Indirect Referencing

[periapt]

OK, I'm stumped. I'm trying to read from a file handle in a while loop. The reference to the file handle is in a hash which itself begins with a hash reference like this ...

my $cfg = {};
open (INFILE,"<testfile.txt") || die "Could not open file\n$!\n";
$$cfg{infile} = \*INFILE;

while(<$$cfg{infile}>){
    print "$_\n";
}
__END__
GLOB(0x20eab18)

However, when I do it this way, it works as expected.

my $cfg = {};
open (INFILE,"<testfile.txt") || die "Could not open file\n$!\n";
$$cfg{infile} = \*INFILE;
my $infh = $$cfg{infile};

while(<$infh}>){
    print "$_\n";
}

I figure that the angle operator is not dereferencing $$cfg{infile} as I want but I cannot come up with a combinations of parens, curly braces and arrows that will make this work. What am I missing?

Update: I knew I was missing the obvious. Thanks Roy. :o)

[Roy Johnson]

From perldoc perlop:

If what's within the angle brackets is neither a filehandle
nor a simple scalar variable containing a filehandle name,
typeglob, or typeglob reference, it is interpreted as a
filename pattern to be globbed, and either a list of
filenames or the next filename in the list is returned,
depending on context. This distinction is determined on
syntactic grounds alone. That means <$x> is always a
readline() from an indirect handle, but <$hash{key}> is
always a glob(). That's because $x is a simple scalar
variable, but $hash{key} is not--it's a hash element.

You can try the readline function rather than the <> operator. It is not clear to me whether you get all the default behaviors when using readline.

[periapt]

Darn, I missed that. I'm not hung up on using the hash element, I just got running down the wrong track. Thanks.

[Aragorn]

Update: Argh! Nevermind, I misunderstood the question.

If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a filehandle name, typeglob, or typeglob reference,

$infh is a typeglob reference, so it should work. Actually, it does, if the while(<$infh}>){ is replaced by while(<$infh>){.

Small change, big results ;-)

Arjen

[demerphq]

Can I just say that you code just screams for a rewrite?

my $cfg = {};
open my $INFILE,"<","testfile.txt" or die "Could not open file:$!";
$cfg->{infile} = $INFILE;
my $infh = $cfg->{infile};

while(<$infh>){
    print "$_\n";
}

Sorry, but if i ever worked on your code before i did anything else i would make conversions like that $$hash{key} is an evil meme that is confusing (it doesnt scale up) easy to misunderstand (what the hell is $$hash {key}, a dereferenced ref followed by a block containing a sub call, a syntax error or a hashref dereference and lookup?) $hash->{key} is hard to mistake for anything but what it is.

[periapt]

Hmmm, I like the open my $INFILE ... construct. I'll have to remember it since I use the

open INFILE,"<","testfile.txt" or die "Could not open file:$!";
$fh = \*INFILE;

code a lot.

On a personal note, I have never felt comfortable with the arrow op. It may just be lack of use although I have been using it more lately. I do agree with you about scalability. I generally end up using the arrow op on more complex structures - although I do have a $$$cfg{infile} line in this same program ;o) - Still, it does look cleaner here...

Thanks for the suggestions.