No const overloads for getting the underlying device of boost::iostream::stream

[LimpSquid]

There seem to be no const overloads for getting the underlying Device of boost::iostream::stream. It might not be as trivial as by adding the following two lines, because sub-sequential calls do not seem to implement the const overloads either.

Device const & operator*() const { return *this->member; }
Device const * operator->() const { return &*this->member; }

I'm not sure if there is some reason why there are no const overloads for these particular methods. As far as I can see there shouldn't be any issue to add them. Currently I can work around this problem by marking the stream in the example as mutable.

In any case here an example: https://coliru.stacked-crooked.com/a/fe70537e096a2b0b. Or use the code below:

#include <iostream>
#include <boost/iostreams/categories.hpp>
#include <boost/iostreams/stream.hpp>

struct stream_sink
{
    using char_type = char;
    using category = boost::iostreams::sink_tag;

    stream_sink(std::ostream & os) :
        stream(os)
    {}

    bool fail() const { return stream.fail(); }

    std::streamsize write(const char * s, std::streamsize n) 
    {
        stream.write(s, n);
        return n;
    }    

    std::ostream & stream;
};

struct cout_writer
{
    using stream_type = boost::iostreams::stream<stream_sink>;

    stream_type stream{std::cout};

    operator bool() { return !stream->fail(); }

    // Does not compile as there are no const overloads like below:
    // - Device const & operator*() const { ... }
    // - Device const * operator->() const { ... }
    operator bool() const { return !stream->fail(); }

    cout_writer & operator<<(std::string_view s)
    {
        stream << s;
        return *this;
    }
};

int main()
{
    cout_writer writer;

    writer << "foo\n";
}