From: M.J.T. Guy <mjtg@cus.cam.ac.uk>
Date: Mon, 24 Mar 1997 07:25:21 +0000 (+0000)
Subject: pods for subroutine argument autovivication
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=db8878faa51a8a1541a40745a8613adb5db155e4;p=p5sagit%2Fp5-mst-13.2.git

pods for subroutine argument autovivication

Here are pod updates for subroutine argument autovivication.
I'll also try to do some additions to the tests.

One small detail noted:

       perl -we 'sub f { print @_ }; f $a[0]'

produces

       Use of uninitialized value at -e line 1.

under Perl 5.003 but is silent under Perl5.003_94.    Do we care?

p5p-msgid: E0w9489-0005YT-00@ursa.cus.cam.ac.uk
---

diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index 958bee3..1e32990 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -27,17 +27,23 @@ might have symbol conflicts if you embed Perl in another application,
 just as in the 5.003 release.  By default, binary compatibility
 is preserved at the expense of symbol table pollution.
 
-=head2 No Autovivification of Subroutine Parameters
-
-In Perl versions 5.002 and 5.003, array and hash elements used as
-subroutine parameters were "autovivified"; that is, they were brought
-into existence if they did not already exist.  For example, calling
-C<func($h{foo})> would create C<$h{foo}> if it did not already exist,
-causing C<exists $h{foo}> to become true and C<keys %h> to return
-C<('foo')>.
-
-Perl 5.004 returns to the pre-5.002 behavior of I<not> autovivifying
-array and hash elements used as subroutine parameters.
+=head2 Creation of Subroutine argumnets only when necessary
+
+In Perl 5.004, array and hash elements used as subroutine parameters
+are brought into existence if they did not already exist only if the
+argument is actually assigned to.  In Perl versions 5.002 and 5.003,
+such arguments were always brought into existence.  Versions before
+5.002 never brought such arguments into existence.
+For example, after
+
+     undef @a; undef %a;
+     sub foo { print $_[0] };
+     sub bar { $_[0]++ };
+     foo($a[2]);
+     bar($a{b});
+
+$a{b} exists but $a[2] does not in Perl5.004; in Perl 5.002 and 5.003
+both would exist.
 
 =head2 Fixed Parsing of $$<digit>, &$<digit>, etc.
 
diff --git a/pod/perlsub.pod b/pod/perlsub.pod
index f2a5b8f..2710774 100644
--- a/pod/perlsub.pod
+++ b/pod/perlsub.pod
@@ -47,9 +47,17 @@ there's really no difference from the language's perspective.)
 
 Any arguments passed to the routine come in as the array @_.  Thus if you
 called a function with two arguments, those would be stored in C<$_[0]>
-and C<$_[1]>.  The array @_ is a local array, but its values are implicit
-references (predating L<perlref>) to the actual scalar parameters.  The
-return value of the subroutine is the value of the last expression
+and C<$_[1]>.  The array @_ is a local array, but its elements are
+aliases for the actual scalar parameters.  In particular, if an element 
+C<$_[0]> is updated, the corresponding argument is updated (or an error
+occurs if it is not updatable).   If an argument is an array or
+hash element which does not exist, the element is created if it is
+assigned to or if a reference to it is taken.   (Some earlier versions
+of Perl created the element whether or not it was assigned to.)
+Note that assigning to the whole array @_ removes the aliasing,
+and does not update any arguments.
+
+The return value of the subroutine is the value of the last expression
 evaluated.  Alternatively, a return statement may be used to specify the
 returned value and exit the subroutine.  If you return one or more arrays
 and/or hashes, these will be flattened together into one large