#include <exception>
...
class exception: public std::runtime_error 

#include <stdexcept>
...
class exception: public std::runtime_error 

template<typename callback_t, typename ...A>
static typename std::result_of<callback_t(A...)>::type invoke(void* target, int cid, A&& ... args)
{
    auto x = dynamic_cast<internal::callback_object<callback_t>*>(reinterpret_cast<callbacks*>(target)->m_lut[cid].get());
    assert(x);
    return x->invoke(std::forward<A>(args)...);
}

template <class Target, class Source>
inline Target polymorphic_downcast(Source* x)
{
    assert(dynamic_cast<Target>(x) == x);
    return static_cast<Target>(x);
}

template<typename callback_t, typename ...A>
static typename std::result_of<callback_t(A...)>::type invoke(void* target, int cid, A&& ... args)
{
    auto x = polymorphic_downcast<internal::callback_object<callback_t>*>(reinterpret_cast<callbacks*>(target)->m_lut[cid].get());
    return x->invoke(std::forward<A>(args)...);
}

    bool write(const std::string& buf, std::function<void(error)> callback)
    {
        return write(buf.c_str(), buf.length(), callback);
    }

    bool write(const std::vector<char>& buf, std::function<void(error)> callback)
    {
        return write(&buf[0], buf.size(), callback);
    }