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>