temp.cpp

    struct P
    {
        T* t;
        std::atomic<std::uint16_t> n;

        template<class DeducedHandler, class... Args>
        P(DeducedHandler&& handler, Args&&... args);

    private:
        P() noexcept;
    };

...

  template<class T>
inline
handler_ptr<T>::P::
P() noexcept
    : n(1)
{
}

template<class T>
template<class DeducedHandler, class... Args>
inline
handler_ptr<T>::P::
P(DeducedHandler&& handler, Args&&... args)
    : P()
{
    t = reinterpret_cast<T*>(
        boost_asio_handler_alloc_helpers::
            allocate(sizeof(T), handler));
    t = new(t) T{
        std::forward<DeducedHandler>(handler),
            std::forward<Args>(args)...};
}