NAME¶
perltie - how to hide an object class in a simple variable
SYNOPSIS¶
tie VARIABLE, CLASSNAME, LIST
$object = tied VARIABLE
untie VARIABLE
DESCRIPTION¶
Prior to release 5.0 of Perl, a programmer could use
dbmopen() to connect
an on-disk database in the standard Unix
dbm(3x) format magically 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. 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(),
TIEHASH(), or
TIEHANDLE(). (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.) You can also retrieve a reference to the underlying
object using the
tied() function.
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 UNTIE and/or 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', $$;
And now whenever either of those variables is accessed, its current system
priority is retrieved and returned. If those variables are set, then the
process's priority is changed!
We'll use Jarkko Hietaniemi <
jhi@iki.fi>'s BSD::Resource class
(not included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
from your system, as well as the
getpriority() and
setpriority()
system calls. Here's the preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
- TIESCALAR classname, LIST
- This is the constructor for the class. That means it is
expected to return a blessed reference to a new scalar (probably
anonymous) that it's creating. For example:
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 0 means me
if ($pid !~ /^\d+$/) {
carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
return undef;
}
unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
carp "Nice::Tie::Scalar 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. Because in this
case we're using just 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. Don't
worry about returning a value from STORE; the semantic of assignment
returning the assigned value is implemented with FETCH.
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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
}
- UNTIE this
- This method will be triggered when the "untie"
occurs. This can be useful if the class needs to know when no further
calls will be made. (Except DESTROY of course.) See "The
"untie" Gotcha" below for more details.
- DESTROY this
- This method will be triggered when the tied variable needs
to be destructed. As with other object classes, such a method is seldom
necessary, because Perl deallocates its moribund object's memory for you
automatically--this isn't C++, you know. We'll use a DESTROY method here
for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
}
That's about all there is to it. Actually, it's more than all there is to it,
because 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, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
FETCHSIZE and STORESIZE are used to provide $#array and equivalent
"scalar(@array)" access.
The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are required
if the perl operator with the corresponding (but lowercase) name is to operate
on the tied array. The
Tie::Array class can be used as a base class to
implement the first five of these in terms of the basic methods above. The
default implementations of DELETE and EXISTS in
Tie::Array simply
"croak".
In addition EXTEND will be called when perl would have pre-extended allocation
in a real array.
For this discussion, we'll implement an array whose elements are a fixed size at
creation. If you try to create an element larger than the fixed size, you'll
take an exception. For example:
use FixedElem_Array;
tie @array, 'FixedElem_Array', 3;
$array[0] = 'cat'; # ok.
$array[1] = 'dogs'; # exception, length('dogs') > 3.
The preamble code for the class is as follows:
package FixedElem_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 "{ELEMSIZE}"
field will store the maximum element size allowed, and the
"{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 $elemsize = shift;
if ( @_ || $elemsize =~ /\D/ ) {
croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
}
return bless {
ELEMSIZE => $elemsize,
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 = shift;
my $index = shift;
return $self->{ARRAY}->[$index];
}
If a negative array index is used to read from an array, the index will be
translated to a positive one internally by calling FETCHSIZE before being
passed to FETCH. You may disable this feature by assigning a true value to
the variable $NEGATIVE_INDICES in the tied array class.
As you may have noticed, the name of the FETCH method (et al.) is the same
for all accesses, even though the constructors differ in names (TIESCALAR
vs TIEARRAY). While in theory you could have the same class servicing
several tied types, in practice this becomes cumbersome, and it's easiest
to keep them at simply one tie type per class.
- STORE this, index, value
- This method will be triggered every time an element in the
tied array is set (written). It takes two arguments beyond its self
reference: the index at which we're trying to store something and the
value we're trying to put there.
In our example, "undef" is really "$self->{ELEMSIZE}"
number of spaces so we have a little more work to do here:
sub STORE {
my $self = shift;
my( $index, $value ) = @_;
if ( length $value > $self->{ELEMSIZE} ) {
croak "length of $value is greater than $self->{ELEMSIZE}";
}
# fill in the blanks
$self->EXTEND( $index ) if $index > $self->FETCHSIZE();
# right justify to keep element size for smaller elements
$self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
}
Negative indexes are treated the same as with FETCH.
