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1995-07-25
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PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))
NNNNAAAAMMMMEEEE
perlref - Perl references and nested data structures
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
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.
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 the
_p_e_r_l_o_b_j manpage 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 *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.
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 _A_N_O_T_H_E_R reference to a variable, since
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:
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PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))
$scalarref = \$foo;
$arrayref = \@ARGV;
$hashref = \%ENV;
$coderef = \&handler;
2. A reference to an anonymous array can be constructed
using square brackets:
$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 elements.
(The multidimensional syntax described later can be used
to access this. For example, after the above,
$arrayref->[2][1] would have the value "b".)
3. A reference to an anonymous hash can be constructed
using curly brackets:
$hashref = {
'Adam' => 'Eve',
'Clyde' => 'Bonnie',
};
Anonymous hash and array constructors can be intermixed
freely to produce as complicated a structure as you
want. The multidimensional syntax described below works
for these too. The values above are literals, but
variables and expressions would work just as well,
because assignment operators in Perl (even within
_l_o_c_a_l() or _m_y()) are executable statements, not
compile-time declarations.
Because curly brackets (braces) are used for several
other things including BLOCKs, you may occasionally have
to disambiguate braces at the beginning of a statement
by putting a + or a return in front so that Perl
realizes the opening brace isn't starting a BLOCK. The
economy and mnemonic value of using curlies is deemed
worth this occasional extra hassle.
For example, if you wanted a function to make a new hash
and return a reference to it, you have these options:
sub hashem { { @_ } } # silently wrong
sub hashem { +{ @_ } } # ok
sub hashem { return { @_ } } # ok
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4. A reference to an anonymous subroutine can be
constructed by using 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 sub
{} is not so much a declaration as it is an operator,
like do{} or eval{}. (However, no matter how many times
you execute that line (unless you're in an eval("...")),
$coderef will still have a reference to the _S_A_M_E
anonymous subroutine.)
For those who worry about these things, the current
implementation uses shallow binding of _l_o_c_a_l()
variables; _m_y() variables are not accessible. This
precludes true closures. However, you can work around
this with a run-time (rather than a compile-time)
_e_v_a_l():
{
my $x = time;
$coderef = eval "sub { \$x }";
}
Normally--if you'd used just sub{} or even eval{}--your
unew sub would only have been able to access the global
$x. But because you've used a run-time _e_v_a_l(), this
will not only generate a brand new subroutine reference
each time called, it will all grant access to the _m_y()
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.)
5. References are often returned by special subroutines
called constructors. Perl objects are just reference 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
customarily named _n_e_w(), but don't have to be:
$objref = new Doggie (Tail => 'short', Ears => 'long');
6. References of the appropriate type can spring into
existence if you dereference them in a context that
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assumes they exist. Since we haven't talked about
dereferencing yet, we can't show you any examples yet.
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 are several basic methods.
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:
$bar = $$scalarref;
push(@$arrayref, $filename);
$$arrayref[0] = "January";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
It's important to understand that we are specifically
_N_O_T dereferencing $arrayref[0] or $hashref{"KEY"} there.
The dereference of the scalar variable happens _B_E_F_O_R_E it
does any key lookups. Anything more complicated than a
simple scalar variable must use methods 2 or 3 below.
However, a "simple scalar" includes an identifier that
itself uses method 1 recursively. Therefore, the
following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier as part of a variable
or subroutine name, you can replace the identifier with
a BLOCK returning a reference of the correct type. In
other words, the previous examples could be written like
this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
Admittedly, it's a little silly to use the curlies in
this case, but the BLOCK can contain any arbitrary
expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple
case of $$x, people often make the mistake of viewing
the dereferencing symbols as proper operators, and
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wonder about their precedence. If they were, though,
you could use parens instead of braces. That's not the
case. Consider the difference below; case 0 is a
short-hand version of case 1, _N_O_T 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.
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:
$arrayref->[0] = "January";
$hashref->{"KEY} = "VALUE";
The left side of the array can be any expression
returning a reference, including a previous dereference.
Note that $array[$x] is _N_O_T the same thing as $array-
>[$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, $array[$x] may have
been undefined. If so, it's automatically defined with
a hash reference so that we can look up {"foo"} in it.
Likewise $array[$x]->{"foo"} will automatically get
defined with an array reference so that we can look up
[0] in it.
One more thing here. The arrow is optional _B_E_T_W_E_E_N
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.
Page 5 (printed 6/30/95)
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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.
The _r_e_f() operator may be used to determine what type of
thing the reference is pointing to. See the _p_e_r_l_f_u_n_c
manpage.
The _b_l_e_s_s() operator may be used to associate a reference
with a package functioning as an object class. See the
_p_e_r_l_o_b_j manpage.
A type glob may be dereferenced the same way a reference
can, since the dereference syntax always indicates the kind
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)}\n";
The way it works is that when the ${...} is seen in the
double-quoted string, it's evaluated as a block. The block
executes the call to 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 ${...} and stuck
into the double-quoted string.
SSSSyyyymmmmbbbboooolllliiiicccc rrrreeeeffffeeeerrrreeeennnncccceeeessss
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_S_N'_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 _N_A_M_E of a variable,
rather than a direct link to a (possibly) anonymous value.
People frequently expect it to work like this. So it does.
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$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 $THAT::foo without eval
This is very powerful, and slightly dangerous, in that it's
possible to intend (with the utmost sincerity) to use a hard
reference, and accidentally use a symbolic reference
instead. To protect against that, you can say
use strict 'refs';
and then only hard references will be allowed for the rest
of the enclosing block. An inner block may countermand that
with
no strict 'refs';
Only package variables are visible to symbolic references.
Lexical variables (declared with _m_y()) aren't in a symbol
table, and thus are invisible to this mechanism. For
example:
local($value) = 10;
$ref = \$value;
{
my $value = 20;
print $$ref;
}
This will still print 10, not 20. Remember that _l_o_c_a_l()
affects package variables, which are all "global" to the
package.
FFFFuuuurrrrtttthhhheeeerrrr RRRReeeeaaaaddddiiiinnnngggg
Besides the obvious documents, source code can be
instructive. Some rather pathological examples of the use
of references can be found in the _t/_o_p/_r_e_f._t regression test
in the Perl source directory.
Page 7 (printed 6/30/95)