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IO::WrapTie(3pm)      User Contributed Perl Documentation     IO::WrapTie(3pm)

       IO::WrapTie - wrap tieable objects in IO::Handle interface

       This is currently Alpha code, released for comments.
         Please give me your feedback!

       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.

       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

       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

       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.

       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, ";

       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::WrapTie::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

       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

       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
       subclass of IO::WrapTie::Master which is an effective master for your
       class, and do the delegation there.

       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 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::Handle 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::WrapTie mixin lets us say:

           $IO = FooHandle->new_tie(&FOO_RDWR, 2);
           print $IO "Hello!\n";

       and so is not too bad.  ":-)"

       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.

       $Id: WrapTie.pm,v 1.2 2005/02/10 21:21:53 dfs Exp $

       Primary Maintainer
           David F. Skoll (dfs@roaringpenguin.com).

       Original Author
           Eryq (eryq@zeegee.com).  President, ZeeGee Software Inc

       Hey! The above document had some coding errors, which are explained

       Around line 481:
           '=item' outside of any '=over'

perl v5.10.0                      2005-02-10                  IO::WrapTie(3pm)