1Test2::Tools::Refcount(U3s)er Contributed Perl DocumentatTieosnt2::Tools::Refcount(3)
2
6 "Test2::Tools::Refcount" - assert reference counts on objects
7
9 use Test2::Tools::Refcount;
10 use Some::Class;
12 my $object = Some::Class->new();
13 is_oneref( $object, '$object has a refcount of 1' );
15 my $otherref = $object;
17 is_refcount( $object, 2, '$object now has 2 references' );
19
21 The Perl garbage collector uses simple reference counting during the
22 normal execution of a program. This means that cycles or unweakened
23 references in other parts of code can keep an object around for longer
24 than intended. To help avoid this problem, the reference count of a new
25 object from its class constructor ought to be 1. This way, the caller
26 can know the object will be properly DESTROYed when it drops all of its
27 references to it.
28 This module provides two test functions to help ensure this property
30 holds for an object class, so as to be polite to its callers.
31 If the assertion fails; that is, if the actual reference count is
33 different to what was expected, either of the following two modules may
34 be used to assist the developer in finding where the references are.
35 • If Devel::MAT is installed, this test module will use it to dump
37 the state of the memory after a failure. It will create a .pmat
38 file named the same as the unit test, but with the trailing .t
39 suffix replaced with -TEST.pmat where "TEST" is the number of the
40 test that failed (in case there was more than one).
41 See the examples below for more information.
43
45 is_refcount
46 is_refcount( $object, $count, $name )
47 Test that $object has $count references to it.
49 is_oneref
51 is_oneref( $object, $name )
52 Assert that the $object has only 1 reference to it.
54 refcount
56 $count = refcount( $object )
57 Returns the reference count of the given object as used by the test
59 functions. This is useful for making tests that don't care what the
60 count is before they start, but simply assert that the count hasn't
61 changed by the end.
62 use Test2::Tools::Refcount import => [qw( is_refcount refcount )];
64 {
65 my $count = refcount( $object );
66 do_something( $object );
68 is_refcount( $object, $count, 'do_something() preserves refcount' );
70 }
71
73 Suppose, having written a new class "MyBall", you now want to check
74 that its constructor and methods are well-behaved, and don't leak
75 references. Consider the following test script:
76 use Test::More tests => 2;
78 use Test2::Tools::Refcount;
79 use MyBall;
81 my $ball = MyBall->new();
83 is_oneref( $ball, 'One reference after construct' );
84 $ball->bounce;
86 # Any other code here that might be part of the test script
88 is_oneref( $ball, 'One reference just before EOF' );
90 The first assertion is just after the constructor, to check that the
92 reference returned by it is the only reference to that object. This
93 fact is important if we ever want "DESTROY" to behave properly. The
94 second call is right at the end of the file, just before the main scope
95 closes. At this stage we expect the reference count also to be one, so
96 that the object is properly cleaned up.
97 Suppose, when run, this produces the following output (presuming
99 Devel::MAT::Dumper is available):
100 1..2
102 ok 1 - One reference after construct
103 not ok 2 - One reference just before EOF
104 # Failed test 'One reference just before EOF'
105 # at ex.pl line 26.
106 # expected 1 references, found 2
107 # SV address is 0x55e14c310278
108 # Writing heap dump to ex-2.pmat
109 # Looks like you failed 1 test of 2.
110 This has written a ex-2.pmat file we can load using the "pmat" shell
112 and use the "identify" command on the given address to find where it
113 went:
114 $ pmat ex-2.pmat
116 Perl memory dumpfile from perl 5.28.1 threaded
117 Heap contains 25233 objects
118 pmat> identify 0x55e14c310278
119 HASH(0)=MyBall at 0x55e14c310278 is:
120 ├─(via RV) the lexical $ball at depth 1 of CODE() at 0x55e14c3104a0=main_cv, which is:
121 │ └─the main code
122 └─(via RV) value {self} of HASH(2) at 0x55e14cacb860, which is (*A):
123 └─(via RV) value {cycle} of HASH(2) at 0x55e14cacb860, which is:
124 itself
125 (This document isn't intended to be a full tutorial on Devel::MAT and
127 the "pmat" shell; for that see Devel::MAT::UserGuide).
128 From this output, we can see that the constructor was well-behaved, but
130 that a reference was leaked by the end of the script - the reference
131 count was 2, when we expected just 1. Reading the trace output, we can
132 see that there were 2 references that could be found - one stored in
133 the $ball lexical in the main program, and one stored in a HASH. Since
134 we expected to find the $ball lexical variable, we know we are now
135 looking for a leak in a hash somewhere in the code. From reading the
136 test script, we can guess this leak is likely to be in the bounce()
137 method. Furthermore, we know that the reference to the object will be
138 stored in a HASH in a member called "self".
139 By reading the code which implements the bounce() method, we can see
141 this is indeed the case:
142 sub bounce
144 {
145 my $self = shift;
146 my $cycle = { self => $self };
147 $cycle->{cycle} = $cycle;
148 }
149 From reading the tracing output, we find that the HASH this object is
151 referenced in also contains a reference to itself, in a member called
152 "cycle". This comes from the last line in this function, a line that
153 purposely created a cycle, to demonstrate the point. While a real
154 program probably wouldn't do anything quite this obvious, the trace
155 would still be useful in finding the likely cause of the leak.
156 If "Devel::MAT::Dumper" is not available, then these detailed traces
158 will not be produced. The basic reference count testing will still take
159 place, but a smaller message will be produced:
160 1..2
162 ok 1 - One reference after construct
163 not ok 2 - One reference just before EOF
164 # Failed test 'One reference just before EOF'
165 # at demo.pl line 16.
166 # expected 1 references, found 2
167 # Looks like you failed 1 test of 2.
168
170 • Temporaries created on the stack
171 Code which creates temporaries on the stack, to be released again
173 when the called function returns does not work correctly on perl
174 5.8 (and probably before). Examples such as
175 is_oneref( [] );
177 may fail and claim a reference count of 2 instead.
179 Passing a variable such as
181 my $array = [];
183 is_oneref( $array );
184 works fine. Because of the intention of this test module; that is,
186 to assert reference counts on some object stored in a variable
187 during the lifetime of the test script, this is unlikely to cause
188 any problems.
189
191 Peter Rabbitson <ribasushi@cpan.org> - for suggesting using core's "B"
192 instead of "Devel::Refcount" to obtain refcounts
193
195 Paul Evans <leonerd@leonerd.org.uk>
196perl v5.36.0 2023-03-23 Test2::Tools::Refcount(3)