=head1 NAME

XS::Framework::Manual::recipe06 - XS::Framework basics



Let's assume that the following type hierarchy in C++:

    struct Base06 {
        virtual const char* method() { return "from base"; }
        virtual ~Base06() { }

    struct Derived06A: public Base06 {
        virtual const char* method() { return "from derived-A"; }
        const char* specific_method() { return "specific-A"; }

    struct Derived06B: public Base06 {
        virtual const char* method() { return "from derived-B"; }
        const char* specific_method() { return "specific-B"; }

The typemaps should also reflect the hierarchy as:

    namespace xs {
        template <typename D>
        //              (1)
        struct Typemap<Base06*, D> : TypemapObject<Base06*, D, ObjectTypePtr, ObjectStorageMG, StaticCast> {
        //                      (2)                         (3)
            static std::string package () { return "MyTest::Cookbook::Base04"; }
        //                      (4)

        template <> struct Typemap<Derived06A*> : Typemap<Base06*, Derived06A*> {
        //                                                  (5)
            static std::string package () { return "MyTest::Cookbook::Derived06A"; }

        template <> struct Typemap<Derived06B*> : Typemap<Base06*, Derived06B*> {
        //                                                  (6)
            static std::string package () { return "MyTest::Cookbook::Derived06B"; }

The base type in C++, which has to be base class in Perl too, have to be defined
as I<partial> template specialization (1). This is important moment: the partial
specialization for base class defines typemap for base class itself B<and>
for all derived classes. This is done when template parameter C<D> is substituted
in (2) and (3) either with C<Base06> class or any child (grand-child,
grand-grand-...-child) derived class.

The base typemap also B<defines and fixes> life time, storage and casting policies
for all hierarchy. In almost all cases this follows DWIM principle.

The default package name (4) is used for base class on C<xs::out> when no C<hint>
parameter is supplied.

The derived classes, as they are final (i.e. no other derived classes expected),
should do full specialization of typemap and B<inherit the specialized> base
typemap (5), (6). If the derived classes are non final, they should be patrially
specialized. In other words, if you are writing library, which is supposed
to be extended in XS/C++, then, make your typemaps extendable (partially

That complexity with typemap<B, D> is needed to perform B<fast> casting right
from Base* pointer to DerivedN* pointer, ommiting all intermediate classes.

The xs-adapters will be:

    MODULE = MyTest                PACKAGE = MyTest::Cookbook::Base06

    const char* Base06::method()

    Base06* Base06::new()
    #             (7)

    MODULE = MyTest                PACKAGE = MyTest::Cookbook::Derived06A

    Derived06A* Derived06A::new()
    #                   (8)

    const char* Derived06A::specific_method()
    #                   (9)

    BOOT {
        //  (10)

    MODULE = MyTest                PACKAGE = MyTest::Cookbook::Derived06B

    Derived06B* Derived06B::new()
    #               (10)

    const char* Derived06B::specific_method()
    #               (11)

    BOOT {
        //          (12)

It is possible to use short-cut constructors (7), (8), (10). It will
automatically call the base constructor of the related class and forward all
parameters. All constructors for all XS-adapters have to be defined,
as they are not inherited.

The xs-adapters B<should also reflect classes hierarhy>. That way the adapted
C<method> can be written once in base xs-adapter only (assuming, that no
additional/specific actions in XS-adapter are needed). That way the following
code works as expected:

    MyTest::Cookbook::Derived06A->new->method;  # => "from derived-A"

The xs-hierarhy is defined as hierarchy between corresponding stashes at
C<BOOT> method (10), (12). The macros C<__PACKAGE__> is substituted to
C string literal of the most recently parsed C<PACKAGE> preambula.

That's way there is need to define only class-specific methods (9), (11)

The short summary: typemaps for base classes should be extensible (patrially
specialized) and typemaps and xs-adapters should reflect original C++