cplusplus-th

C++ Foreign Import Generation https://github.com/nicta/cplusplus-th

Latest on Hackage:1.0.0.0

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow stackage.org to host generated Haddocks.

BSD3 licensed by Maxwell Swadling

cplusplus-th

cplusplus-th allows you to foreign import C++ functions that are compatible with the ccall calling convention. It also includes some standard library abstractions.

Example

With the following C++ function in the object file cbits/string.o:

namespace haskell {
string* fromCString(char const* x, int length) {
  return new string(x, length);
}
}

We can import it into Haskell with:

cplusplus "haskell::fromCString(char const*, int)" "cbits/string.o"
          [t|CString -> Int -> IO Std__basic_string|]

Building

It works by looking up the symbol in the object file. When using cabal, it is recommended you include the C++ file in your c-sources and compile it in a build hook. For example:

import Distribution.Simple
import Distribution.Simple.Setup
import Distribution.Simple.Program
import Distribution.Simple.Program.Types
import Distribution.Simple.LocalBuildInfo
import Distribution.PackageDescription

cc_flags = ["-stdlib=libc++", "-o", "cbits/string.o", "-c", "cbits/string.cc"]

main :: IO ()
main = defaultMainWithHooks simpleUserHooks {
    buildHook = buildCPlusPlus
  }

buildCPlusPlus :: PackageDescription -> LocalBuildInfo -> UserHooks -> BuildFlags -> IO ()
buildCPlusPlus pkg buildInfo hooks flags = do
  let verb = fromFlag (buildVerbosity flags)
  clang <- findProgramLocation verb "clang++"
  let clang' = case clang of
                Just x -> x
                Nothing -> error "clang++ not on path"
  runProgram verb (simpleConfiguredProgram "clang++" (FoundOnSystem clang')) cc_flags
  buildHook simpleUserHooks pkg buildInfo hooks flags

Standard Library

Foreign.CPlusPlusStdLib exports the following type class:

class CPlusPlusLand a {- haskell side -} b {- c++ side -} where
  to :: a -> IO b
  from :: b -> IO a

To avoid orphan instances, it implements instances for some numeric types, String and ByteString.

Compatability

  • Static functions are simply the arguments.
  • Static member functions take the object as the first argument.
  • Functions via a vtable are not possible.
  • Inline functions are not possible.
  • instantiating templates is not possible.
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