This is currently Alpha code, released for comments.
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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.
Normally, users of your class would have two options:
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.
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.
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.
$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.
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";
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.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.