Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
IO::WrapTie(3)	      User Contributed Perl Documentation	IO::WrapTie(3)

       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 "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.

       Suppose you have	a class	"FooHandle", where...

       o   "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).

       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

       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.

   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 $WT directly.
		      |	    ,---isa--> IO::WrapTie::Master >--isa--> IO::Handle
		      V	   /
	       |	     |
	       |	     |	 * Perl	i/o operators work on the tied object,
	       |  "Master"   |	   invoking the	C<TIEHANDLE> methods.
	       |	     |	 * Method invocations are delegated to the tied
	       |	     |	   slave.
	   tied(*$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

   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

       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	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.  ":-)"

       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.

       Eryq (	President, ZeeGee Software Inc

       Dianne Skoll (

       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 v5.32.1			  2020-01-17			IO::WrapTie(3)


Want to link to this manual page? Use this URL:

home | help