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

Creating artificial hierarchy

Let's assume there is a different C++ libraries, offering similar capabilities, e.g. WAV-files playing library and multimedia-files (ogg, aac, mp3) playing library. Their interfaces are like:

/* is able to hold only files in WAV-format */
struct WAVFile {
    WAVFile(const char* name): name_{name} {}
    const char* name() const noexcept { return name_; }
private:
    const char* name_;
};

/* is able to hold any files only in ogg, mp3 and aac formats */
struct MultimediaFile {
    MultimediaFile(const char* name, const char* format): name_{name}, format_{format} {}
    const char* name() const noexcept { return name_; }
    const char* format() const noexcept { return format_; }
private:
    const char* name_;
    const char* format_;
};

struct WAVPlayer {
  WAVPlayer(double preferred_bitrate): preferred_bitrate_{preferred_bitrate} {}
  std::string play_wav(WAVFile* file) {
    std::string result = "wav-player is playing ";
    result += file->name();
    result += " with bitrate ";
    result += std::to_string(preferred_bitrate_);
    return result;
  }
  double preferred_bitrate() const noexcept { return preferred_bitrate_; }
  WAVPlayer* clone() const noexcept { return new WAVPlayer(preferred_bitrate_); }
private:
  double preferred_bitrate_;
};

struct MultimediaPlayer {
  MultimediaPlayer(int quality): quality_{quality} {}
  std::string play_file(MultimediaFile* file) {
    std::string result = "player is playing ";
    result += file->name();
    result += " (";
    result += file->format();
    result += ")";
    result += " with quality ";
    result += std::to_string(quality_);
    return result;
  }
  int quality() const noexcept { return quality_; }
  MultimediaPlayer* clone() const noexcept { return new MultimediaPlayer(quality_); }
private:
  int quality_;
};

Their typemaps are trivial without inheritance and are omitted here (see t/cookbook/recipe08.xsi for full sources).

What we would like to achive is to "fix" C++ hierarchy in Perl: as WAVPlayer and MultimediaPlayer almost the same interface, and as MultimediaPlayer looks as the most generic one, let's have xs-adapter for MultimediaPlayer Perl, and let it inherits WAVPlayer xs-adapter, i.e. offers capabilities of the both C++ classes. (The xs-adapters for WAVFile and MultimediaFile are omitted)

MODULE = MyTest                PACKAGE = MyTest::Cookbook::WAVPlayer
PROTOTYPES: DISABLE

std::string WAVPlayer::play_wav(WAVFile* file)

double WAVPlayer::preferred_bitrate()

WAVPlayer* WAVPlayer::new(double preferred_bitrate) # (1)

WAVPlayer* WAVPlayer::clone() { // (2)
    Object self{ST(0)};
    PROTO = self.stash();   // (3)
    RETVAL = THIS->clone(); // (4)
}

The auto-generated constructor (1) will forward all provided parameters to the underlying C++ class; it is also aware of 1st argument CLASS/PROTO, i.e. SV* blessing will be performed into final class.

The clone (2) method performs acutally the same, hovewer we can't leave it as:

WAVPlayer* WAVPlayer::clone()

because the PROTO hint will be empty, and by TypemapObject rules it will be blessed into TypemapObject::package, i.e. to MyTest::Cookbook::WAVPlayer, in other words it is not inheritance-aware. To fix that we have to manually write the clone method, which will forward to clone method of underlying C++ object (4) and bless it it to the actual Perl object package (3).

Let's write xs-adapter for MultimediaPlayer, which fixes C++ class hierarchy:

MODULE = MyTest                PACKAGE = MyTest::Cookbook::MultimediaPlayer
PROTOTYPES: DISABLE

MultimediaPlayer* MultimediaPlayer::new(double preferred_bitrate, int quality) {
    (void)preferred_bitrate;    // silence warning
    PROTO = Stash::from_name(CLASS).call_next(cv, &ST(1), 1);   // (5)
    if (!PROTO.defined()) XSRETURN_UNDEF;
    RETVAL = new MultimediaPlayer(quality);                     // (6)
}

std::string MultimediaPlayer::play_file(MultimediaFile* file)

int MultimediaPlayer::quality()

MultimediaPlayer* MultimediaPlayer::clone() {
    Object self{ST(0)};
    PROTO = self.call_next(cv);         // (7)
    RETVAL = THIS->clone();             // (8)
}

BOOT {
    auto stash = Stash(__PACKAGE__, GV_ADD);
    stash.inherit("MyTest::Cookbook::WAVPlayer");   // (9)
}

First, in the constructor new the base SV* wrapper have to be created in (5). It actually forwards call to the new method of WAVPlayer xs-adapter (1). Then it creates MultimediaPlayer C++ object in (6) and, as the PROTO variable already contains SV* wrapper, by XS::Framework rules the MultimediaPlayer C++ object will attached to SV*. Please, note that SV* wrapper will be already blessed into the right package after (5).

The clone method (7)..(8) is similar to the new constructor, i.e. it first clones (7) XS-adapter for WAVPlayer (which clones C++ class WAVPlayer), and after (8) the pointer to C++ MultimediaPlayer object will be attached to it. Please, note, that THIS variable is C++ MultimediaPlayer at line (8), and it is WAVPlayer at line (4).

It should be noted, that in (5) and (7) the call_next is used. It could be changed to call_SUPER, but call_next is somewhat more general.

In the line (9) we should specify that MultimediaPlayer xs-adapter inherits from WAVPlayer adapter. The following test proves correctness:

my $wav = MyTest::Cookbook::WAVFile->new('sample.wav');
my $ogg = MyTest::Cookbook::MultimediaFile->new('sample.ogg', 'ogg');
my $player = MyTest::Cookbook::MultimediaPlayer->new(44100, 6);
my $clone = $player->clone;
is $clone->quality, 6;
is $clone->preferred_bitrate, 44100;
is $clone->play_file($ogg), 'player is playing sample.ogg (ogg) with quality 6';
is $clone->play_wav($wav), 'wav-player is playing sample.wav with bitrate 44100.000000';

The short summary: if needed it is possible to fix/enrich C++ class hierarchry in Perl classes (xs-adapters).