CPlusPlus from Haskell
From HaskellWiki
Contents |
1 Abstract
This page describes some of the black magic needed to call C++ from Haskell (on Linux, compiled with g++). I refer to it as the "The Hard Way" because it is a tedious, hand done method of generating C++ bindings. The information on this page is pretty spotty and probably wrong in places. But hopefully it will be of some use. You may also want to look at hacanon-light. It is currently unmaintained, but the idea of using Template Haskell is pretty appealing.
gcc-xml might be a good way to get the information needed to do more automatic C++ binding generation.
(BTW, this was written by JeremyShaw, know as stepcut on irc.freenode.net/#haskell if you have questions).
2 The basic steps
There are two phases to calling C++ from Haskell
- making the objects accessible to Haskell programs
- creating objects at run-time
2.1 Making objects accessible to Haskell
According to the Foreign Function Interface section of the Haskell 2010 report, you import a C++ function by simply specifying the calling convention cplusplus. Unfortunately, no Haskell compiler actually supports this calling convention, so we have to use the C calling convention instead.
The standard method for dealing with this is to write C++ code that uses extern "C" to export the methods unmangled. Of course, you still have to uniquify the method names, but *you* get to pick the names instead of letting the compiler do whatever wacky method it wants.
The method presented on this page does not use that method — it just calls the mangled names directly. So far this seems to work OK...
2.1.1 Finding the mangled name
C++ allows you to have many methods with the same name, but different arguments. However, the underlying linker does not — so C++ mangles the methods names to ensure that each method name is unique. To import these methods into Haskell, we need to know the mangled name. Since there is no standard for mangling names, we must figure out how our C++ compiler decides to do things.
Let's say I want to import the constructor
KApplication::KApplication (bool allowStyles=true, bool GUIenabled=true)
from /usr/lib/libkdecore.so
First I use nm -C to find the unmangled version of the constructor.
# nm -D -C /usr/lib/libkdecore.so | grep '::KApplication' 000a9c40 T KApplication::KApplication(bool, bool) 000aa780 T KApplication::KApplication(bool, bool, KInstance*) 000aa000 T KApplication::KApplication(_XDisplay*, unsigned long, unsigned long, bool) 000aa3c0 T KApplication::KApplication(_XDisplay*, unsigned long, unsigned long, bool, KInstance*) 000aab70 T KApplication::KApplication(_XDisplay*, int&, char**, QCString const&, bool, bool) 000a9850 T KApplication::KApplication(int&, char**, QCString const&, bool, bool) 000a9a60 T KApplication::KApplication(bool, bool) 000aa5a0 T KApplication::KApplication(bool, bool, KInstance*) 000a9e20 T KApplication::KApplication(_XDisplay*, unsigned long, unsigned long, bool) 000aa1e0 T KApplication::KApplication(_XDisplay*, unsigned long, unsigned long, bool, KInstance*) 000aa960 T KApplication::KApplication(_XDisplay*, int&, char**, QCString const&, bool, bool) 000a9640 T KApplication::KApplication(int&, char**, QCString const&, bool, bool) 001c7780 W KApplicationPrivate::KApplicationPrivate() #
The instance I want is '000a9c40'. So now I grep the mangled symbols for that number:
# nm -D ./usr/lib/libkdecore.so | grep '000a9c40' 000a9c40 T _ZN12KApplicationC1Ebb #
2.1.2 Importing the method into Haskell
I then import that into Haskell like this:
data KApplication = KApplication foreign import ccall "_ZN12KApplicationC1Ebb" kapplication_KApplication :: (Ptr KApplication) -> CInt -> CInt -> IO ()
'kapplication_KApplication' is what I want to call this method in Haskell — I can name it anything I want, as long as it is a valid Haskell identifier.
C does not have a bool type, so I guessed (and it seems to work) that a C++ bool is just a CInt.
You will also notice that the first argument to the constructor is a pointer to KApplication, however this argument was not shown in the C++ type signature:
KApplication::KApplication (bool allowStyles=true, bool GUIenabled=true)
All the methods in the KApplication class take a pointer to KApplication as their first argument. Normally C++ manages this for you — so you never see it. The pointer is, of course, the 'this' pointer.
2.1.3 Calculating the object size
We also need to know how big the object is, so we can allocate space for it with new() later.
Currently I use this little C++ program to figure that out:
#include <qstring.h> #include <kapplication.h> #include <klocale.h> #include <iostream> using namespace std; main () { cout << sizeof(KApplication) << endl; }
# g++ size.cpp -I/usr/include/qt3 -I/usr/include/kde -lkdecore -o size && ./size 204
2.2 Creating objects at run-time
The steps for creating and destroying an object are:
- Call new() on the object to allocate space for the object.
- Call the object constructor
- do whatever you want with the object
- Call the object destructor
- Call delete() on the object
Note that we have to explicitly call the Constructor.
2.3 How to link the program
Most programming languages, especially Haskell and C++, link by calling the system linker, passing it appropriate libraries. If you link Haskell and C++ code into the same binary, you cannot have this convenience for both languages. Also, one of both runtime systems may not get initialized correctly.
