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标 题: perl(24)
发信站: 白山黑水BBS (Wed Apr 2 17:41:48 1997)
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出 处: conger.neu.edu.cn
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NAME
perltie - how to hide an object class in a simple variable
---------------------------------------------------------------------------
SYNOPSIS
tie VARIABLE, CLASSNAME, LIST
untie VARIABLE
---------------------------------------------------------------------------
DESCRIPTION
Prior to release 5.0 of Perl, a programmer could use dbmopen() to magically
connect an on-disk database in the standard Unix dbm(3x) format to a %HASH
in their program. However, their Perl was either built with one particular
dbm library or another, but not both, and you couldn't extend this
mechanism to other packages or types of variables.
Now you can.
The tie() function binds a variable to a class (package) that will provide
the implementation for access methods for that variable. Once this magic
has been performed, accessing a tied variable automatically triggers method
calls in the proper class. All of the complexity of the class is hidden
behind magic methods calls. The method names are in ALL CAPS, which is a
convention that Perl uses to indicate that they're called implicitly rather
than explicitly--just like the BEGIN() and END() functions.
In the tie() call, VARIABLE is the name of the variable to be enchanted.
CLASSNAME is the name of a class implementing objects of the correct type.
Any additional arguments in the LIST are passed to the appropriate
constructor method for that class--meaning TIESCALAR() , TIEARRAY() , or
TIEHASH() . (Typically these are arguments such as might be passed to the
dbminit() function of C.) The object returned by the ``new'' method is also
returned by the tie() function, which would be useful if you wanted to
access other methods in CLASSNAME. (You don't actually have to return a
reference to a right ``type'' (e.g. HASH or CLASSNAME) so long as it's a
properly blessed object.)
Unlike dbmopen() , the tie() function will not use or require a module for
you--you need to do that explicitly yourself.
---------------------------------------------------------------------------
Tying Scalars
A class implementing a tied scalar should define the following methods:
TIESCALAR, FETCH, STORE, and possibly DESTROY.
Let's look at each in turn, using as an example a tie class for scalars
that allows the user to do something like:
tie $his_speed, 'Nice', getppid();
tie $my_speed, 'Nice', $$;
And now whenever either of those variables is accessed, its current system
priority is retrieved and returned. If those variables are set, then the
process's priority is changed!
We'll use Jarkko Hietaniemi <Jarkko.Hietaniemi@hut.fi>'s BSD::Resource
class (not included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX
constants from your system, as well as the getpriority() and setpriority()
system calls. Here's the preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
TIESCALAR classname, LIST
This is the constructor for the class. That means it is expected to
return a blessed reference to a new scalar (probably anonymous) that
it's creating. For example:
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 0 means me
if ($pid !~ /^\d+$/) {
carp "Nice::TieScalar got non-numeric pid $pid" if $^W;
return undef;
}
unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
carp "Nice::TieScalar got bad pid $pid: $!" if $^W;
return undef;
}
return bless \$pid, $class;
}
This tie class has chosen to return an error rather than raising an
exception if its constructor should fail. While this is how dbmopen()
works, other classes may well not wish to be so forgiving. It checks
the global variable $^ W to see whether to emit a bit of noise anyway.
FETCH this
This method will be triggered every time the tied variable is accessed
(read). It takes no arguments beyond its self reference, which is the
object representing the scalar we're dealing with. Since in this case
we're just using a SCALAR ref for the tied scalar object, a simple $$
self allows the method to get at the real value stored there. In our
example below, that real value is the process ID to which we've tied
our variable.
sub FETCH {
my $self = shift;
confess "wrong type" unless ref $self;
croak "usage error" if @_;
my $nicety;
local($!) = 0;
$nicety = getpriority(PRIO_PROCESS, $$self);
if ($!) { croak "getpriority failed: $!" }
return $nicety;
}
This time we've decided to blow up (raise an exception) if the renice
fails--there's no place for us to return an error otherwise, and it's
probably the right thing to do.
