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
-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
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.
=head2 Making References
X<\> X<backslash>
By using the backslash operator on a variable, subroutine, or value.
-(This works much like the & (address-of) operator in C.)
+(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
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>,
brackets:
$hashref = {
- 'Adam' => 'Eve',
- 'Clyde' => 'Bonnie',
+ 'Adam' => 'Eve',
+ 'Clyde' => 'Bonnie',
};
Anonymous hash and array composers like these can be intermixed freely to
On the other hand, if you want the other meaning, you can do this:
- sub showem { { @_ } } # ambiguous (currently ok, but may change)
+ sub showem { { @_ } } # ambiguous (currently ok,
+ # but may change)
sub showem { {; @_ } } # ok
sub showem { { return @_ } } # ok
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");
$ioref = *STDIN{IO};
$globref = *foo{GLOB};
$formatref = *foo{FORMAT};
+ $globname = *foo{NAME}; # "foo"
+ $pkgname = *foo{PACKAGE}; # "main"
-All of these are self-explanatory except for C<*foo{IO}>. It returns
+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 deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
-of its use.
+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.
+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.
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); # 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";
+ my $fh = shift;
+ print $fh "her um well a hmmm\n";
}
- $rec = get_rec(*STDIN); # pass the whole glob
+ $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>;
}
=back
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<$$x>,
people often make the mistake of viewing the dereferencing symbols as
Consider the difference below; case 0 is a short-hand version of case 1,
I<not> case 2:
- $$hashref{"KEY"} = "VALUE"; # CASE 0
- ${$hashref}{"KEY"} = "VALUE"; # CASE 1
- ${$hashref{"KEY"}} = "VALUE"; # CASE 2
- ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
+ $$hashref{"KEY"} = "VALUE"; # CASE 0
+ ${$hashref}{"KEY"} = "VALUE"; # CASE 1
+ ${$hashref{"KEY"}} = "VALUE"; # CASE 2
+ ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
Case 2 is also deceptive in that you're accessing a variable
called %hashref, not dereferencing through $hashref to the hash
X<reference, numeric context>
if ($ref1 == $ref2) { # cheap numeric compare of references
- print "refs 1 and 2 refer to the same thing\n";
+ print "refs 1 and 2 refer to the same thing\n";
}
Using a reference as a string produces both its referent's type,
People frequently expect it to work like this. So it does.
$name = "foo";
- $$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)
+ $$name = 1; # Sets $foo
+ ${$name} = 2; # Sets $foo
+ ${$name x 2} = 3; # Sets $foofoo
+ $name->[0] = 4; # Sets $foo[0]
+ @$name = (); # Clears @foo
+ &$name(); # Calls &foo()
$pack = "THAT";
- ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
+ ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
This is powerful, and slightly dangerous, in that it's possible
to intend (with the utmost sincerity) to use a hard reference, and
local $value = 10;
$ref = "value";
{
- my $value = 20;
- print $$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", even though push is
-a reserved word. This has been generalized to work the same outside
-of quotes, so that
+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.) 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" }
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.
+trying to build.
@colors = qw(red blue green yellow orange purple violet);
for my $name (@colors) {
- no strict 'refs'; # allow symbol table manipulation
+ no strict 'refs'; # allow symbol table manipulation
*$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
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.
+shared" due to the reasons explained above.
For example, this won't work:
sub outer {
This has the interesting effect of creating a function local to another
function, something not normally supported in Perl.
-=head1 WARNING
+=head1 WARNING: Don't use references as hash keys
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
The standard Tie::RefHash module provides a convenient workaround to this.
+=head2 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.
+
+=head2 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.
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<perltoot>, L<perlobj>, and L<perlbot>
+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