=head1 DESCRIPTION
-In Perl 4 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.
+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.
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. (Note: the 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 a kind of symbolic reference. (Symbolic
+references are sometimes called "soft references", but please don't call
+them that; references are confusing enough without useless synonyms.)
+
+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
+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.
+
+References can be constructed in several ways.
=over 4
=item 1.
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.) Note
+that 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 = \*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.)
=item 2.
$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
+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, $arrayref->[2][1] would have
+access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
the value "b".)
+Note that 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!
+
+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>.
+
=item 3.
A reference to an anonymous hash can be constructed using curly
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
+
+Note how the leading C<+{> and C<{;> always serve to disambiguate
+the expression to mean either the HASH reference, or the BLOCK.
+
=item 4.
A reference to an anonymous subroutine can be constructed by using
C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
anonymous subroutine.)
-For those who worry about these things, the current implementation
-uses shallow binding of local() variables; my() variables are not
-accessible. This precludes true closures. However, you can work
-around this with a run-time (rather than a compile-time) eval():
+Anonymous subroutines act as closures with respect to my() variables,
+that is, variables visible lexically 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.
- {
- my $x = time;
- $coderef = eval "sub { \$x }";
+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 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:
+
+ sub newprint {
+ my $x = shift;
+ return sub { my $y = shift; print "$x, $y!\n"; };
}
+ $h = newprint("Howdy");
+ $g = newprint("Greetings");
+
+ # Time passes...
+
+ &$h("world");
+ &$g("earthlings");
-Normally--if you'd used just C<sub{}> or even C<eval{}>--your unew sub
-would only have been able to access the global $x. But because you've
-used a run-time eval(), this will not only generate a brand new subroutine
-reference each time called, it will all grant access to the my() variable
-lexically above it rather than the global one. The particular $x
-accessed will be different for each new sub you create. This mechanism
-yields deep binding of variables. (If you don't know what closures, deep
-binding, or shallow binding are, don't worry too much about it.)
+This prints
+
+ 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.
+
+This applies to only 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.
References are often returned by special subroutines called constructors.
-Perl objects are just reference a special kind of object that happens to know
+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
=item 6.
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.
+
+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};
+
+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
+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.
+
+ splutter(*STDOUT{IO});
+ sub splutter {
+ my $fh = shift;
+ print $fh "her um well a hmmm\n";
+ }
+
+ $rec = get_rec(*STDIN{IO});
+ sub get_rec {
+ my $fh = shift;
+ return scalar <$fh>;
+ }
+
+Beware, though, that you can't do this with a routine which is going 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
That's it for creating references. By now you're probably dying to
=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 = $$scalarref;
push(@$arrayref, $filename);
$$arrayref[0] = "January";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
+ print $globref "output\n";
It's important to understand that we are specifically I<NOT> dereferencing
C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
=item 2.
-Anywhere you'd put an identifier as part of a variable or subroutine
-name, you can replace the identifier with a BLOCK returning a reference
-of the correct type. In other words, the previous examples could be
-written like this:
+Anywhere you'd put an identifier (or chain of identifiers) as part of a
+variable or subroutine name, you can replace the identifier with a
+BLOCK returning a reference of the correct type. In other words, the
+previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
+ $globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in this case, but
the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:
- &{ $dispatch{$index} }(1,2,3); # call correct routine
+ &{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of C<$$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:
=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
+ $coderef->(1,2,3); # Subroutine call
-The left side of the array can be any expression returning a reference,
+The left side of the arrow can be any expression returning a reference,
including a previous dereference. Note that C<$array[$x]> is I<NOT> the
same thing as C<$array-E<gt>[$x]> here:
The bless() operator may be used to associate a reference with a package
functioning as an object class. See L<perlobj>.
-A type glob may be dereferenced the same way a reference can, since
+A typeglob may be dereferenced the same way a reference can, because
the dereference syntax always indicates the kind of reference desired.
So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
Here's a trick for interpolating a subroutine call into a string:
- print "My sub returned ${\mysub(1,2,3)}\n";
+ print "My sub returned @{[mysub(1,2,3)]} that time.\n";
+
+The way it works is that when the C<@{...}> is seen in the double-quoted
+string, it's evaluated as a block. The block creates a reference to an
+anonymous array containing the results of the call to C<mysub(1,2,3)>. So
+the whole block returns a reference to an array, which is then
+dereferenced by C<@{...}> and stuck into the double-quoted string. This
+chicanery is also useful for arbitrary expressions:
-The way it works is that when the C<${...}> is seen in the double-quoted
-string, it's evaluated as a block. The block executes the call to
-C<mysub(1,2,3)>, and then takes a reference to that. So the whole block
-returns a reference to a scalar, which is then dereferenced by C<${...}>
-and stuck into the double-quoted string.
+ print "That yields @{[$n + 5]} widgets\n";
=head2 Symbolic references
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';
{
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 Further Reading
+=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,
+
+ $push = "pop on ";
+ print "${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
+
+ print ${push} . "over";
+
+and even
+
+ 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
+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.
+
+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
+
+ $array{ "aaa" }{ "bbb" }{ "ccc" }
+
+you can write just
+
+ $array{ aaa }{ bbb }{ ccc }
+
+and not worry about whether the subscripts are reserved words. In the
+rare event that you do wish to do something like
+
+ $array{ shift }
+
+you can force interpretation as a reserved word by adding anything that
+makes it more than a bareword:
+
+ $array{ shift() }
+ $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, because the
+string is effectively quoted.
+
+=head1 WARNING
+
+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 attempting. You might want to do something
+more like
+
+ $r = \@a;
+ $x{ $r } = $r;
+
+And then at least you can use the values(), which will be
+real refs, instead of the keys(), which won't.
+
+=head1 SEE ALSO
Besides the obvious documents, source code can be instructive.
Some rather 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.
projects / p5sagit/p5-mst-13.2.git / blobdiff