How do I convert an io.Reader to a std::istream with SWIG & CGO?

Question

I'm trying to use SWIG to create a Go wrapper for a C++ API that uses std::istream to read binary data. I'd like to be able to pass an io.Reader to these APIs, but I'm not sure how to create a mapping between it and std::istream . I know I need to implement a std::streambuf subclass and I assume the rest will involve directors and typemaps, but I'm not familiar enough with SWIG to figure out the right combination.

Any ideas?

Answer 1

io.Reader is too general to be passed to C functions -- it might not be backed on a real file at all (it's just a class that implements a Read(...) function)

What you could do (so long as you aren't on windows) is use os.Pipe() to give you a ture FH object, but sadly the stock std::*stream doesn't have any way of creating a stream from an open file handle.

The pipe bit looks something like this:

func wrapReader(r io.Reader) uintptr {
    pr, pw, err := os.Pipe()
    if err != nil {
        panic(err)
    }

    go func () {
        _, _ io.Copy(pw, r)
        _ = pw.Close()
    }()

    return pr
}

If you combine that some of the code in this answer How to construct a c++ fstream from a POSIX file descriptor? you might get what you need