1Devel::Refcount(3)    User Contributed Perl Documentation   Devel::Refcount(3)
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4

NAME

6       "Devel::Refcount" - obtain the REFCNT value of a referent
7

SYNOPSIS

9        use Devel::Refcount qw( refcount );
10
11        my $anon = [];
12
13        print "Anon ARRAY $anon has " . refcount( $anon ) . " reference\n";
14
15        my $otherref = $anon;
16
17        print "Anon ARRAY $anon now has " . refcount( $anon ) . " references\n";
18
19        assert_oneref $otherref; # This will throw an exception at runtime
20

DESCRIPTION

22       This module provides a single function which obtains the reference
23       count of the object being pointed to by the passed reference value. It
24       also provides a debugging assertion that asserts a given reference has
25       a count of only 1.
26

FUNCTIONS

28   $count = refcount( $ref )
29       Returns the reference count of the object being pointed to by $ref.
30
31   assert_oneref( $ref )
32       Asserts that the given object reference has a reference count of only
33       1. If this is true the function does nothing. If it has more than 1
34       reference then an exception is thrown. Additionally, if Devel::FindRef
35       is available, it will be used to print a more detailed trace of where
36       the references are found.
37
38       Typically this would be useful in debugging to track down cases where
39       objects are still being referenced beyond the point at which they are
40       supposed to be dropped. For example, if an element is delete from a
41       hash that ought to be the last remaining reference, the return value of
42       the "delete" operator can be asserted on
43
44        assert_oneref delete $self->{some_item};
45
46       If at the time of deleting there are any other references to this
47       object then the assertion will fail; and if "Devel::FindRef" is
48       available the other locations will be printed.
49

COMPARISON WITH SvREFCNT

51       This function differs from "Devel::Peek::SvREFCNT" in that SvREFCNT()
52       gives the reference count of the SV object itself that it is passed,
53       whereas refcount() gives the count of the object being pointed to. This
54       allows it to give the count of any referent (i.e. ARRAY, HASH, CODE,
55       GLOB and Regexp types) as well.
56
57       Consider the following example program:
58
59        use Devel::Peek qw( SvREFCNT );
60        use Devel::Refcount qw( refcount );
61
62        sub printcount
63        {
64           my $name = shift;
65
66           printf "%30s has SvREFCNT=%d, refcount=%d\n",
67              $name, SvREFCNT( $_[0] ), refcount( $_[0] );
68        }
69
70        my $var = [];
71
72        printcount 'Initially, $var', $var;
73
74        my $othervar = $var;
75
76        printcount 'Before CODE ref, $var', $var;
77        printcount '$othervar', $othervar;
78
79        my $code = sub { undef $var };
80
81        printcount 'After CODE ref, $var', $var;
82        printcount '$othervar', $othervar;
83
84       This produces the output
85
86                       Initially, $var has SvREFCNT=1, refcount=1
87                 Before CODE ref, $var has SvREFCNT=1, refcount=2
88                             $othervar has SvREFCNT=1, refcount=2
89                  After CODE ref, $var has SvREFCNT=2, refcount=2
90                             $othervar has SvREFCNT=1, refcount=2
91
92       Here, we see that SvREFCNT() counts the number of references to the SV
93       object passed in as the scalar value - the $var or $othervar
94       respectively, whereas refcount() counts the number of reference values
95       that point to the referent object - the anonymous ARRAY in this case.
96
97       Before the CODE reference is constructed, both $var and $othervar have
98       SvREFCNT() of 1, as they exist only in the current lexical pad. The
99       anonymous ARRAY has a refcount() of 2, because both $var and $othervar
100       store a reference to it.
101
102       After the CODE reference is constructed, the $var variable now has an
103       SvREFCNT() of 2, because it also appears in the lexical pad for the new
104       anonymous CODE block.
105

PURE-PERL FALLBACK

107       An XS implementation of this function is provided, and is used by
108       default. If the XS library cannot be loaded, a fallback implementation
109       in pure perl using the "B" module is used instead. This will behave
110       identically, but is much slower.
111
112               Rate   pp   xs
113        pp 225985/s   -- -66%
114        xs 669570/s 196%   --
115

SEE ALSO

117       •   Test::Refcount - assert reference counts on objects
118

AUTHOR

120       Paul Evans <leonerd@leonerd.org.uk>
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124perl v5.34.0                      2022-01-21                Devel::Refcount(3)
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