perlref
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perlref - Perl references and nested data structures
形式
Please see following description for synopsis
説明
Perl Programmers Reference Guide PERLREF(1)
NAME
perlref - Perl references and nested data structures
NOTE
This is complete documentation about all aspects of
references. For a shorter, tutorial introduction to just
the essential features, see perlreftut.
DESCRIPTION
Before release 5 of Perl it was difficult to represent
complex data structures, because all references had to be
symbolic--and even then it was difficult to refer to a
variable instead of a symbol table entry. Perl now not only
makes it easier to use symbolic references to variables, but
also lets you have "hard" references to any piece of data or
code. 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. (Reference counts
for values in self-referential or cyclic data structures may
not go to zero without a little help; see "Two-Phased
Garbage Collection" in perlobj for a detailed explanation.)
If that thing happens to be an object, the object is
destructed. See 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 *glob notation is something of a
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, as in the
following paragraph, it is usually 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.
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Making References
References can be created in several ways.
1. By using the backslash operator on a variable,
subroutine, or value. (This works much like the &
(address-of) operator in C.) This typically creates
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:
$scalarref = \$foo;
$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. The most you can get is a reference to a
typeglob, which is actually a complete symbol table
entry. But see the explanation of the *foo{THING}
syntax below. However, you can still use type globs and
globrefs as though they were IO handles.
2. A reference to an anonymous array can be created using
square brackets:
$arrayref = [1, 2, ['a', 'b', 'c']];
Here we've created a reference to an anonymous array of
three elements whose final element is itself a reference
to another anonymous array of three elements. (The
multidimensional syntax described later can be used to
access this. For example, after the above,
"$arrayref->[2][1]" would have the value "b".)
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, "\(@foo)" returns a list of
references to the contents of @foo, not a reference to
@foo itself. Likewise for %foo, except that the key
references are to copies (since the keys are just
strings rather than full-fledged scalars).
3. A reference to an anonymous hash can be created using
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curly brackets:
$hashref = {
'Adam' => 'Eve',
'Clyde' => 'Bonnie',
};
Anonymous hash and array composers like these 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 local() or my()) 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
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
The leading "+{" and "{;" always serve to disambiguate
the expression to mean either the HASH reference, or the
BLOCK.
4. A reference to an anonymous subroutine can be created by
using "sub" without a subname:
$coderef = sub { print "Boink!\n" };
Note the semicolon. Except for the code inside not
being immediately executed, 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 particular line (unless you're in an
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"eval("...")"), $coderef will still have a reference to
the same anonymous subroutine.)
Anonymous subroutines act as closures with respect to
my() variables, that is, variables lexically visible
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 the
context.
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 perlobj.
You might also think of closure as a way to write a
subroutine template without using eval(). Here's a
small example of how closures work:
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");
This prints
Howdy, world!
Greetings, earthlings!
Note particularly that $x continues to refer to the
value passed into newprint() despite "my $x" having gone
out of scope by the time the anonymous subroutine runs.
That's what a closure is all about.
This applies only to 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.
5. References are often returned by special subroutines
called constructors. Perl objects are just references
to a special type of object that happens to know which
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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 often
named "new()". You can call them indirectly:
$objref = new Doggie( Tail => 'short', Ears => 'long' );
But that can produce ambiguous syntax in certain cases,
so it's often better to use the direct method invocation
approach:
$objref = Doggie->new(Tail => 'short', Ears => 'long');
use Term::Cap;
$terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
use Tk;
$main = MainWindow->new();
$menubar = $main->Frame(-relief => "raised",
-borderwidth => 2)
6. References of the appropriate type can spring into
existence if you 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.
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};
$formatref = *foo{FORMAT};
All of these are self-explanatory except for *foo{IO}.
It returns the IO handle, used for file handles ("open"
in perlfunc), sockets ("socket" in perlfunc and
"socketpair" in perlfunc), and directory handles
("opendir" in perlfunc). For compatibility with
previous versions of Perl, *foo{FILEHANDLE} is a synonym
for *foo{IO}, though it is deprecated as of 5.8.0. If
deprecation warnings are in effect, it will warn of its
use.
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*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.
*foo{IO} is an alternative to the *HANDLE mechanism
given in "Typeglobs and Filehandles" in perldata for
passing filehandles into or out of subroutines, or
storing into larger data structures. Its disadvantage
is that it won't create a new filehandle for you. Its
advantage is that you have less risk of clobbering more
than you want to with a typeglob assignment. (It still
conflates file and directory handles, though.) However,
if you assign the incoming value to a scalar instead of
a typeglob as we do in the examples below, there's no
risk of that happening.
splutter(*STDOUT); # pass the whole glob
splutter(*STDOUT{IO}); # pass both file and dir handles
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(*STDIN); # pass the whole glob
$rec = get_rec(*STDIN{IO}); # pass both file and dir handles
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
Using References
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 (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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically
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not dereferencing $arrayref[0] or $hashref{"KEY"} there.
