=head1 NAME

XS::Framework::Manual::recipe06 - XS::Framework advanced topic



Let's assume that there is external C++ library (out of our control), which
offers the following API:

    struct DateRecipe10;

    struct TimezoneRecipe10 {
        const char* get_name() const { return name.c_str(); }
        TimezoneRecipe10(const char* name_): name{name_} { }
        std::string name;
        friend class DateRecipe10;

    struct DateRecipe10 {
        DateRecipe10(const char* tz_ = "Europe/Minsk"): tz(tz_) { update(); }
        ~DateRecipe10() { std::cout << "~DateRecipe10()\n"; }
        void update()   { epoch = std::time(nullptr); }

        int get_epoch() const { return epoch; }
        TimezoneRecipe10& get_timezone() { return tz; }
        std::time_t epoch;
        TimezoneRecipe10 tz;

It allows to create C<Date> object, and offres the C<Timezone> object by
reference. In other words, the I<lifetime of Timezone object is limited
by lifetme of Date object>. Let's imagine to possible use-cases of the library
from Perl perspecitve:

    my $date = MyTest::Cookbook::DateRecipe10->new;
    my $tz = $date->get_timezone;
    print "now is ", $date->epoch, " at ", $tz->get_name, "\n";
    undef $date;
    print "mytimezone is ", $tz->get_name, "\n";    # SEGFAULT (1)

The common Perl programmer expectations are: if there is a valid reference to
an objcet (C<$tz>), the object is valid, and all it's allowed to invoke it's
methods etc. May be some error might occur, but Perl interpreter core dump
is not expected: it is hard to debug, it should be not allowed to have that
code in production system. Let's reformulate: here there is a conflict
between C++ API objects lifetimes and possible perl script lifetimes.

Let's check what are the available options:

1) In C<Date> xs-adapter do not expose the C<Timezone> object, but instead
just return by value timezone name as string. That good solution, but it is
rather limited for simple objects, which can be reduced to string names
and do not expose any other methods.

2) Let C<Date> xs-adapter return "detached" copy of C<Timezone> object.
As the new clone has independent from C<Date> lifetime the problem would
be solved. Hovewer, this is not always possible: the c++ object might
do not have clone method or copy-constructor, or the cloning operation
might be a bit heavy-weight.

3) Let the C<Timezone> xs-adapter somehow "prolongs" lifetime of it's
owner C<Date> as long as needed.

        +-----------------+           +---------------------+
  +---->|Date(C++ pointer)|---------->|TimeZone(C++ pointer)|<------------+
  |     +-----------------+           +---------------------+             |
  |                                                                       |
  |                                                                       |
  |                                                                       |
  |    +-------------------+         +----------------------------+       |
  |    |Date (xs-adapter)  |         |TimeZone(xs-adapter)        |       |
  +----|* C++ date pointer |<---     |  * C++ TimeZone pointer    |-------+
       +-------------------+    \----|  * XS adapter Date pointer |

in other words, C<Timezone> xs-adapter will hold C++ C<Timezone> pointer
B<and> D<Date> xs-adapter, which holds C++ C<Date> pointer. So, invoking
methods on C<TimeZone> C++ object is guaranteed to be safe and match
Perl developer expectations, i.e. s/he might store/enclose C<$timezone> object
as long as needed and methods invocations will be safe.

Let's show how this can be achived using magic payload, provided by SV-api
of L<XS::Framework>:

    namespace xs {
        template <>
        struct Typemap<DateRecipe10*> : TypemapObject<DateRecipe10*, DateRecipe10*, ObjectTypePtr, ObjectStorageMG> {
            static std::string package () { return "MyTest::Cookbook::DateRecipe10"; }

        template <>
        struct Typemap<TimezoneRecipe10*> : TypemapObject<TimezoneRecipe10*, TimezoneRecipe10*, ObjectTypeForeignPtr, ObjectStorageMG> {
            //                                                                                      (2)
            static std::string package () { return "MyTest::Cookbook::TimezoneRecipe10"; }

    static xs::Sv::payload_marker_t payload_marker_10{};

    MODULE = MyTest                PACKAGE = MyTest::Cookbook::TimezoneRecipe10

    const char* TimezoneRecipe10::get_name()

    MODULE = MyTest                PACKAGE = MyTest::Cookbook::DateRecipe10

    DateRecipe10* DateRecipe10::new(const char* name)

    void DateRecipe05::update()

    std::time_t DateRecipe10::get_epoch()

    Sv DateRecipe10::get_timezone() {
        Object self {ST(0)};
        Object tz = xs::out<>(&THIS->get_timezone());                   // (3)
        auto self_ref = Ref::create(self);                              // (4)
        tz.payload_attach(self_ref, &payload_marker_10);  // (5)
        RETVAL = tz.ref();  // (6)

As the lifetime of C<Timezone> B<C++ object> isn't managed by Perl the
C<ObjectTypeForeignPtr> (2) should be specified as lifetime policy, i.e.
the C<delete> operation will never be invoked on C++ object.

When a user asks C<Date> xs-wrapper for C<Timezone> object, xs-adapter for it
is lazily created at (3). Than, reference to the original C<Date> object is
taken (4) and stored in C<Timezone> Perl SV* wrapper (5) as payload.

It is possible to achive the same without C<XS::Framework>, i.e. via inheritance,
storing SV* wrapper to C<Date> in the Derived class; and doing C<inc> on the
pointer in Constructor, and C<dec> in the destructor. Hovewer, using
L<XS::Framework> magic payload seems a bit more easier and convenient in the

As usual, we should return reference to the object (6) instead of object

Short summary: if C++ API offers two different objects, where lifetime of one
is bounded to the lifetime of the other, it is possible to "decouple" them
on XS-layer via using payload. However, if you have control on C++ API,
it would be better to "share" objects lifetimes between C++ and Perl using
refcounter mechanism as more generic one.