STORE this, value
This method will be triggered every time the tied variable is set
(assigned). Beyond its self reference, it also expects one (and only
one) argument--the new value the user is trying to assign.
sub STORE {
my $self = shift;
confess "wrong type" unless ref $self;
my $new_nicety = shift;
croak "usage error" if @_;
if ($new_nicety < PRIO_MIN) {
carp sprintf
"WARNING: priority %d less than minimum system priority %d",
$new_nicety, PRIO_MIN if $^W;
$new_nicety = PRIO_MIN;
}
if ($new_nicety > PRIO_MAX) {
carp sprintf
"WARNING: priority %d greater than maximum system priority %d",
$new_nicety, PRIO_MAX if $^W;
$new_nicety = PRIO_MAX;
}
unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
return $new_nicety;
}
DESTROY this
This method will be triggered when the tied variable needs to be
destructed. As with other object classes, such a method is seldom
ncessary, since Perl deallocates its moribund object's memory for you
automatically--this isn't C++, you know. We'll use a DESTROY method
here for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
}
That's about all there is to it. Actually, it's more than all there.is to
it, since we've done a few nice things here for the sake of completeness,
robustness, and general aesthetics. Simpler TIESCALAR classes are certainly
possible.
---------------------------------------------------------------------------
Tying Arrays
A class implementing a tied ordinary array should define the following
methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.
WARNING: Tied arrays are incomplete. They are also distinctly lacking
something for the $# ARRAY access (which is hard, as it's an lvalue), as
well as the other obvious array functions, like push() , pop() , shift() ,
unshift() , and splice() .
For this discussion, we'll implement an array whose indices are fixed at
its creation. If you try to access anything beyond those bounds, you'll
take an exception. (Well, if you access an individual element; an aggregate
assignment would be missed.) For example:
require Bounded_Array;
tie @ary, Bounded_Array, 2;
$| = 1;
for $i (0 .. 10) {
print "setting index $i: ";
$ary[$i] = 10 * $i;
$ary[$i] = 10 * $i;
print "value of elt $i now $ary[$i]\n";
}
The preamble code for the class is as follows:
package Bounded_Array;
use Carp;
use strict;
TIEARRAY classname, LIST
This is the constructor for the class. That means it is expected to
return a blessed reference through which the new array (probably an
anonymous ARRAY ref) will be accessed.
In our example, just to show you that you don't really have to return
an ARRAY reference, we'll choose a HASH reference to represent our
object. A HASH works out well as a generic record type: the {BOUND}
field will store the maximum bound allowed, and the C<{ARRAY} field
will hold the true ARRAY ref. If someone outside the class tries to
dereference the object returned (doubtless thinking it an ARRAY ref),
they'll blow up. This just goes to show you that you should respect an
object's privacy.
sub TIEARRAY {
my $class = shift;
my $bound = shift;
confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
if @_ || $bound =~ /\D/;
return bless {
BOUND => $bound,
ARRAY => [],
}, $class;
}
FETCH this, index
This method will be triggered every time an individual element the
tied array is accessed (read). It takes one argument beyond its self
reference: the index whose value we're trying to fetch.
sub FETCH {
my($self,$idx) = @_;
if ($idx > $self->{BOUND}) {
confess "Array OOB: $idx > $self->{BOUND}";
}
return $self->{ARRAY}[$idx];
}
As you may have noticed, the name of the FETCH method (et al.) is the
same for all accesses, even though the constructors differ in names
(TIESCALAR vs TIEARRAY). While in theory you could have the same class
servicing several tied types, in practice this becomes cumbersome, and
it's easiest to simply keep them at one tie type per class.
STORE this, index, value
This method will be triggered every time an element in the tied array
is set (written). It takes two arguments beyond its self reference:
the index at which we're trying to store something and the value we're
trying to put there. For example:
sub STORE {
my($self, $idx, $value) = @_;
print "[STORE $value at $idx]\n" if _debug;
if ($idx > $self->{BOUND} ) {
confess "Array OOB: $idx > $self->{BOUND}";
}
return $self->{ARRAY}[$idx] = $value;
}
DESTROY this
This method will be triggered when the tied variable needs to be
destructed. As with the sclar tie class, this is almost never needed
in a language that does its own garbage collection, so this time we'll
just leave it out.
