What is the type of the underlying stream of a boost::iostreams::filtering_stream?

Question

I have this stream that performs decompression using Boost.Iostreams:

struct istream_zlib
  : public boost::iostreams::filtering_stream<boost::iostreams::input, char>
{
  istream_zlib(std::istream& in)
  {
    push(boost::iostreams::zlib_decompressor());
    push(in);
  }
};

Now, I would like to access the underlying stream (std::istream& in) later on. Naively, I thought that requesting a std::istream through component() would do it, but the pointer I get back is null:

auto ptr = component<std::istream>(1); // ptr is null!

What type should I provide to component() for doing that?

Answer 1

It's not real, since not istream will be pushed into filtering_stream (for my boost 1.48 it will be boost::iostreams::detail::mode_adapter<boost::iostreams::input, std::istream> for example), you can check it type by component_type function. However, I have no ideas, why you need to get stream from filtering_stream, since you send reference - you should have this object in places, where you use this filtering_stream.

Also, you can use reference_wrapper for this case (i.e push(boost::ref(in));) and then get it by component using

auto ptr = component<boost::reference_wrapper<std::istream>>(1);