IO::WrapTie(3pm) User Contributed Perl Documentation IO::WrapTie(3pm)
NAME
IO::WrapTie - wrap tieable objects in IO::Handle interface
This is currently Alpha code, released for comments.
Please give me your feedback!
SYNOPSIS
First of all, you'll need tie(), so:
require 5.004;
Function interface (experimental). Use this with any existing class...
use IO::WrapTie;
use FooHandle; ### implements TIEHANDLE interface
### Suppose we want a "FooHandle->new(&FOO_RDWR, 2)".
### We can instead say...
$FH = wraptie('FooHandle', &FOO_RDWR, 2);
### Now we can use...
print $FH "Hello, "; ### traditional operator syntax...
$FH->print("world!\n"); ### ...and OO syntax as well!
OO interface (preferred). You can inherit from the "Slave" in
IO::WrapTie mixin to get a nifty new_tie() constructor...
#------------------------------
package FooHandle; ### a class which can TIEHANDLE
use IO::WrapTie;
@ISA = qw(IO::WrapTie::Slave); ### inherit new_tie()
...
#------------------------------
package main;
$FH = FooHandle->new_tie(&FOO_RDWR, 2); ### $FH is an IO::WrapTie::Master
print $FH "Hello, "; ### traditional operator syntax
$FH->print("world!\n"); ### OO syntax
See IO::Scalar as an example. It also shows you how to create classes
which work both with and without 5.004.
DESCRIPTION
Suppose you have a class "FooHandle", where...
• "FooHandle" does not inherit from IO::Handle. That is, it performs
file handle-like I/O, but to something other than an underlying file
descriptor. Good examples are IO::Scalar (for printing to a string)
and IO::Lines (for printing to an array of lines).
• "FooHandle" implements the "TIEHANDLE" interface (see perltie).
That is, it provides methods "TIEHANDLE", "GETC", "PRINT", "PRINTF",
"READ", and "READLINE".
• "FooHandle" implements the traditional OO interface of FileHandle
and IO::Handle. i.e., it contains methods like "getline", "read",
"print", "seek", "tell", "eof", etc.
Normally, users of your class would have two options:
• Use only OO syntax, and forsake named I/O operators like "print".
• Use with tie, and forsake treating it as a first-class object (i.e.,
class-specific methods can only be invoked through the underlying
object via "tied"... giving the object a "split personality").
But now with IO::WrapTie, you can say:
$WT = wraptie('FooHandle', &FOO_RDWR, 2);
$WT->print("Hello, world\n"); ### OO syntax
print $WT "Yes!\n"; ### Named operator syntax too!
$WT->weird_stuff; ### Other methods!
And if you're authoring a class like "FooHandle", just have it inherit
from "IO::WrapTie::Slave" and that first line becomes even prettier:
$WT = FooHandle->new_tie(&FOO_RDWR, 2);
The bottom line: now, almost any class can look and work exactly like an
IO::Handle and be used both with OO and non-OO file handle syntax.
HOW IT ALL WORKS
The data structures
Consider this example code, using classes in this distribution:
use IO::Scalar;
use IO::WrapTie;
$WT = wraptie('IO::Scalar',\$s);
print $WT "Hello, ";
$WT->print("world!\n");
In it, the "wraptie" function creates a data structure as follows:
* $WT is a blessed reference to a tied filehandle
$WT glob; that glob is tied to the "Slave" object.
| * You would do all your i/o with $WT directly.
|
|
| ,---isa--> IO::WrapTie::Master >--isa--> IO::Handle
V /
.-------------.
| |
| | * Perl i/o operators work on the tied object,
| "Master" | invoking the C<TIEHANDLE> methods.
| | * Method invocations are delegated to the tied
| | slave.
`-------------'
|
tied(*$WT) | .---isa--> IO::WrapTie::Slave
V /
.-------------.
| |
| "Slave" | * Instance of FileHandle-like class which doesn't
| | actually use file descriptors, like IO::Scalar.
| IO::Scalar | * The slave can be any kind of object.
| | * Must implement the C<TIEHANDLE> interface.
`-------------'
NOTE: just as an IO::Handle is really just a blessed reference to a
traditional file handle glob. So also, an "IO::WrapTie::Master" is
really just a blessed reference to a file handle glob which has been
tied to some "slave" class.
How "wraptie" works
1. The call to function "wraptie(SLAVECLASS, TIEARGS...)" is passed
onto IO::WrapTie::Master::new(). Note that class
"IO::WrapTie::Master" is a subclass of IO::Handle.
2. The "IO::WrapTie::Master->new" method creates a new IO::Handle
object, re-blessed into class "IO::WrapTie::Master". This object is
the master, which will be returned from the constructor. At the same
time...