The code we presented at the top of the tied array class accesses
many.elements of the array, far more than we've set the bounds to.
Therefore, it will blow up once they try to access beyond the 2nd element
of @ary, as the following output demonstrates:
setting index 0: value of elt 0 now 0
setting index 1: value of elt 1 now 10
setting index 2: value of elt 2 now 20
setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
Bounded_Array::FETCH called at testba line 12
---------------------------------------------------------------------------
Tying Hashes
As the first Perl data type to be tied (see dbmopen() ), associative arrays
have the most complete and useful tie() implementation. A class
implementing a tied associative array should define the following methods:
TIEHASH is the constructor. FETCH and STORE access the key and value pairs.
EXISTS reports whether a key is present in the hash, and DELETE deletes
one. CLEAR empties the hash by deleting all the key and value pairs.
FIRSTKEY and NEXTKEY implement the keys() and each() functions to iterate
over all the keys. And DESTROY is called when the tied variable is garbage
collected.
If this seems like a lot, then feel free to merely inherit from the
standard TieHash module for most of your methods, redefining only the
interesting ones. See TieHash for details.
Remember that Perl distinguishes between a key not existing in the hash,
and the key existing in the hash but having a corresponding value of undef
The two possibilities can be tested with the exists() and defined()
functions.
Here's an example of a somewhat interesting tied hash class: it gives you a
hash representing a particular user's dotfiles. You index into the hash
with the name of the file (minus the dot) and you get back that dotfile's
contents. For example:
use DotFiles;
tie %dot, DotFiles;
if ( $dot{profile} =~ /MANPATH/ ||
$dot{login} =~ /MANPATH/ ||
$dot{cshrc} =~ /MANPATH/ )
{
print "you seem to set your manpath\n";
}
Or here's another sample of using our tied class:
tie %him, DotFiles, 'daemon';
foreach $f ( keys %him ) {
printf "daemon dot file %s is size %d\n",
$f, length $him{$f};
}
In our tied hash DotFiles example, we use a regular hash for the object
containing several important fields, of which only the {LIST} field will be
what the user thinks of as the real hash.
USER
whose dot files this object represents
HOME
where those dotfiles live
CLOBBER
whether we should try to change or remove those dot files
LIST
the hash of dotfile names and content mappings
Here's the start of Dotfiles.pm:. package DotFiles; use Carp; sub whowasi {
(caller(1))[3] . '()' } my $DEBUG = 0; sub debug { $DEBUG = @_ ? shift : 1
}
For our example, we want to able to emit debugging info to help in tracing
during development. We keep also one convenience function around internally
to help print out warnings; whowasi() returns the function name that calls
it.
Here are the methods for the DotFiles tied hash.
TIEHASH classname, LIST
This is the constructor for the class. That means it is expected to
return a blessed reference through which the new object (probably but
not necessarily an anonymous hash) will be accessed.
Here's the constructor:
sub TIEHASH {
my $self = shift;
my $user = shift || $>;
my $dotdir = shift || '';
croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
$user = getpwuid($user) if $user =~ /^\d+$/;
my $dir = (getpwnam($user))[7]
|| croak "@{[&whowasi]}: no user $user";
$dir .= "/$dotdir" if $dotdir;
my $node = {
USER => $user,
HOME => $dir,
LIST => {},
CLOBBER => 0,
};
opendir(DIR, $dir)
|| croak "@{[&whowasi]}: can't opendir $dir: $!";
foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
$dot =~ s/^\.//;
$node->{LIST}{$dot} = undef;
}
closedir DIR;
return bless $node, $self;
}
It's probably worth mentioning that if you're going to filetest the
return values out of a readdir, you'd better prepend the directory in
question. Otherwise, since we didn't chdir() there, it would have been
testing the wrong file.
FETCH this, key
This method will be triggered every time an element in the tied hash
is accessed (read). It takes one argument beyond its self reference:
the key whose value we're trying to fetch.
