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
+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. (Note: The reference counts for values in self-referential
-or cyclic data structures may not go to zero without a little help; see
+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.)
-
-
-A symbolic reference contains the name of a variable, just as a
-symbolic link in the filesystem contains merely 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.
+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
+(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
$coderef = \&handler;
$globref = \*foo;
-It isn't possible to create a reference to an IO handle (filehandle or
+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.
+*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.
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've constructed a reference to an anonymous array of three elements
-whose final element is itself reference to another anonymous array of three
+whose final element is itself a reference to another anonymous array of three
elements. (The multidimensional syntax described later can be used to
access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
the value "b".)
as using square brackets--instead it's the same as creating
a list of references!
- @list = (\$a, \@b, \%c);
+ @list = (\$a, \@b, \%c);
@list = \($a, @b, %c); # same thing!
-As a special case, C<\(@foo)> returns a list of references to the contents
+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.
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>.
+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
(L<perlfunc/opendir>). For compatibility with previous versions of
Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}.
-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.
+*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 $foo hasn't been used yet. This might change in a
+future release.
+
+The use of *foo{IO} is the best way to pass bareword filehandles into or
+out of subroutines, or to store them in larger data structures.
splutter(*STDOUT{IO});
sub splutter {
return scalar <$fh>;
}
-The best way to do this used to be to use the entire *foo typeglob (or a
-reference to it), so you'll probably come across old code which does it
-that way.
+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
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<$$x>,
people often make the mistake of viewing the dereferencing symbols as
=item 3.
-The case of individual array elements arises often enough that it gets
-cumbersome to use method 2. As a form of syntactic sugar, the two
-lines like that above can be written:
+Subroutine calls and lookups of individual array elements arise often
+enough that it gets cumbersome to use method 2. As a form of
+syntactic sugar, the examples for method 2 may be written:
- $arrayref->[0] = "January";
- $hashref->{"KEY"} = "VALUE";
+ $arrayref->[0] = "January"; # Array element
+ $hashref->{"KEY"} = "VALUE"; # Hash element
+ $coderef->(1,2,3); # Subroutine call
-The left side of the array can be any expression returning a reference,
+The left side of the arrow can be any expression returning a reference,
including a previous dereference. Note that C<$array[$x]> is I<NOT> the
same thing as C<$array-E<gt>[$x]> here:
use strict 'refs';
and then only hard references will be allowed for the rest of the enclosing
-block. An inner block may countermand that with
+block. An inner block may countermand that with
no strict 'refs';
{
my $value = 20;
print $$ref;
- }
+ }
This will still print 10, not 20. Remember that local() affects package
variables, which are all "global" to the package.
=head2 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";
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.) Note that this
construct is I<not> considered to be a symbolic reference when you're
using strict refs:
$x{ \$a } = $a;
-If you try to dereference the key, it won't do a hard dereference, and
+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
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