[Haskell-cafe] FFI: C-side object not destructed

[Yves Parès]

I assume you are right.
The weirdest thing here is that getY() returns the Y value of the destructed
object while getX() returns always 0.


2011/2/26 Miguel Mitrofanov <miguelimo38 at yandex.ru>

> Well, this code in C++ would probably work too:
>
> Klass *k = new Klass(4,5);
> delete k;
> std::cout << k->getY() << std::endl;
>
> though smart compiler would probably issue a warning. See, when you delete
> something, C++ doesn't automagically mark your pointer as "invalid"; in
> fact, it preserves all the data in your deleted class. If you didn't provide
> a destructor, then the only outcome of "delete" would be that the same
> memory can be assigned to another object by "new" operator, but it doesn't
> get cleared or invalidated in any way.
>
> Seems to me, Haskell works in the same way.
>
> On 26 Feb 2011, at 13:59, Yves Parès wrote:
>
> > Hello,
> > I'm trying to use a C++ class in Haskell through C exports.
> > It works all very well, except that when I call the function that deletes
> the object, it is still valid, I can still call methods on it.
> >
> > Here is my Haskell code:
> >
> > {-# LANGUAGE ForeignFunctionInterface #-}
> >
> > import Foreign
> > import Foreign.C.Types
> >
> > newtype PKlass = PKlass (Ptr PKlass)
> >
> > foreign import ccall unsafe "Klass_Create"
> >   kCreate :: CInt -> CInt -> IO PKlass
> >
> > foreign import ccall unsafe "Klass_Destroy"
> >   kDestroy :: PKlass -> IO ()
> >
> > foreign import ccall unsafe "Klass_GetX"
> >   kGetX :: PKlass -> IO CInt
> > foreign import ccall unsafe "Klass_GetY"
> >   kGetY :: PKlass -> IO CInt
> >
> > foreign import ccall unsafe "Klass_AddKlass"
> >   kAdd :: PKlass -> PKlass -> IO PKlass
> >
> >
> > main = do
> >   k <- kCreate 4 5
> >   kDestroy k
> >   kGetY k >>= print   -- This shouldn't work
> >   k' <- kCreate 2 8
> >   k'' <- k `kAdd` k'
> >   kDestroy k''
> >   kGetY k'' >>= print   -- This neither
> >
> >
> > So it is very basic, and I can't understand why the supposedly destroyed
> objects are still here.
> > Enclosed is all the source code (C++ class, C exportation and the Haskell
> main hereabove).
> >
> > I compile it this way:
> > ghc --make main.hs *.cpp -lstdc++
> > <test.zip>_______________________________________________
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>
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