Path: utzoo!mnetor!tmsoft!torsqnt!jarvis.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Passing scalars by reference
Message-ID: <7293@jpl-devvax.JPL.NASA.GOV>
Date: 6 Mar 90 18:23:48 GMT
References: <1220@frankland-river.aaii.oz.au>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 44

In article <1220@frankland-river.aaii.oz.au> pem@frankland-river.aaii.oz.au (Paul E. Maisano) writes:
: Could someone please tell me how I can pass scalars by reference in the
: same way arrays are. I know I can refer to $_[nnn] like its says in the
: manual, but I would like to do something like:
: 
: sub foo {
:     local($x_by_ref_with_meaningful_name) = $_[0];
:     ...
:     $x_by_ref_with_meaningful_name .= $foo;	# update the original scalar
: }
: 
: I tried this and it seemed to work on the first call only. Thereafter
: it updated a global variable called $x_by_ref_with_meaningful_name.

That won't do what you want, because local($scalar) = @_ will always copy the
value--it's how you turn call-by-reference into call-by-value in Perl.

There are several ways for you to reference the original scalar:

	1) Give up on giving a meaningful name and refer to $_[0].

	2) Invoke the C preprocessor and say

		#define meaningful_name _[0]

	3) Use the type glob *name to pass in the symbol name:

		sub foo {
		    local(*meaningful_name) = $_[0];
		    $meaningful_name .= pop(@meaningful_name);
		}

		&foo(*original_name);

This last method is the most powerful, because you can pass anything that has
a name in the symbol table, including filehandles, subroutines and formats.
What you're really doing is passing in the symbol table entry for everything
with that name and giving it a new name locally.

Note that mechanism 1 can only give you references into an array's elements,
while mechanism 3 can give you the array as a whole, so you can push, pop,
shift, etc. on it.

Larry