#  You may distribute under the terms of either the GNU General Public License
#  or the Artistic License (the same terms as Perl itself)
#
#  (C) Paul Evans, 2008-2013 -- leonerd@leonerd.org.uk

package Devel::Refcount;

use strict;
use warnings;

our $VERSION = '0.10';

use Exporter 'import';
our @EXPORT_OK = qw( refcount assert_oneref );

require XSLoader;
if( !eval { XSLoader::load( __PACKAGE__, $VERSION ) } ) {
   *refcount = \&_refcount_pp;
   require B;
}

use Carp;
use Scalar::Util qw( weaken );

=head1 NAME

C<Devel::Refcount> - obtain the REFCNT value of a referent

=head1 SYNOPSIS

 use Devel::Refcount qw( refcount );

 my $anon = [];

 print "Anon ARRAY $anon has " . refcount( $anon ) . " reference\n";

 my $otherref = $anon;

 print "Anon ARRAY $anon now has " . refcount( $anon ) . " references\n";

 assert_oneref $otherref; # This will throw an exception at runtime

=head1 DESCRIPTION

This module provides a single function which obtains the reference count of
the object being pointed to by the passed reference value. It also provides a
debugging assertion that asserts a given reference has a count of only 1.

=cut

=head1 FUNCTIONS

=cut

=head2 $count = refcount( $ref )

Returns the reference count of the object being pointed to by $ref.

=cut

# This normally isn't used if the XS code is loaded
sub _refcount_pp
{
   B::svref_2object( shift )->REFCNT;
}

=head2 assert_oneref( $ref )

Asserts that the given object reference has a reference count of only 1. If
this is true the function does nothing. If it has more than 1 reference then
an exception is thrown. Additionally, if L<Devel::FindRef> is available, it
will be used to print a more detailed trace of where the references are found.

Typically this would be useful in debugging to track down cases where objects
are still being referenced beyond the point at which they are supposed to be
dropped. For example, if an element is delete from a hash that ought to be the
last remaining reference, the return value of the C<delete> operator can be
asserted on

 assert_oneref delete $self->{some_item};

If at the time of deleting there are any other references to this object then
the assertion will fail; and if C<Devel::FindRef> is available the other
locations will be printed.

=cut

sub assert_oneref
{
   my $object = shift;
   weaken $object;

   my $refcount = refcount( $object );
   return if $refcount == 1;

   my $message = Carp::shortmess( "Expected $object to have only one reference, found $refcount" );

   if( eval { require Devel::FindRef } ) {
      my $track = Devel::FindRef::track( $object );
      die "$message\n$track\n";
   }
   else {
      die $message;
   }
}

=head1 COMPARISON WITH SvREFCNT

This function differs from C<Devel::Peek::SvREFCNT> in that SvREFCNT() gives
the reference count of the SV object itself that it is passed, whereas
refcount() gives the count of the object being pointed to. This allows it to
give the count of any referent (i.e. ARRAY, HASH, CODE, GLOB and Regexp types)
as well.

Consider the following example program:

 use Devel::Peek qw( SvREFCNT );
 use Devel::Refcount qw( refcount );

 sub printcount
 {
    my $name = shift;

    printf "%30s has SvREFCNT=%d, refcount=%d\n",
       $name, SvREFCNT( $_[0] ), refcount( $_[0] );
 }

 my $var = [];

 printcount 'Initially, $var', $var;

 my $othervar = $var;

 printcount 'Before CODE ref, $var', $var;
 printcount '$othervar', $othervar;

 my $code = sub { undef $var };

 printcount 'After CODE ref, $var', $var;
 printcount '$othervar', $othervar;

This produces the output

                Initially, $var has SvREFCNT=1, refcount=1
          Before CODE ref, $var has SvREFCNT=1, refcount=2
                      $othervar has SvREFCNT=1, refcount=2
           After CODE ref, $var has SvREFCNT=2, refcount=2
                      $othervar has SvREFCNT=1, refcount=2

Here, we see that SvREFCNT() counts the number of references to the SV object
passed in as the scalar value - the $var or $othervar respectively, whereas
refcount() counts the number of reference values that point to the referent
object - the anonymous ARRAY in this case.

Before the CODE reference is constructed, both $var and $othervar have
SvREFCNT() of 1, as they exist only in the current lexical pad. The anonymous
ARRAY has a refcount() of 2, because both $var and $othervar store a reference
to it.

After the CODE reference is constructed, the $var variable now has an
SvREFCNT() of 2, because it also appears in the lexical pad for the new
anonymous CODE block.

=cut

=head1 PURE-PERL FALLBACK

An XS implementation of this function is provided, and is used by default. If
the XS library cannot be loaded, a fallback implementation in pure perl using
the C<B> module is used instead. This will behave identically, but is much
slower.

        Rate   pp   xs
 pp 225985/s   -- -66%
 xs 669570/s 196%   --

=head1 SEE ALSO

=over 4

=item *

L<Test::Refcount> - assert reference counts on objects

=back

=head1 AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

=cut

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