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7.6. Writing a Subroutine That Takes Filehandles as Built-ins Do
7.6.1. Problem
You can pass a bareword filehandle to Perl functions like
eof, and you''d like to write similar subroutines
of your own.
7.6.2. Solution
Use the
Symbol module''s qualify_to_ref in conjunction with
a * prototype on the subroutine:
use Symbol qw(qualify_to_ref);
sub my_eof (*) {
my $handle = shift;
$handle = qualify_to_ref($handle, caller( ));
# use $handle
}
7.6.3. Discussion
The * prototype tells Perl that the function
expects a bareword filehandle as its argument. This lets you call the
function like so:
my_eof(HANDLE);
This works even when use strict ''subs'' is in
effect. The function receives a string as its argument, though. To
safely use the argument as a filehandle, you need the Symbol module
to turn it into a reference to a typeglob. And since typeglob refs
can be used wherever you''d use named filehandles, store that
reference in a scalar variable and use the variable as an indirect
filehandle within your subroutine.If you pass in a filehandle that is already a reference to a
typeglob, like those autovivified by open, Perl
and qualify_to_ref still do the right thing:
open(my $fh, "<", $filename) or die;
my_eof($fh);
This technique is used in Recipe 7.23.
7.6.4. See Also
The documentation for the standard module Symbol (also in Chapter 32
of Programming Perl); the "Prototypes" section
in the perlsub(1) manpage (also in Chapter 6 of
Programming Perl); Recipe 7.23
