Size of buffer

buffer_size.cpp

#include <sstream>
#include <iostream>

template<typename Tag, typename Tag::type M>
struct grant_access
{
    friend typename Tag::type get(Tag) { return M; }
};

template<class cT, long n>
struct streambuf_tag
{
    typedef cT* (std::basic_streambuf<cT>::*type)() const;
    friend type get(streambuf_tag<cT, n>);
};

template struct grant_access<streambuf_tag<char, 0>, &std::streambuf::pbase>;
template struct grant_access<streambuf_tag<char, 1>, &std::streambuf::pptr>;

template<class cT>
std::ptrdiff_t buffer_size(std::basic_ostream<cT> const& os)
{
    std::basic_streambuf<cT>* rdbuf = os.rdbuf();
    auto pbase = (rdbuf->*get(streambuf_tag<cT, 0>{}))(),
         pptr  = (rdbuf->*get(streambuf_tag<cT, 1>{}))();
        
    return pptr - pbase;
}

template<class cT>
std::ptrdiff_t buffer_size(std::basic_istringstream<cT> const& is)
{
    return const_cast<std::basic_istringstream<cT>&>(is).rdbuf()->in_avail();
}

int main()
{
    std::istringstream os("fdfd");
    std::cout << buffer_size(os);
}