The dereference of the scalar variable happens before it
does any key lookups. Anything more complicated than a
simple scalar variable must use methods 2 or 3 below.
However, a "simple scalar" includes an identifier that
itself uses method 1 recursively. Therefore, the
following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
2. Anywhere you'd put an identifier (or chain of
identifiers) as part of a variable or subroutine name,
you can replace the identifier with a BLOCK returning a
reference of the correct type. In other words, the
previous examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in
this case, but the BLOCK can contain any arbitrary
expression, in particular, subscripted expressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple
case of $$x, people often make the mistake of viewing
the dereferencing symbols as proper operators, and
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, 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
$hashref to the hash it's presumably referencing. That
would be case 3.
3. Subroutine calls and lookups of individual array
elements arise often enough that it gets cumbersome to
use method 2. As a form of syntactic sugar, the
examples for method 2 may be written:
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$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 $array[$x] is not 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. This process is called autovivification.
One more thing here. The arrow is optional 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.
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.
Using a string or number as a reference produces a symbolic
reference, as explained above. Using a reference as a
number produces an integer representing its storage location
in memory. The only useful thing to be done with this is to
compare two references numerically to see whether they refer
to the same location.
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if ($ref1 == $ref2) { # cheap numeric compare of references
print "refs 1 and 2 refer to the same thing\n";
}
Using a reference as a string produces both its referent's
type, including any package blessing as described in
perlobj, as well as the numeric address expressed in hex.
The ref() operator returns just the type of thing the
reference is pointing to, without the address. See "ref" in
perlfunc for details and examples of its use.
The bless() operator may be used to associate the object a
reference points to with a package functioning as an object
class. See perlobj.
A typeglob may be dereferenced the same way a reference can,
because the dereference syntax always indicates the type of
reference desired. So "${*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";
Similarly, an expression that returns a reference to a
scalar can be dereferenced via "${...}". Thus, the above
expression may be written as:
print "That yields ${\($n + 5)} widgets\n";
Symbolic references
We said that references spring into existence as necessary
if they are undefined, but we didn't say what happens if a
value used as a reference is already defined, but isn't a
hard reference. If you use it as a reference, it'll be
treated as a symbolic reference. That is, the value of the
scalar is taken to be the name of a variable, rather than a
direct link to a (possibly) anonymous value.
People frequently expect it to work like this. So it does.
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$name = "foo";
$$name = 1; # Sets $foo
${$name} = 2; # Sets $foo
${$name x 2} = 3; # Sets $foofoo
$name->[0] = 4; # Sets $foo[0]
@$name = (); # Clears @foo
&$name(); # Calls &foo() (as in Perl 4)
$pack = "THAT";
${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
This is powerful, and slightly dangerous, in that it's
possible to intend (with the utmost sincerity) to use a hard
reference, and accidentally use a symbolic reference
instead. To protect against that, you can say
use strict 'refs';
and then only hard references will be allowed for the rest
of the enclosing block. An inner block may countermand that
with
no strict 'refs';
Only package variables (globals, even if localized) are
visible to symbolic references. Lexical variables (declared
with my()) aren't in a symbol table, and thus are invisible
to this mechanism. For example:
local $value = 10;
$ref = "value";
{
my $value = 20;
print $$ref;
}
This will still print 10, not 20. Remember that local()
affects package variables, which are all "global" to the
package.
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", even though push is
a reserved word. This has been generalized to work the same
outside of quotes, so that
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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.
Pseudo-hashes: Using an array as a hash
Pseudo-hashes have been removed from Perl. The 'fields'
pragma remains available.
Function Templates
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
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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 time--like those in the
"for" loop above, basically aliases to elements from the
surrounding lexical scopes-- only works with anonymous subs,
not with named subroutines. Generally said, named
subroutines do not nest properly and should only be declared
in the main package scope.
This is because named subroutines are created at compile
time so their lexical variables get assigned to the parent
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lexicals from the first execution of the parent block. If a
parent scope is entered a second time, its lexicals are
created again, while the nested subs still reference the old
ones.
Anonymous subroutines get to capture each time you execute
the "sub" operator, as they are created on the fly. 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" due to the reasons explained
above. 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 anonymous subroutine.
But when it does, it has normal access to the lexical
variable $x from the scope of outer() at the time outer is
invoked.
This has the interesting effect of creating a function local
to another function, something not normally supported in
Perl.
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;
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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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
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.
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.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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