-only)
+ # @harry is (1,2,3)
+
+ my $type = ref $thingy;
+ ($type ? $type eq 'ARRAY' ? \@foo : \$bar : $baz) = $thingy;
+
+The C<foreach> loop can also take a reference constructor for its loop
+variable, though the syntax is limited to one of the following, with an
+optional C<my>, C<state>, or C<our> after the backslash:
+
+ \$s
+ \@a
+ \%h
+ \&c
+
+No parentheses are permitted. This feature is particularly useful for
+arrays-of-arrays, or arrays-of-hashes:
+
+ foreach \my @a (@array_of_arrays) {
+ frobnicate($a[0], $a[-1]);
+ }
+
+ foreach \my %h (@array_of_hashes) {
+ $h{gelastic}++ if $h{type} eq 'funny';
+ }
+
+B<CAVEAT:> Aliasing does not work correctly with closures. If you try to
+alias lexical variables from an inner subroutine or C<eval>, the aliasing
+will only be visible within that inner sub, and will not affect the outer
+subroutine where the variables are declared. This bizarre behavior is
+subject to change.
+
+=head1 Declaring a Reference to a Variable
+
+Beginning in v5.26.0, the referencing operator can come after C<my>,
+C<state>, C<our>, or C<local>. This syntax must be enabled with C<use
+feature 'declared_refs'>. It is experimental, and will warn by default
+unless C<no warnings 'experimental::refaliasing'> is in effect.
+
+This feature makes these:
+
+ my \$x;
+ our \$y;
+
+equivalent to:
+
+ \my $x;
+ \our $x;
+
+It is intended mainly for use in assignments to references (see
+L</Assigning to References>, above). It also allows the backslash to be
+used on just some items in a list of declared variables:
+
+ my ($foo, \@bar, \%baz); # equivalent to: my $foo, \my(@bar, %baz);
+
=head1 SEE ALSO
Besides the obvious documents, source code can be instructive.
-Some rather pathological examples of the use of references can be found
+Some pathological examples of the use of references can be found
in the F<t/op/ref.t> regression test in the Perl source directory.
See also L<perldsc> and L<perllol> for how to use references to create
-complex data structures, and L<perlobj> for how to use them to create
-objects.
+complex data structures, and L<perlootut> and L<perlobj>
+for how to use them to create objects.
https://perl5.git.perl.org / perl5.git / blobdiff