=head1 NAME
+X<reference> X<pointer> X<data structure> X<structure> X<struct>
perlref - Perl references and nested data structures
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.)
If that thing happens to be an object, the object is destructed. See
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, 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
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 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:
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 created using square
brackets:
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!
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 created using curly
brackets:
sub showem { {; @_ } } # ok
sub showem { { return @_ } } # ok
-Note how the leading C<+{> and C<{;> always serve to disambiguate
+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 created by using
C<sub> without a subname:
$coderef = sub { print "Boink!\n" };
-Note the presence of the semicolon. Except for the fact that the code
-inside isn't executed immediately, a C<sub {}> is not so much a
+Note the semicolon. Except for the code
+inside not being immediately executed, a C<sub {}> is not so much a
declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
matter how many times you execute that particular line (unless you're in an
-C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
+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;
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 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 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 often
-named new() and called indirectly:
+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');
+ $objref = new Doggie( Tail => 'short', Ears => 'long' );
-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 = Doggie->new(Tail => 'short', Ears => 'long');
-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};
+
+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.
-*foo{IO} is an alternative to the \*HANDLE mechanism given in
+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 no risk of clobbering more than you want
-to with a typeglob assignment, although if you assign to a scalar instead
-of a typeglob, you're ok.
+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);
- splutter(*STDOUT{IO});
+ splutter(*STDOUT); # pass the whole glob
+ splutter(*STDOUT{IO}); # pass both file and dir handles
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
- $rec = get_rec(*STDIN);
- $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 $fh = shift;
=back
=head2 Using References
+X<reference, use> X<dereferencing> X<dereference>
That's it for creating references. By now you're probably dying to
know how to use references to get back to your long-lost data. There
&$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".
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
$coderef->(1,2,3); # Subroutine call
The left side of the arrow can be any expression returning a reference,
-including a previous dereference. Note that C<$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 == $ref2) { # cheap numeric compare of references
+ print "refs 1 and 2 refer to the same thing\n";
+ }
+
+Using a reference as a string produces both its referent's type,
+including any package blessing as described in L<perlobj>, as well
+as the numeric address expressed in hex. The ref() operator returns
+just the type of thing the reference is pointing to, without the
+address. See L<perlfunc/ref> for details and examples of its use.
+X<reference, string context>
The bless() operator may be used to associate the object a reference
points to with a package functioning as an object class. See L<perlobj>.
A typeglob may be dereferenced the same way a reference can, because
-the dereference syntax always indicates the kind of reference desired.
+the dereference syntax always indicates the type of reference desired.
So C<${*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 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
$push = "pop on ";
print "${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
-Perl 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{ +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>
-WARNING: This section describes an experimental feature. Details may
-change without notice in future versions.
-
-Beginning with release 5.005 of Perl you can use an array reference
-in some contexts that would normally require a hash reference. This
-allows you to access array elements using symbolic names, as if they
-were fields in a structure.
-
-For this to work, the array must contain extra information. The first
-element of the array has to be a hash reference that maps field names
-to array indices. Here is an example:
-
- $struct = [{foo => 1, bar => 2}, "FOO", "BAR"];
-
- $struct->{foo}; # same as $struct->[1], i.e. "FOO"
- $struct->{bar}; # same as $struct->[2], i.e. "BAR"
-
- keys %$struct; # will return ("foo", "bar") in some order
- values %$struct; # will return ("FOO", "BAR") in same some order
-
- while (my($k,$v) = each %$struct) {
- print "$k => $v\n";
- }
-
-Perl will raise an exception if you try to delete keys from a pseudo-hash
-or try to access nonexistent fields. For better performance, Perl can also
-do the translation from field names to array indices at compile time for
-typed object references. See L<fields>.
-
+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, a closure is an anonymous function with access to the
-lexical variables visible when that function was compiled. 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.
+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
print "Be ", red("careful"), "with that ", green("light");
-The red() and green() functions would be very similar. To create these,
+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.
putting the whole loop of assignments within a BEGIN block, forcing it
to occur during compilation.
-Access to lexicals that change over type--like those in the C<for> loop
-above--only works with closures, not general subroutines. In the general
-case, then, named subroutines do not nest properly, although anonymous
-ones do. 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 kind of thing
-incurs mysterious warnings about ``will not stay shared''. For example,
-this won't work:
+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:
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 closure (anonymous subroutine). But when
-it does, it has normal access to the lexical variable $x from the scope
-of outer().
+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:
=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
https://perl5.git.perl.org / perl5.git / blobdiff