Currently, I'm using Ghc to do the linking, and instruct it to link the c++ runtime library:
g++ -c nifty_code.cc -o nifty_code.o ghc --make main.hs -o hybrid_program -lstdc++ nifty_code.o
This seems to work (with GHC 6.4.1 and G++ 3.3.5). However, I'm pretty certain, the C++ runtime doesn't get initialized, so as soon as someone touches, say {{{std::cin}}}, all hell breaks lose. I dare not think about what happens, should I ever feel the need to bring a third language into the mix.
AnswerMe: Can anyone confirm that this works, whether it is supposed to work and how to do this correctly? — UdoStenzel
2.4 An example
Here is an example where I create enough bindings to launch a small KDE app. This is actually a bad example because it has a lot of stuff that is unrelated to C++ bindings. Also, it does some things wrong. Things I know of include:
- Should probably use Finalizers and ForeignPtr's
- I read somewhere that you need to call the C++ main() to ensure that some C++ stdlib stuff is properly initialized.
Also of note is the ->> operator. The entire purpose of the operator is to make things look more C++ like.
instead of writing:
kapplication_KApplication kapp 1 1
I can use the more C++ like syntax:
(kapp->> kapplication_KApplication) 1 1
Though, I think I actually prefer the former.
To compile the code on a Debian system you need:
ghc6 and the kdelibs4-dev
My kdelibs is version 3.3.2 and was compiled with the g++ 3.x compiler. I think the name mangling scheme changed with g++ 4.0, so if you are using that version you will have to figure out the new mangled names.
Also, I don't believe the buttons are actually hooked up yet. I think it just sits there looking pretty.
# ghc --make KApp3.hs -fglasgow-exts -lkdecore -o kapp3
module Main where import Word import Control.Monad import Control.Monad.State import Foreign.C import Foreign.Ptr import Foreign.Marshal.Array -- * Heterogeneous lists data HNil = HNil data HCons a b = HCons a b instance (Show a, Show b, HList b) => Show (HCons a b) where show (HCons a b) = show a ++ " .:. " ++ show b {- instance Show HNil where show HNil = "[]" -} class HList a -- instance HList HNil -- instance (HList b) => HList (HCons a b) infixr 5 .:. (.:.) :: a -> b -> HCons a b a .:. b = HCons a b {- instance CreateWidgets HNil where createWidgets _ HNil = return () -} data HTrue data HFalse class HBool x instance HBool HTrue instance HBool HFalse data Proxy e proxy :: Proxy e proxy = undefined; toProxy :: e -> Proxy e toProxy _ = undefined class HBool b => TypeEq x y b | x y -> b proxyEq :: TypeEq t t' b => Proxy t -> Proxy t' -> b proxyEq _ _ = undefined -- * Some Basic Widgets data QPushButton = QPushButton String deriving Show -- * Some Layout Widgets data (QWidgetList a) => QHBox a = QHBox a deriving Show data (QWidgetList a) => QVBox a = QVBox a deriving Show -- * Add Widgets to Widget Class class QWidget a instance QWidget QPushButton instance QWidget (QHBox a) instance QWidget (QVBox a) data NoParent instance QWidget NoParent noParent = nullPtr :: Ptr NoParent -- * Widget List constraint class QWidgetList a instance (QWidget a, QWidgetList b) => (QWidgetList (HCons a b)) instance QWidgetList QPushButton instance (QWidgetList a) => QWidgetList (QHBox a) instance (QWidgetList a) => QWidgetList (QVBox a) instance QWidgetList NoParent -- instance QWidgetList HNil qwidgets :: (Show a, QWidgetList a) => a -> IO () qwidgets a = print $ a class CreateWidgets a where createWidgets :: (QWidget p) => (Ptr p) -> a -> IO () instance (CreateWidget a, CreateWidgets b) => CreateWidgets (HCons a b) where createWidgets parent (HCons w ws) = do createWidget parent w createWidgets parent ws instance (CreateWidgets QPushButton) where createWidgets parent (QPushButton str) = do qstr <- qstring str qpb <- new (qpb->>qpushbutton) qstr (castPtr parent) 0 showWidget qpb return () instance (QWidgetList a, CreateWidgets a) => (CreateWidgets (QHBox a)) where createWidgets parent (QHBox children) = do qhbox <- new (qhbox->>qhbox_QHBox) (castPtr parent) 0 0 createWidgets qhbox children showWidget qhbox return () instance (QWidgetList a, CreateWidgets a) => (CreateWidgets (QVBox a)) where createWidgets parent (QVBox children) = do qvbox <- new (qvbox->>qvbox_QVBox) (castPtr