IO::WrapTie

Section: User Contributed Perl Documentation (3)
Updated: 2020-01-30
Page Index
 

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.


 

Index

NAME
SYNOPSIS
DESCRIPTION
HOW IT ALL WORKS
The data structures
How wraptie works
How I/O operators work (on the master)
How methods work (on the master)
NOTES
WARNINGS
AUTHOR
CONTRIBUTORS
COPYRIGHT & LICENSE
LinuxReviews : manual page archive : man3pm