- FETCHSIZE this
- Returns the total number of items in the tied array
associated with object this. (Equivalent to
"scalar(@array)"). For example:
sub FETCHSIZE {
my $self = shift;
return scalar @{$self->{ARRAY}};
}
- STORESIZE this, count
- Sets the total number of items in the tied array associated
with object this to be count. If this makes the array larger
then class's mapping of "undef" should be returned for new
positions. If the array becomes smaller then entries beyond count should
be deleted.
In our example, 'undef' is really an element containing
"$self->{ELEMSIZE}" number of spaces. Observe:
sub STORESIZE {
my $self = shift;
my $count = shift;
if ( $count > $self->FETCHSIZE() ) {
foreach ( $count - $self->FETCHSIZE() .. $count ) {
$self->STORE( $_, '' );
}
} elsif ( $count < $self->FETCHSIZE() ) {
foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
$self->POP();
}
}
}
- EXTEND this, count
- Informative call that array is likely to grow to have
count entries. Can be used to optimize allocation. This method need
do nothing.
In our example, we want to make sure there are no blank ("undef")
entries, so "EXTEND" will make use of "STORESIZE" to
fill elements as needed:
sub EXTEND {
my $self = shift;
my $count = shift;
$self->STORESIZE( $count );
}
- EXISTS this, key
- Verify that the element at index key exists in the
tied array this.
In our example, we will determine that if an element consists of
"$self->{ELEMSIZE}" spaces only, it does not exist:
sub EXISTS {
my $self = shift;
my $index = shift;
return 0 if ! defined $self->{ARRAY}->[$index] ||
$self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
return 1;
}
- DELETE this, key
- Delete the element at index key from the tied array
this.
In our example, a deleted item is "$self->{ELEMSIZE}" spaces:
sub DELETE {
my $self = shift;
my $index = shift;
return $self->STORE( $index, '' );
}
- CLEAR this
- Clear (remove, delete, ...) all values from the tied array
associated with object this. For example:
sub CLEAR {
my $self = shift;
return $self->{ARRAY} = [];
}
- PUSH this, LIST
- Append elements of LIST to the array. For example:
sub PUSH {
my $self = shift;
my @list = @_;
my $last = $self->FETCHSIZE();
$self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
return $self->FETCHSIZE();
}
- POP this
- Remove last element of the array and return it. For
example:
sub POP {
my $self = shift;
return pop @{$self->{ARRAY}};
}
- SHIFT this
- Remove the first element of the array (shifting other
elements down) and return it. For example:
sub SHIFT {
my $self = shift;
return shift @{$self->{ARRAY}};
}
- UNSHIFT this, LIST
- Insert LIST elements at the beginning of the array, moving
existing elements up to make room. For example:
sub UNSHIFT {
my $self = shift;
my @list = @_;
my $size = scalar( @list );
# make room for our list
@{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
= @{$self->{ARRAY}};
$self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
}
- SPLICE this, offset, length, LIST
- Perform the equivalent of "splice" on the array.
offset is optional and defaults to zero, negative values count back
from the end of the array.
length is optional and defaults to rest of the array.
LIST may be empty.
Returns a list of the original length elements at offset.
In our example, we'll use a little shortcut if there is a LIST:
sub SPLICE {
my $self = shift;
my $offset = shift || 0;
my $length = shift || $self->FETCHSIZE() - $offset;
my @list = ();
if ( @_ ) {
tie @list, __PACKAGE__, $self->{ELEMSIZE};
@list = @_;
}
return splice @{$self->{ARRAY}}, $offset, $length, @list;
}
- UNTIE this
- Will be called when "untie" happens. (See
"The "untie" Gotcha" below.)
- DESTROY this
- This method will be triggered when the tied variable needs
to be destructed. As with the scalar 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.
Tying Hashes¶
Hashes were the first Perl data type to be tied (see
dbmopen()). A class
implementing a tied hash 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. SCALAR is triggered when the tied hash is evaluated in scalar
context. UNTIE is called when "untie" happens, and DESTROY is called
when the tied variable is garbage collected.
