perlref.pod

ARM上的如果你对底层感兴趣

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 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:

    $$hashref{"KEY"}   = "VALUE";	# CASE 0
    ${$hashref}{"KEY"} = "VALUE";	# CASE 1
    ${$hashref{"KEY"}} = "VALUE";	# CASE 2
    ${$hashref->{"KEY"}} = "VALUE";	# CASE 3

Case 2 is also deceptive in that you're accessing a variable
called %hashref, not dereferencing through $hashref to the hash
it's presumably referencing.  That would be case 3.

=item 3.

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";   # Array element
    $hashref->{"KEY"} = "VALUE";  # Hash element
    $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:

    $array[$x]->{"foo"}->[0] = "January";

This is one of the cases we mentioned earlier in which references could
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
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
subscripts, so you can shrink the above down to

    $array[$x]{"foo"}[0] = "January";

Which, in the degenerate case of using only ordinary arrays, gives you
multidimensional arrays just like C's:

    $score[$x][$y][$z] += 42;

Well, okay, not entirely like C's arrays, actually.  C doesn't know how
to grow its arrays on demand.  Perl does.

=item 4.

If a reference happens to be a reference to an object, then there are
probably methods to access the things referred to, and you should probably
stick to those methods unless you're in the class package that defines the
object's methods.  In other words, be nice, and don't violate the object's
encapsulation without a very good reason.  Perl does not enforce
encapsulation.  We are not totalitarians here.  We do expect some basic
civility though.

=back

The ref() operator may be used to determine what type of thing the
reference is pointing to.  See L<perlfunc>.

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.
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)]} 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:

    print "That yields @{[$n + 5]} widgets\n";

=head2 Symbolic references

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>
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 very 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.

=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.

=head2 Pseudo-hashes: Using an array as a hash

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>.


=head2 Function Templates

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.

Using a closure as a function template allows us to generate many functions
that act similarly.  Suppopose 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 very 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 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:

    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.

=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.

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
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<perltoot>, L<perlobj>, and L<perlbot>
for how to use them to create objects.