parent) 0 0 createWidgets qvbox children showWidget qvbox return () {- instance (CreateWidget a, QWidget b) => CreateWidgets (HCons a b) where createWidgets parent (HCons a b) = do createWidget parent a createWidget parent b -} {- createWidgets :: (QWidgetList a, QWidget p) => p -> a -> IO () createWidgets _ HNil = return () createWidgets parent (HCons a b) = do createWidget parent a createWidgets parent b -} class (QWidget w) => (CreateWidget w) where createWidget :: (QWidget p) => (Ptr p) -> w -> IO (Ptr w) instance (CreateWidget QPushButton) where createWidget parent (QPushButton str) = do qstr <- qstring str qpb <- new (qpb->>qpushbutton) qstr (castPtr parent) 0 showWidget qpb return qpb instance (QWidgetList a, CreateWidgets a) => (CreateWidget (QHBox a)) where createWidget parent (QHBox children) = do qhbox <- new (qhbox->>qhbox_QHBox) (castPtr parent) 0 0 createWidgets qhbox children showWidget qhbox return qhbox instance (QWidgetList a, CreateWidgets a) => (CreateWidget (QVBox a)) where createWidget parent (QVBox children) = do qvbox <- new (qvbox->>qvbox_QVBox) (castPtr parent) 0 0 createWidgets qvbox children showWidget qvbox return qvbox -- garbage collection ? qstring :: String -> IO (Ptr QString) qstring str = do cstr <- newCString str qstr <- new (qstr->>qstring_QString) cstr return qstr -- * Other data KApplication = KApplication data KCmdLineArgs = KCmdLineArgs data QString = QString class New a where new :: IO (Ptr a) instance New KApplication where new = hnew kapplicationSize instance New QString where new = hnew qstringSize instance New QPushButton where new = hnew qpushButtonSize instance New (QHBox a) where new = hnew qhboxsize instance New (QVBox a) where new = hnew qvboxsize kapplicationSize = 204 qstringSize = 4 qpushButtonSize = 140 qhboxsize = 152 qvboxsize = 152 foreign import ccall "_ZN12KApplicationC1Ebb" kapplication_KApplication :: (Ptr KApplication) -> CInt -> CInt -> IO () foreign import ccall "_ZN12QApplication4execEv" qapplication_exec :: (Ptr KApplication) -> IO Int foreign import ccall "_Znwj" cnew :: Word -> IO (Ptr ()) foreign import ccall "_ZN12KCmdLineArgs4initEiPPcPKcS3_S3_b" kCmdLineArgs_init :: Int -> Ptr CString -> CString -> CString -> CString -> Int -> IO () foreign import ccall "_ZN7QStringC1EPKc" qstring_QString :: (Ptr QString) -> CString -> IO () foreign import ccall "_ZN11QPushButtonC1ERK7QStringP7QWidgetPKc" qpushbutton :: (Ptr QPushButton) -> (Ptr QString) -> (Ptr ()) -> Int -> IO () foreign import ccall "_ZN12QApplication13setMainWidgetEP7QWidget" setMainWidget :: (Ptr KApplication) -> (Ptr ()) -> IO () foreign import ccall "_ZN7QWidget4showEv" qshow :: (Ptr ()) -> IO () foreign import ccall "_ZN7QObject7connectEPKS_PKcS1_S3_" cconnect :: (Ptr ()) -> CString -> (Ptr ()) -> CString -> IO () foreign import ccall "_ZN5QHBoxC1EP7QWidgetPKcj" qhbox_QHBox :: (Ptr (QHBox a)) -> Ptr () -> Int -> Int -> IO () foreign import ccall "_ZN5QVBoxC1EP7QWidgetPKcj" qvbox_QVBox :: (Ptr (QVBox a)) -> Ptr () -> Int -> Int -> IO () showWidget :: (QWidget w) => (Ptr w) -> IO () showWidget w = qshow (castPtr w) hnew :: (New a) => Word -> IO (Ptr a) hnew size = do o <- cnew size return (castPtr o) connect :: Ptr a -> String -> Ptr b -> String -> IO () connect a signal b slot = withCString ('2':signal) $ \sigStr -> withCString ('1':slot) $ \slotStr -> cconnect (castPtr a) sigStr (castPtr b) slotStr infixl 0 ->> c ->> m = m c kapplication :: String -> String -> String -> IO (Ptr KApplication) kapplication name description version = do arg <- newCString name cargv <- newArray [ arg] appname <- newCString "appname" desc <- newCString "desc" ver <- newCString "ver" kCmdLineArgs_init 1 cargv appname desc ver 0 kapp <- new :: IO (Ptr KApplication) (kapp->> kapplication_KApplication) 1 1 return kapp -- * My Application layout = QVBox ( QHBox ((QPushButton "1") .:. (QPushButton "2") .:. (QPushButton "3")) .:. QHBox ((QPushButton "4") .:. (QPushButton "5") .:. (QPushButton "6")) .:. QHBox ((QPushButton "7") .:. (QPushButton "8") .:. (QPushButton "9")) ) main = do kapp <- kapplication "Calculator" "A Calculator in Haskell" "1.0" mainWidget <- createWidget noParent layout (kapp ->> setMainWidget) (castPtr mainWidget) (kapp ->> qapplication_exec)