If this seems like a lot, then feel free to inherit from merely the standard
Tie::StdHash module for most of your methods, redefining only the interesting
ones. See Tie::Hash 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 dot files. You index into the hash with the
name of the file (minus the dot) and you get back that dot file'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 dot files live
- CLOBBER
- whether we should try to change or remove those dot
files
- LIST
- the hash of dot file 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 be 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, because 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, because 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(my $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";
Another way to lay hands on a reference to the underlying object is to use
the tied() function, so they might alternately have set clobber
using:
tie %daemon_dots, 'daemon';
tied(%daemon_dots)->clobber(1);
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};
my $success = unlink($file);
carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
$success;
}
The value returned by DELETE becomes the return value of the call to
delete(). If you want to emulate the normal behavior of
delete(), you should return whatever FETCH would have returned for
this key. In this example, we have chosen instead to return a value which
tells the caller whether the file was successfully deleted.
- 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 dot files! 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 dot files 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}}; # reset each() iterator
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're using a real hash so we'll do just the simple thing,
but we'll have to go through the LIST field indirectly.
sub NEXTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
return each %{ $self->{LIST} }
}
- SCALAR this
- This is called when the hash is evaluated in scalar
context. In order to mimic the behaviour of untied hashes, this method
should return a false value when the tied hash is considered empty. If
this method does not exist, perl will make some educated guesses and
return true when the hash is inside an iteration. If this isn't the case,
FIRSTKEY is called, and the result will be a false value if FIRSTKEY
returns the empty list, true otherwise.
However, you should not blindly rely on perl always doing the right
thing. Particularly, perl will mistakenly return true when you clear the
hash by repeatedly calling DELETE until it is empty. You are therefore
advised to supply your own SCALAR method when you want to be absolutely
sure that your hash behaves nicely in scalar context.
In our example we can just call "scalar" on the underlying hash
referenced by "$self->{LIST}":
sub SCALAR {
carp &whowasi if $DEBUG;
my $self = shift;
return scalar %{ $self->{LIST} }
}
- UNTIE this
- This is called when "untie" occurs. See "The
"untie" Gotcha" below.
- 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 $DEBUG;
}
Note that functions such as
keys() and
values() may return huge
lists when used on large objects, like DBM files. You may prefer to use the
each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
Tying FileHandles¶
This is partially implemented now.
A class implementing a tied filehandle should define the following methods:
TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC, READ, and
possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE, OPEN,
EOF, FILENO, SEEK, TELL - if the corresponding perl operators are used on the
handle.
When STDERR is tied, its PRINT method will be called to issue warnings and error
messages. This feature is temporarily disabled during the call, which means
you can use "warn()" inside PRINT without starting a recursive loop.
And just like "__WARN__" and "__DIE__" handlers, STDERR's
PRINT method may be called to report parser errors, so the caveats mentioned
under "%SIG" in perlvar apply.
All of this is especially useful when perl is embedded in some other program,
where output to STDOUT and STDERR may have to be redirected in some special
way. See nvi and the Apache module for examples.
When tying a handle, the first argument to "tie" should begin with an
asterisk. So, if you are tying STDOUT, use *STDOUT. If you have assigned it to
a scalar variable, say $handle, use *$handle. "tie $handle" works,
too, but that is considered a bug and will be fixed in Perl 5.16. It is
supposed to tie the scalar $handle, not the handle inside it. "tie
$handle" emits a deprecation warning as of Perl 5.14.
In our example we're going to create a shouting handle.
package Shout;
- TIEHANDLE classname, LIST
- This is the constructor for the class. That means it is
expected to return a blessed reference of some sort. The reference can be
used to hold some internal information.
sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
- WRITE this, LIST
- This method will be called when the handle is written to
via the "syswrite" function.
sub WRITE {
$r = shift;
my($buf,$len,$offset) = @_;
print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
}
- PRINT this, LIST
- This method will be triggered every time the tied handle is
printed to with the "print()" or "say()" functions.
