in paragraph 101.Using a reference as a string produces both its referent's type,
including any package blessing as described in 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 perlfunc/ref for details and examples of its use.
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 102.
The bless() operator may be used to associate the object a reference
points to with a package functioning as an object class. See perlobj.
A typeglob may be dereferenced the same way a reference can, because
the dereference syntax always indicates the type of reference desired.
So ${*foo} and ${\$foo} both indicate the same scalar variable.
Here's a trick for interpolating a subroutine call into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
The way it works is that when the @{...} is seen in the double-quoted
string, it's evaluated as a block. The block creates a reference to an
anonymous array containing the results of the call to mysub(1,2,3). So
the whole block returns a reference to an array, which is then
dereferenced by @{...} and stuck into the double-quoted string. This
chicanery is also useful for arbitrary expressions:
print "That yields @{[$n + 5]} widgets\n";
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 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 name
of a variable, rather than a direct link to a (possibly) anonymous
value.
People frequently expect it to work like this. So it does.
$name = "foo";
$$name = 1; # Sets $foo
${$name} = 2; # Sets $foo
${$name x 2} = 3; # Sets $foofoo
$name->[0] = 4; # Sets $foo[0]
@$name = (); # Clears @foo
&$name(); # Calls &foo() (as in Perl 4)
$pack = "THAT";
${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
This is powerful, and slightly dangerous, in that it's possible
to intend (with the utmost sincerity) to use a hard reference, and
accidentally use a symbolic reference instead. To protect against
that, you can say
use strict 'refs';
and then only hard references will be allowed for the rest of the enclosing
block. An inner block may countermand that with
no strict 'refs';
Only package variables (globals, even if localized) are visible to
symbolic references. Lexical variables (declared with my()) aren't in
a symbol table, and thus are invisible to this mechanism. For example:
local $value = 10;
$ref = "value";
{
my $value = 20;
print $$ref;
}
This will still print 10, not 20. Remember that local() affects package
variables, which are all ``global'' to the package.
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'', even though 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.) This
construct is 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 use warnings pragma or the -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.
WARNING: This section describes an experimental feature. Details may
change without notice in future versions.
NOTE: The current user-visible implementation of pseudo-hashes
(the weird use of the first array element) is deprecated starting from
Perl 5.8.0 and will be removed in Perl 5.10.0, and the feature will be
implemented differently. Not only is the current interface rather ugly,
but the current implementation slows down normal array and hash use quite
noticeably. The 'fields' pragma interface will remain available.Beginning with release 5.005 of Perl, you may 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 access nonexistent fields.
To avoid inconsistencies, always use the fields::phash() function
provided by the fields pragma.
use fields;
$pseudohash = fields::phash(foo => "FOO", bar => "BAR");
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 148.For better performance, Perl can also do the translation from field
names to array indices at compile time for typed object references.
See fields.
There are two ways to check for the existence of a key in a
pseudo-hash. The first is to use exists(). This checks to see if the
given field has ever been set. It acts this way to match the behavior
of a regular hash. For instance:
use fields;
$phash = fields::phash([qw(foo bar pants)], ['FOO']);
$phash->{pants} = undef;
print exists $phash->{foo}; # true, 'foo' was set in the declaration
print exists $phash->{bar}; # false, 'bar' has not been used.
print exists $phash->{pants}; # true, your 'pants' have been touched
The second is to use exists() on the hash reference sitting in the
first array element. This checks to see if the given key is a valid
field in the pseudo-hash.
print exists $phash->[0]{bar}; # true, 'bar' is a valid field
print exists $phash->[0]{shoes};# false, 'shoes' can't be used
delete() on a pseudo-hash element only deletes the value corresponding
to the key, not the key itself. To delete the key, you'll have to
explicitly delete it from the first hash element.
print delete $phash->{foo}; # prints $phash->[1], "FOO"
print exists $phash->{foo}; # false
print exists $phash->[0]{foo}; # true, key still exists
print delete $phash->[0]{foo}; # now key is gone
print $phash->{foo}; # runtime exception
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 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 type--like those in the 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. Thus is because named subroutines are created (and capture any
outer lexicals) only once at compile time, whereas anonymous subroutines
get to capture each time you execute the 'sub' operator. 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''. 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 closure (anonymous subroutine). But when
it does, it has normal access to the lexical variable $x from the scope
of outer().
This has the interesting effect of creating a function local to another
function, something not normally supported in Perl.
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.
The standard Tie::RefHash module provides a convenient workaround to this.
Besides the obvious documents, source code can be instructive.
Some pathological examples of the use of references can be found
in the t/op/ref.t regression test in the Perl source directory.
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 181.
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 181.
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 181.
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 181.
perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlref.pod: cannot resolve L in paragraph 181.
See also perldsc and perllol for how to use references to create
complex data structures, and perltoot, perlobj, and perlbot
for how to use them to create objects.
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