WrapTie(3)
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 IO::WrapTie::Slave
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...
o FooHandle does not inherit from IO::Handle; that is, it performs
filehandle-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).
o FooHandle implements the TIEHANDLE interface (see perltie); that
is, it provides methods TIEHANDLE, GETC, PRINT, PRINTF, READ, and
READLINE.
o 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:
o Use only OO syntax, and forsake named I/O operators like 'print'.
o 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 person-
ality").
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 filehandle 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 $WT directly.
|
|
| ,---isa--> IO::WrapTie::Master >--isa--> IO::Handle
V /
.-------------.
| |
| | * Perl i/o operators work on the tied object,
| "Master" | invoking the TIEHANDLE methods.
| | * Method invocations are delegated to the tied
| | slave.
`-------------'
|
tied(*$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 TIEHANDLE interface.
`-------------'
NOTE: just as an IO::Handle is really just a blessed reference to a
traditional filehandle glob... so also, an IO::WrapTie::Master is
really just a blessed reference to a filehandle 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::Wrap-
Tie::Master is a subclass of IO::Handle.
2. The "IO::WrapTie::Master::new" method creates a new IO::Handle
object, reblessed 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(HANDLE, SLAVECLASS, TIEARGS...)". This call to
"tie()" creates the slave in the following manner:
4. Class SLAVECLASS is sent the message "TIEHANDLE(TIEARGS...)"; 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 IO::Scalar::PRINT 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
IO::Scalar::print().
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 superclass,
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 sub-
class 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 ttiiee(())?
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 FooHandle to inherit from IO::Handle?
I tried this, with an implementation similar to that of IO::Socket.
The problem is that the whole point is to use this with objects that
don't have an underlying file/socket descriptor.. Subclassing IO::Han-
dle will work fine for the OO stuff, and fine with named operators if
you 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::Wrap-
Tie 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 descrip-
tor, you're better off just inheriting from IO::Handle.
Be aware that nneeww_ttiiee(()) 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.
VERSION
$Id: WrapTie.pm,v 2.102 2001/08/17 02:06:33 eryq Exp $
AUTHOR
Eryq (eryq@zeegee.com). President, ZeeGee Software Inc
(http://www.zeegee.com).
perl v5.8.6 2001-08-09 IO::WrapTie(3)
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