Evan Laforge wrote:
To threadjack a little bit, I've been interfacing haskell with c++.
It gets awkward when the c++ structures use STL types like string and
vector. Of course those are too complex for haskell to marshal to.
What I've been doing is defining an XMarshal variant of the X c++
class, that uses plain c arrays. Then I marshal to that, and
construct the c++ object properly from XMarshal in the c->c++ wrapper
layer. On a few occasions, when the c++ class is really big and only
has one STL member, I make a partially constructed c++ object, pass
the array separately, and then construct the proper c++ class from the
broken haskell generated one. Possibly dangerous as all get-out
because I'm dealing with "unconstructed" c++ objects, but it seems to
work.
you mean, you hack around with the internal representation of those
structures? Well, if you want to avoid double-copying, C++ can't access
Haskell sequences, and Haskell can't access C++ sequences, I guess I
don't see an alternative.
Passing back to haskell is easier since I can use "&*vec.begin()",
which according to the internet should be safe because STL guarantees
that vector contents are contiguous.
safe until either:
the vector's contents change, if Haskell is assuming it's immutable,
or more seriously, if the vector's length is changed, the pointers are
invalidated and it might crash (due to reallocating for a bigger
continuous memory chunk)
I'm only saved by the fact that I don't have that many different kinds
of classes to pass. This would be much more drudgery if I had more.
Does anyone have a better solution or convention for marshalling c++
objects?
not "better", but, you could wrap the methods of the class and call back
into C++ (through C wrappers) to do anything with the class, if it
suited your purposes better and wasn't too slow
I've also noticed warnings from g++ about hsc2hs's use of the OFFSETOF
macro on c++ classes, but some googling of g++ mailing lists implied
that it's harmless if you don't have virtual bases, and what sane
person does, so I suppress it now :)
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