Provided by: libtest-refcount-perl_0.08-3_all 
NAME
"Test::Refcount" - assert reference counts on objects
SYNOPSIS
use Test::More tests => 2;
use Test::Refcount;
use Some::Class;
my $object = Some::Class->new();
is_oneref( $object, '$object has a refcount of 1' );
my $otherref = $object;
is_refcount( $object, 2, '$object now has 2 references' );
DESCRIPTION
The Perl garbage collector uses simple reference counting during the normal execution of a program. This
means that cycles or unweakened references in other parts of code can keep an object around for longer
than intended. To help avoid this problem, the reference count of a new object from its class constructor
ought to be 1. This way, the caller can know the object will be properly DESTROYed when it drops all of
its references to it.
This module provides two test functions to help ensure this property holds for an object class, so as to
be polite to its callers.
If the assertion fails; that is, if the actual reference count is different to what was expected, either
of the following two modules may be used to assist the developer in finding where the references are.
• If Devel::FindRef module is installed, a reverse-references trace is printed to the test output.
• If Devel::MAT is installed, this test module will use it to dump the state of the memory after a
failure. It will create a .pmat file named the same as the unit test, but with the trailing .t suffix
replaced with -TEST.pmat where "TEST" is the number of the test that failed (in case there was more
than one).
See the examples below for more information.
FUNCTIONS
is_refcount( $object, $count, $name )
Test that $object has $count references to it.
is_oneref( $object, $name )
Assert that the $object has only 1 reference to it.
EXAMPLE
Suppose, having written a new class "MyBall", you now want to check that its constructor and methods are
well-behaved, and don't leak references. Consider the following test script:
use Test::More tests => 2;
use Test::Refcount;
use MyBall;
my $ball = MyBall->new();
is_oneref( $ball, 'One reference after construct' );
$ball->bounce;
# Any other code here that might be part of the test script
is_oneref( $ball, 'One reference just before EOF' );
The first assertion is just after the constructor, to check that the reference returned by it is the only
reference to that object. This fact is important if we ever want "DESTROY" to behave properly. The second
call is right at the end of the file, just before the main scope closes. At this stage we expect the
reference count also to be one, so that the object is properly cleaned up.
Suppose, when run, this produces the following output (presuming "Devel::FindRef" is available):
1..2
ok 1 - One reference after construct
not ok 2 - One reference just before EOF
# Failed test 'One reference just before EOF'
# at demo.pl line 16.
# expected 1 references, found 2
# MyBall=ARRAY(0x817f880) is
# +- referenced by REF(0x82c1fd8), which is
# | in the member 'self' of HASH(0x82c1f68), which is
# | referenced by REF(0x81989d0), which is
# | in the member 'cycle' of HASH(0x82c1f68), which was seen before.
# +- referenced by REF(0x82811d0), which is
# in the lexical '$ball' in CODE(0x817fa00), which is
# the main body of the program.
# Looks like you failed 1 test of 2.
From this output, we can see that the constructor was well-behaved, but that a reference was leaked by
the end of the script - the reference count was 2, when we expected just 1. Reading the trace output, we
can see that there were 2 references that "Devel::FindRef" could find - one stored in the $ball lexical
in the main program, and one stored in a HASH. Since we expected to find the $ball lexical variable, we
know we are now looking for a leak in a hash somewhere in the code. From reading the test script, we can
guess this leak is likely to be in the bounce() method. Furthermore, we know that the reference to the
object will be stored in a HASH in a member called "self".
By reading the code which implements the bounce() method, we can see this is indeed the case:
sub bounce
{
my $self = shift;
my $cycle = { self => $self };
$cycle->{cycle} = $cycle;
}
From reading the "Devel::FindRef" output, we find that the HASH this object is referenced in also
contains a reference to itself, in a member called "cycle". This comes from the last line in this
function, a line that purposely created a cycle, to demonstrate the point. While a real program probably
wouldn't do anything quite this obvious, the trace would still be useful in finding the likely cause of
the leak.
If "Devel::FindRef" is unavailable, then these detailed traces will not be produced. The basic reference
count testing will still take place, but a smaller message will be produced:
1..2
ok 1 - One reference after construct
not ok 2 - One reference just before EOF
# Failed test 'One reference just before EOF'
# at demo.pl line 16.
# expected 1 references, found 2
# Looks like you failed 1 test of 2.
BUGS
• Temporaries created on the stack
Code which creates temporaries on the stack, to be released again when the called function returns
does not work correctly on perl 5.8 (and probably before). Examples such as
is_oneref( [] );
may fail and claim a reference count of 2 instead.
Passing a variable such as
my $array = [];
is_oneref( $array );
works fine. Because of the intention of this test module; that is, to assert reference counts on some
object stored in a variable during the lifetime of the test script, this is unlikely to cause any
problems.
ACKNOWLEDGEMENTS
Peter Rabbitson <ribasushi@cpan.org> - for suggesting using core's "B" instead of "Devel::Refcount" to
obtain refcounts
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.20.2 2015-10-31 Test::Refcount(3pm)