=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 5 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. Since 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. 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 have been officially "blessed" into a class package.)
-
-A symbolic reference contains the name of a variable, just as a
-symbolic link in the filesystem merely contains the name of a file.
-The C<*glob> notation is a kind of symbolic reference. Hard references
-are more like hard links in the file system: merely another way
-at getting at the same underlying object, irrespective of its name.
-
-"Hard" 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 scalar. It doesn't magically start being an array or a hash
-unless you tell it so explicitly by dereferencing it.
-
-References can be constructed several ways.
+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.)
+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
+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 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, 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
+tell it explicitly to do so, by dereferencing it.
+
+=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 works in C.) Note
-that this typically creates I<ANOTHER> reference to a variable, since
+(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:
$arrayref = \@ARGV;
$hashref = \%ENV;
$coderef = \&handler;
- $globref = \*STDOUT;
+ $globref = \*foo;
+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
-whose final element is itself reference to another anonymous array of three
+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, $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);
+ @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>,
+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 = {
'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
little bits of code to run later, such as callbacks. You can even
-do object-oriented stuff with it, though Perl provides a different
-mechanism to do that already--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.)
+do object-oriented stuff with it, though Perl already provides a different
+mechanism to do that--see L<perlobj>.
-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;
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 only applies to 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' );
+
+But that can produce ambiguous syntax in certain cases, so it's often
+better to use the direct method invocation approach:
-References are often returned by special subroutines called constructors.
-Perl objects are just references to a special kind of object that happens to know
-which package it's associated with. Constructors are just special
-subroutines that know how to create that association. They do so by
-starting with an ordinary reference, and it remains an ordinary reference
-even while it's also being an object. Constructors are customarily
-named new(), but don't have to be:
+ $objref = Doggie->new(Tail => 'short', Ears => 'long');
- $objref = new Doggie (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. Since we haven't
+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
+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 C<*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, 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.
+
+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{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
-References to filehandles can be created by taking a reference to
-a typeglob. This is currently the best way to pass filehandles into or
-out of subroutines, or to store them in larger data structures.
-
- splutter(\*STDOUT);
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
- $rec = get_rec(\*STDIN);
+ $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>;
=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.
=item 1.
-Anywhere you'd put an identifier 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:
+Anywhere you'd put an identifier (or chain of identifiers) as part
+of a variable or subroutine name, you can replace the identifier with
+a simple scalar variable containing a reference of the correct type:
$bar = $$scalarref;
push(@$arrayref, $filename);
&$coderef(1,2,3);
print $globref "output\n";
-It's important to understand that we are specifically I<NOT> dereferencing
+It's important to understand that we are specifically I<not> dereferencing
C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
-scalar variable happens I<BEFORE> it does any key lookups. Anything more
+scalar variable happens I<before> it does any key lookups. Anything more
complicated than a simple scalar variable must use methods 2 or 3 below.
However, a "simple scalar" includes an identifier that itself uses method
1 recursively. Therefore, the following prints "howdy".
=item 2.
-Anywhere you'd put an identifier as part of a variable or subroutine
-name, you can replace the identifier with a BLOCK returning a reference
-of the correct type. In other words, the previous examples could be
-written like this:
+Anywhere you'd put an identifier (or chain of identifiers) as part of a
+variable or subroutine name, you can replace the identifier with a
+BLOCK returning a reference of the correct type. In other words, the
+previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
- $globref->print("output\n"); # iff you use FileHandle
+ $globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in this case, but
the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:
- &{ $dispatch{$index} }(1,2,3); # call correct routine
+ &{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of C<$$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 parens instead of braces. That's not the case.
+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
=item 3.
