select and IO::Handle

[John M. Dlugosz]

I was suprised to find the following

my $string;
my $SH= new IO::Scalar;
$SH->open (\$string);

select ($SH);  ## here

choked, complaing that $SH was not a reference to a glob.

Then I also found that $SH->select isn't a member, either.

Why doesn't this work? I thought IO::Handle objects were glob refs, which is why you can use print {$handle} "hello world".

Looking at IO::Scalar.pm, it seems that it's a blessed hash. It doesn't inherit from IO::Handle, even though the docs say "This class implements objects which behave just like FileHandle (or IO::Handle) objects..." is that incorrect?

Just how do you select one of these?

—John

[danger]

I found that using either of the tied interfaces worked as you expect (both documented in the POD's for Perl-5.004 or later):

#!/usr/bin/perl -w
use strict;
use IO::Scalar;

my $string;
tie *OUT, 'IO::Scalar', \$string;

select OUT;
print "line 1\nline 2\n", "line 3\n";

select STDOUT;
print "The string contains:\n$string\n";

### OR this way:

$string = '';
my $SH = IO::Scalar->new_tie(\$string);
$SH->print("line 1\n");  # use the OO method

print $SH "line 2\n";    # use it like a filehandle

select $SH;              # select it as default output handle
print "line 3\n";

select STDOUT;
print "The string contains:\n$string"
__END__

[runrig]

IO::Scalar does not return an IO::Handle, it returns a blessed hash reference. The author said he tried to subclass IO::Handle, but ran into 'xxx on unopened filehandle' warnings. You have to use methods on an IO::Scalar object, unless you are using the tied interface, but then you would be using a tied filehandle/glob.

I'm actually working on a patch for IO::Scalar so that it will return a blessed glob reference with a tied glob so that it will behave more like a real filehandle, and things that return IO::Scalar objects can be passed to things which expect filehandles. If there's a better fix, I'd be glad to hear of it :-)

Update: IO::Scalar now returns a blessed glob reference and so the returned object behaves more like a real filehandle. However, modern perls let you open a filehandle directly to a scalar, which makes these sorts of modules obsolete.

[John M. Dlugosz]

I was just talking to the author, too.

A better fix: Only implement actual methods for the TIE. Also have a function to create one and return an IO::Handle to it as one call.

While you're at it, note that WRITE method is missing.

—John