3. The "new" method also creates the slave: this is an instance of
"SLAVECLASS" which is created by tying the master's IO::Handle to
"SLAVECLASS" via "tie". This call to "tie" creates the slave in the
following manner:
4. Class "SLAVECLASS" is sent the message "TIEHANDLE"; it will usually
delegate this to "SLAVECLASS->new(TIEARGS)", resulting in a new
instance of "SLAVECLASS" being created and returned.
5. Once both master and slave have been created, the master is returned
to the caller.
How I/O operators work (on the master)
Consider using an i/o operator on the master:
print $WT "Hello, world!\n";
Since the master $WT is really a "blessed" reference to a glob, the
normal Perl I/O operators like "print" may be used on it. They will
just operate on the symbol part of the glob.
Since the glob is tied to the slave, the slave's "PRINT" method (part of
the "TIEHANDLE" interface) will be automatically invoked.
If the slave is an IO::Scalar, that means "PRINT" in IO::Scalar will be
invoked, and that method happens to delegate to the "print" method of
the same class. So the real work is ultimately done by "print" in
IO::Scalar.
How methods work (on the master)
Consider using a method on the master:
$WT->print("Hello, world!\n");
Since the master $WT is blessed into the class "IO::WrapTie::Master",
Perl first attempts to find a "print" method there. Failing that, Perl
next attempts to find a "print" method in the super class, IO::Handle.
It just so happens that there is such a method; that method merely
invokes the "print" I/O operator on the self object... and for that,
see above!
But let's suppose we're dealing with a method which isn't part of
IO::Handle... for example:
my $sref = $WT->sref;
In this case, the intuitive behavior is to have the master delegate the
method invocation to the slave (now do you see where the designations
come from?). This is indeed what happens: "IO::WrapTie::Master"
contains an "AUTOLOAD" method which performs the delegation.
So: when "sref" can't be found in IO::Handle, the "AUTOLOAD" method of
"IO::WrapTie::Master" is invoked, and the standard behavior of
delegating the method to the underlying slave (here, an IO::Scalar) is
done.
Sometimes, to get this to work properly, you may need to create a
subclass of "IO::WrapTie::Master" which is an effective master for your
class, and do the delegation there.
NOTES
Why not simply use the object's OO interface?
Because that means forsaking the use of named operators like "print",
and you may need to pass the object to a subroutine which will attempt
to use those operators:
$O = FooHandle->new(&FOO_RDWR, 2);
$O->print("Hello, world\n"); ### OO syntax is okay, BUT....
sub nope { print $_[0] "Nope!\n" }
X nope($O); ### ERROR!!! (not a glob ref)
Why not simply use tie()?
Because (1) you have to use "tied" to invoke methods in the object's
public interface (yuck), and (2) you may need to pass the tied symbol to
another subroutine which will attempt to treat it in an OO-way... and
that will break it:
tie *T, 'FooHandle', &FOO_RDWR, 2;
print T "Hello, world\n"; ### Operator is okay, BUT...
tied(*T)->other_stuff; ### yuck! AND...
sub nope { shift->print("Nope!\n") }
X nope(\*T); ### ERROR!!! (method "print" on unblessed ref)
Why a master and slave?
Why not simply write C<FooHandle> to inherit from L<IO::Handle?>
I tried this, with an implementation similar to that of L<IO::Socket>.
The problem is that I<the whole point is to use this with objects
that don't have an underlying file/socket descriptor.>.
Subclassing L<IO::Handle> will work fine for the OO stuff, and fine with
named operators I<if> you C<tie>... but if you just attempt to say:
$IO = FooHandle->new(&FOO_RDWR, 2);
print $IO "Hello!\n";
you get a warning from Perl like:
Filehandle GEN001 never opened
because it's trying to do system-level I/O on an (unopened) file
descriptor. To avoid this, you apparently have to "tie" the handle...
which brings us right back to where we started! At least the
IO::WrapTie mixin lets us say:
$IO = FooHandle->new_tie(&FOO_RDWR, 2);
print $IO "Hello!\n";
and so is not too bad. ":-)"
WARNINGS
Remember: this stuff is for doing FileHandle-like I/O on things without
underlying file descriptors. If you have an underlying file descriptor,
you're better off just inheriting from IO::Handle.
Be aware that new_tie() always returns an instance of a kind of
IO::WrapTie::Master... it does not return an instance of the I/O class
you're tying to!
Invoking some methods on the master object causes "AUTOLOAD" to delegate
them to the slave object... so it looks like you're manipulating a
"FooHandle" object directly, but you're not.
I have not explored all the ramifications of this use of "tie". Here
there be dragons.
AUTHOR
Eryq (eryq@zeegee.com). President, ZeeGee Software Inc
(http://www.zeegee.com).
CONTRIBUTORS
Dianne Skoll (dfs@roaringpenguin.com).
COPYRIGHT & LICENSE
Copyright (c) 1997 Erik (Eryq) Dorfman, ZeeGee Software, Inc. All rights
reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.38.2 2024-07-31 IO::WrapTie(3pm)
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