Here's the fetch for our DotFiles example.
sub FETCH {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
my $dir = $self->{HOME};
my $file = "$dir/.$dot";
unless (exists $self->{LIST}->{$dot} || -f $file) {
carp "@{[&whowasi]}: no $dot file" if $DEBUG;
return undef;
}
if (defined $self->{LIST}->{$dot}) {
return $self->{LIST}->{$dot};
} else {
return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
}
}
It was easy to write by having it call the Unix cat(1) command, but it
would probably be more portable to open the file manually (and
somewhat more efficient). Of course, since dot files are a Unixy
concept, we're not that concerned.
STORE this, key, value
This method will be triggered every time an element in the tied hash
is set (written). It takes two arguments beyond its self reference:
the index at which we're trying to store something, and the value
we're trying to put there.
Here in our DotFiles example, we'll be careful not to let them try to
overwrite the file unless they've called the clobber() method on the
original object reference returned by tie() .
sub STORE {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
my $value = shift;
my $file = $self->{HOME} . "/.$dot";
my $user = $self->{USER};
croak "@{[&whowasi]}: $file not clobberable"
unless $self->{CLOBBER};
open(F, "> $file") || croak "can't open $file: $!";
print F $value;
close(F);
}
If they wanted to clobber something, they might say:
$ob = tie %daemon_dots, 'daemon';
$ob->clobber(1);
$daemon_dots{signature} = "A true daemon\n";
Where the clobber method is simply:
sub clobber {
my $self = shift;
$self->{CLOBBER} = @_ ? shift : 1;
}
DELETE this, key
This method is triggered when we remove an element from the hash,
typically by using the delete() function. Again, we'll be careful to
check whether they really want to clobber files.
sub DELETE {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
my $file = $self->{HOME} . "/.$dot";
croak "@{[&whowasi]}: won't remove file $file"
unless $self->{CLOBBER};
delete $self->{LIST}->{$dot};
unlink($file) || carp "@{[&whowasi]}: can't unlink $file: $!";
}
CLEAR this
This method is triggered when the whole hash is to be cleared, usually
by assigning the empty list to it.
In our example, that would remove all the user's dotfiles! It's such a
dangerous thing that they'll have to set CLOBBER to something higher
than 1 to make it happen.
sub CLEAR {
carp &whowasi if $DEBUG;
my $self = shift;
croak "@{[&whowasi]}: won't remove all dotfiles for $self->{USER}"
unless $self->{CLOBBER} > 1;
my $dot;
foreach $dot ( keys %{$self->{LIST}}) {
$self->DELETE($dot);
}
}
EXISTS this, key
This method is triggered when the user uses the exists() function on a
particular hash. In our example, we'll look at the {LIST} hash element
for this:
sub EXISTS {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
return exists $self->{LIST}->{$dot};
}
FIRSTKEY this
This method will be triggered when the user is going to iterate
through the hash, such as via a keys() or each() call.
sub FIRSTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
my $a = keys %{$self->{LIST}};
each %{$self->{LIST}}
}
NEXTKEY this, lastkey
This method gets triggered during a keys() or each() iteration. It has
a second argument which is the last key that had been accessed. This
is useful if you're carrying about ordering or calling the iterator
from more than one sequence, or not really storing things in a hash
anywhere.
For our example, we our using a real hash so we'll just do the simple
thing, but we'll have to indirect through the LIST field.
sub NEXTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
return each %{ $self->{LIST} }
}
DESTROY this
This method is triggered when a tied hash is about to go out of scope.
You don't really need it unless you're trying to add debugging or have
auxiliary state to clean up. Here's a very simple function:
sub DESTROY {
carp &whowasi if $DEBUG;
}
Note that functions such as keys() and values() may return huge
array.values when used on large objects, like DBM files. You may prefer to
use the each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
---------------------------------------------------------------------------
Tying FileHandles
This isn't implemented yet. Sorry; maybe someday.
---------------------------------------------------------------------------
SEE ALSO
See DB_File or Config for some interesting tie() implementations.
---------------------------------------------------------------------------
BUGS
Tied arrays are incomplete. They are also distinctly lacking something for
the $# ARRAY access (which is hard, as it's an lvalue), as well as the
other obvious array functions, like push() , pop() , shift() , unshift() ,
and splice() .
---------------------------------------------------------------------------
AUTHOR
Tom Christiansen
--------------7DAFFA0409F--
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