Beyond its self reference it also expects the list that was passed to the
print function.
sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
"say()" acts just like "print()" except $\ will be
localized to "\n" so you need do nothing special to handle
"say()" in "PRINT()".
- PRINTF this, LIST
- This method will be triggered every time the tied handle is
printed to with the "printf()" function. Beyond its self
reference it also expects the format and list that was passed to the
printf function.
sub PRINTF {
shift;
my $fmt = shift;
print sprintf($fmt, @_);
}
- READ this, LIST
- This method will be called when the handle is read from via
the "read" or "sysread" functions.
sub READ {
my $self = shift;
my $bufref = \$_[0];
my(undef,$len,$offset) = @_;
print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
# add to $$bufref, set $len to number of characters read
$len;
}
- READLINE this
- This method is called when the handle is read via
"<HANDLE>" or "readline HANDLE".
As per "readline", in scalar context it should return the next
line, or "undef" for no more data. In list context it should
return all remaining lines, or an empty list for no more data. The strings
returned should include the input record separator $/ (see perlvar),
unless it is "undef" (which means "slurp" mode).
sub READLINE {
my $r = shift;
if (wantarray) {
return ("all remaining\n",
"lines up\n",
"to eof\n");
} else {
return "READLINE called " . ++$$r . " times\n";
}
}
- GETC this
- This method will be called when the "getc"
function is called.
sub GETC { print "Don't GETC, Get Perl"; return "a"; }
- EOF this
- This method will be called when the "eof"
function is called.
Starting with Perl 5.12, an additional integer parameter will be passed. It
will be zero if "eof" is called without parameter; 1 if
"eof" is given a filehandle as a parameter, e.g.
"eof(FH)"; and 2 in the very special case that the tied
filehandle is "ARGV" and "eof" is called with an empty
parameter list, e.g. "eof()".
sub EOF { not length $stringbuf }
- CLOSE this
- This method will be called when the handle is closed via
the "close" function.
sub CLOSE { print "CLOSE called.\n" }
- UNTIE this
- As with the other types of ties, this method will be called
when "untie" happens. It may be appropriate to "auto
CLOSE" when this occurs. See "The "untie" Gotcha"
below.
- DESTROY this
- As with the other types of ties, this method will be called
when the tied handle is about to be destroyed. This is useful for
debugging and possibly cleaning up.
sub DESTROY { print "</shout>\n" }
Here's how to use our little example:
tie(*FOO,'Shout');
print FOO "hello\n";
$a = 4; $b = 6;
print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
print <FOO>;
UNTIE this¶
You can define for all tie types an UNTIE method that will be called at
untie(). See "The "untie" Gotcha" below.
The "untie" Gotcha¶
If you intend making use of the object returned from either
tie() or
tied(), and if the tie's target class defines a destructor, there is a
subtle gotcha you
must guard against.
As setup, consider this (admittedly rather contrived) example of a tie; all it
does is use a file to keep a log of the values assigned to a scalar.
package Remember;
use strict;
use warnings;
use IO::File;
sub TIESCALAR {
my $class = shift;
my $filename = shift;
my $handle = IO::File->new( "> $filename" )
or die "Cannot open $filename: $!\n";
print $handle "The Start\n";
bless {FH => $handle, Value => 0}, $class;
}
sub FETCH {
my $self = shift;
return $self->{Value};
}
sub STORE {
my $self = shift;
my $value = shift;
my $handle = $self->{FH};
print $handle "$value\n";
$self->{Value} = $value;
}
sub DESTROY {
my $self = shift;
my $handle = $self->{FH};
print $handle "The End\n";
close $handle;
}
1;
Here is an example that makes use of this tie:
use strict;
use Remember;
my $fred;
tie $fred, 'Remember', 'myfile.txt';
$fred = 1;
$fred = 4;
$fred = 5;
untie $fred;
system "cat myfile.txt";
This is the output when it is executed:
The Start
1
4
5
The End
So far so good. Those of you who have been paying attention will have spotted
that the tied object hasn't been used so far. So lets add an extra method to
the Remember class to allow comments to be included in the file; say,
something like this:
sub comment {
my $self = shift;
my $text = shift;
my $handle = $self->{FH};
print $handle $text, "\n";
}
And here is the previous example modified to use the "comment" method
(which requires the tied object):
use strict;
use Remember;
my ($fred, $x);
$x = tie $fred, 'Remember', 'myfile.txt';
$fred = 1;
$fred = 4;
comment $x "changing...";
$fred = 5;
untie $fred;
system "cat myfile.txt";
When this code is executed there is no output. Here's why:
When a variable is tied, it is associated with the object which is the return
value of the TIESCALAR, TIEARRAY, or TIEHASH function. This object normally
has only one reference, namely, the implicit reference from the tied variable.