-The case of individual array elements arises often enough that it gets
-cumbersome to use method 2. As a form of syntactic sugar, the two
-lines like that above can be written:
+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";
- $hashref->{"KEY"} = "VALUE";
+ $arrayref->[0] = "January"; # Array element
+ $hashref->{"KEY"} = "VALUE"; # Hash element
+ $coderef->(1,2,3); # Subroutine call
-The left side of the array can be any expression returning a reference,
-including a previous dereference. Note that C<$array[$x]> is I<NOT> the
-same thing as C<$array-E<gt>[$x]> here:
+The left side of the arrow can be any expression returning a reference,
+including a previous dereference. Note that C<$array[$x]> is I<not> the
+same thing as C<< $array->[$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>
-The bless() operator may be used to associate a reference with a package
-functioning as an object class. See L<perlobj>.
+ if ($ref1 == $ref2) { # cheap numeric compare of references
+ print "refs 1 and 2 refer to the same thing\n";
+ }
-A typeglob may be dereferenced the same way a reference can, since
-the dereference syntax always indicates the kind of reference desired.
+Using a reference as a string produces both its referent's type,
+including any package blessing as described in L<perlobj>, as well
+as the numeric address expressed in hex. The ref() operator returns
+just the type of thing the reference is pointing to, without the
+address. See L<perlfunc/ref> for details and examples of its use.
+X<reference, string context>
+
+The bless() operator may be used to associate the object a reference
+points to with a package functioning as an object class. See L<perlobj>.
+
+A typeglob may be dereferenced the same way a reference can, because
+the dereference syntax always indicates the type of reference desired.
So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
Here's a trick for interpolating a subroutine call into a string:
dereferenced by C<@{...}> and stuck into the double-quoted string. This
chicanery is also useful for arbitrary expressions:
- print "That yeilds @{[$n + 5]} widgets\n";
+ 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 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.
$pack = "THAT";
${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
-This is very powerful, and slightly dangerous, in that it's possible
+This is powerful, and slightly dangerous, in that it's possible
to intend (with the utmost sincerity) to use a hard reference, and
accidentally use a symbolic reference instead. To protect against
that, you can say
use strict 'refs';
and then only hard references will be allowed for the rest of the enclosing
-block. An inner block may countermand that with
+block. An inner block may countermand that with
no strict 'refs';
-Only package variables are visible to symbolic references. Lexical
-variables (declared with my()) aren't in a symbol table, and thus are
-invisible to this mechanism. For example:
+Only package variables (globals, even if localized) are visible to
+symbolic references. Lexical variables (declared with my()) aren't in
+a symbol table, and thus are invisible to this mechanism. For example:
- local($value) = 10;
- $ref = \$value;
+ local $value = 10;
+ $ref = "value";
{
my $value = 20;
print $$ref;
- }
+ }
This will still print 10, not 20. Remember that local() affects package
variables, which are all "global" to the package.
=head2 Not-so-symbolic references
-A new feature contributing to readability in 5.001 is that the brackets
-around a symbolic reference behave more like quotes, just as they
+A new feature contributing to readability in perl version 5.001 is that the
+brackets around a symbolic reference behave more like quotes, just as they
always have within a string. That is,
$push = "pop on ";
print "${push}over";
-has always meant to print "pop on over", despite the fact that push is
+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";
will have the same effect. (This would have been a syntax error in
-5.000, though Perl 4 allowed it in the spaceless form.) Note that this
+Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
construct is I<not> considered to be a symbolic reference when you're
using strict refs:
$array{ "aaa" }{ "bbb" }{ "ccc" }
-you can just write
+you can write just
$array{ aaa }{ bbb }{ ccc }
$array{ +shift }
$array{ shift @_ }
-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, since the
+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:
$x{ \$a } = $a;
-If you try to dereference the key, it won't do a hard dereference, and
-you won't accomplish what you're attemping. You might want to do something
+If you try to dereference the key, it won't do a hard dereference, and
+you won't accomplish what you're attempting. You might want to do something
more like
$r = \@a;
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 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<perltoot>, L<perlobj>, and L<perlbot>
+for how to use them to create objects.
https://perl5.git.perl.org / perl5.git / blobdiff