When
untie() is called, that reference is destroyed. Then, as in the
first example above, the object's destructor (DESTROY) is called, which is
normal for objects that have no more valid references; and thus the file is
closed.
In the second example, however, we have stored another reference to the tied
object in $x. That means that when
untie() gets called there will still
be a valid reference to the object in existence, so the destructor is not
called at that time, and thus the file is not closed. The reason there is no
output is because the file buffers have not been flushed to disk.
Now that you know what the problem is, what can you do to avoid it? Prior to the
introduction of the optional UNTIE method the only way was the good old
"-w" flag. Which will spot any instances where you call
untie() and there are still valid references to the tied object. If the
second script above this near the top "use warnings 'untie'" or was
run with the "-w" flag, Perl prints this warning message:
untie attempted while 1 inner references still exist
To get the script to work properly and silence the warning make sure there are
no valid references to the tied object
before untie() is called:
undef $x;
untie $fred;
Now that UNTIE exists the class designer can decide which parts of the class
functionality are really associated with "untie" and which with the
object being destroyed. What makes sense for a given class depends on whether
the inner references are being kept so that non-tie-related methods can be
called on the object. But in most cases it probably makes sense to move the
functionality that would have been in DESTROY to the UNTIE method.
If the UNTIE method exists then the warning above does not occur. Instead the
UNTIE method is passed the count of "extra" references and can issue
its own warning if appropriate. e.g. to replicate the no UNTIE case this
method can be used:
sub UNTIE
{
my ($obj,$count) = @_;
carp "untie attempted while $count inner references still exist" if $count;
}
SEE ALSO¶
See DB_File or Config for some interesting
tie() implementations. A good
starting point for many
tie() implementations is with one of the
modules Tie::Scalar, Tie::Array, Tie::Hash, or Tie::Handle.
BUGS¶
The bucket usage information provided by "scalar(%hash)" is not
available. What this means is that using %tied_hash in boolean context doesn't
work right (currently this always tests false, regardless of whether the hash
is empty or hash elements).
Localizing tied arrays or hashes does not work. After exiting the scope the
arrays or the hashes are not restored.
Counting the number of entries in a hash via "scalar(keys(%hash))" or
"scalar(values(%hash)") is inefficient since it needs to iterate
through all the entries with FIRSTKEY/NEXTKEY.
Tied hash/array slices cause multiple FETCH/STORE pairs, there are no tie
methods for slice operations.
You cannot easily tie a multilevel data structure (such as a hash of hashes) to
a dbm file. The first problem is that all but GDBM and Berkeley DB have size
limitations, but beyond that, you also have problems with how references are
to be represented on disk. One module that does attempt to address this need
is DBM::Deep. Check your nearest CPAN site as described in perlmodlib for
source code. Note that despite its name, DBM::Deep does not use dbm. Another
earlier attempt at solving the problem is MLDBM, which is also available on
the CPAN, but which has some fairly serious limitations.
Tied filehandles are still incomplete.
sysopen(),
truncate(),
flock(),
fcntl(),
stat() and -X can't currently be
trapped.
AUTHOR¶
Tom Christiansen
TIEHANDLE by Sven Verdoolaege <
skimo@dns.ufsia.ac.be> and Doug
MacEachern <
dougm@osf.org>
UNTIE by Nick Ing-Simmons <
nick@ing-simmons.net>
SCALAR by Tassilo von Parseval <
tassilo.von.parseval@rwth-aachen.de>
Tying Arrays by Casey West <
casey@geeknest.com>