feat/add_eventbus (#9)

Co-authored-by: tqcq <99722391+tqcq@users.noreply.github.com>
Reviewed-on: #9

3party/eventbus/include/detail/leftright.h

@@ -0,0 +1,388 @@
+#pragma once
+
+#include <algorithm>
+#include <array>
+#include <atomic>
+#include <cassert>
+#include <cstdint>
+#include <mutex>
+#include <thread>
+#include <type_traits>
+
+#ifndef MPM_LEFTRIGHT_CACHE_LINE_SIZE
+#define MPM_LEFTRIGHT_CACHE_LINE_SIZE 64
+#endif
+
+namespace mpm {
+//! \defgroup Concepts
+//! Concept is a term that describes a named set of requirements for a type.
+
+//! \defgroup ReaderRegistry
+//! \ingroup Concepts
+//! \{
+//!
+//! Keeps track of active readers such that it can efficiently
+//! indicate whether there are any active readers when queried.
+//!
+//! \par Extends
+//! DefaultConstructible
+//!
+//! \par Requirements
+//! Given:\n
+//! R, an implementation of the ReaderRegistry concept \n
+//! r, an instance of R
+//!
+//! |Expression  | Requirements                                             | Return type  |
+//! |:-----------|:---------------------------------------------------------|:-------------|
+//! | R()        | R is default constructible.                              | R            |
+//! | r.arrive() | Notes the arival of a reader. Wait-free and noexcept.    | void         |
+//! | r.depart() | Notes the departure of a reader. Wait-free and noexcept. | void         |
+//! | r.empty()  | const and noexcept.    | true if there are no readers; false otherwise. |
+//! \}
+
+struct in_place_t {};
+
+constexpr in_place_t in_place{};
+
+//! Wrap any single-threaded datastructure with Left-Right
+//! concurrency control
+//!
+//! Left-Right concurrency allows wait-free population-oblivious reads
+//! and blocking writes. Writers never block readers.
+//!
+//! Instances of this class maintain two full copies of the underlying
+//! datastructure and all modifications are performed twice. Consequently,
+//! uses of this class should be limited to small amounts of data where
+//! the number of reads dominates the number of writes.
+//!
+//! Left-Right concurrency control is described in depth in
+//!   A. Correia and P. Ramalhete. Left-Right: A Concurrency Control
+//!   Technique with Wait-Free Population Oblivious Reads
+template<typename T, typename ReaderRegistry>
+class basic_leftright {
+    static_assert(noexcept(std::declval<ReaderRegistry>().arrive()),
+                  "ReaderRegistry::arrive() must be noexcept");
+    static_assert(noexcept(std::declval<ReaderRegistry>().depart()),
+                  "ReaderRegistry::depart() must be noexcept");
+    static_assert(noexcept(std::declval<const ReaderRegistry>().empty()),
+                  "ReaderRegistry::empty() must be noexcept");
+
+public:
+    using value_type = T;
+    using reference = value_type &;
+    using const_reference = const value_type &;
+
+    basic_leftright() = default;
+
+    //! Construct the two underlying instances of T
+    //! by moving a seed instance
+    basic_leftright(T &&seed) noexcept(
+        std::is_nothrow_copy_constructible<T>::value
+        && std::is_nothrow_move_constructible<T>::value);
+
+    //! Construct the two underlying instances of T
+    //! by copying a seed instance
+    basic_leftright(const value_type &seed) noexcept(
+        std::is_nothrow_copy_constructible<T>::value);
+
+    //! Construct the two underlying instances of T in place, forwarding
+    //! the args after the mpm::in_place tag
+    template<typename... Args>
+    basic_leftright(in_place_t, Args &&...) noexcept(
+        std::is_nothrow_copy_constructible<T>::value
+        && std::is_nothrow_constructible<T, Args...>::value);
+
+    //! \internal
+    //!  Need a use-case for these. It seems that you would never want to
+    //!  move/copy/swap the full leftright instance but rather apply those
+    //!  operations to the encapsulated instance, in which case the relevant
+    //!  operation is accessed via modify()
+
+    basic_leftright(const basic_leftright &other) = delete;
+    basic_leftright(basic_leftright &&other) = delete;
+    basic_leftright &operator=(const basic_leftright &rhs) = delete;
+    basic_leftright &operator=(basic_leftright &&rhs) = delete;
+    void swap(basic_leftright &other) noexcept = delete;
+
+    //! Modify the state of the managed datastructure
+    //!
+    //! Blocks/is-blocked-by other concurrent writers; does not
+    //! block concurrent readers
+    //!
+    //! This function requires that execution of the supplied functor
+    //! be noexcept.
+    //!
+    //! The function passed will be executed twice and *must*
+    //! result in the exact same mutation operation being applied
+    //! in both cases. For example it would be incorrect to supply
+    //! a function here that inserted a random number into the
+    //! underlying datastructure if said random number were calculated
+    //! for each invocation (i.e. each invocation would insert a
+    //! different value).
+    //!
+    //! \throws std::system_error on failure to lock internal mutex
+    //!
+    //! \internal I wanted the declaration to be as below so that this
+    //!           function doesn't even exist for non-noexcept functors,
+    //!           but g++ has not yet implemented noexcept mangling
+    //!           as of version 5.2.1. Instead we just static_assert the
+    //!           noexcept-ness of the functor in the body of the function
+    //!
+    //! template <typename F>
+    //! auto modify(F f)
+    //!     -> typename std::enable_if<noexcept(f(std::declval<T&>())),
+    //!             typename std::result_of<F(T&)>::type>::type;
+    template<typename F>
+    typename std::result_of<F(T &)>::type modify(F f);
+
+    //! Observe the state of the managed datastructure
+    //!
+    //! Always wait-free provided ReaderRegistry::arrive() and
+    //! ReaderRegistry::depart() are truly wait-free
+    //!
+    //! \throws Whatever the provided functor throws and nothing else
+    template<typename F>
+    typename std::result_of<F(const T &)>::type observe(F f) const
+        noexcept(noexcept(f(std::declval<const T &>())));
+
+private:
+    class scoped_read_indication {
+    public:
+        scoped_read_indication(ReaderRegistry &rr) noexcept;
+        ~scoped_read_indication() noexcept;
+
+    private:
+        ReaderRegistry &m_reg;
+    };
+
+    template<typename Lock>
+    void toggle_reader_registry(Lock &l) noexcept;
+
+    enum lr { read_left, read_right };
+
+    mutable std::array<ReaderRegistry, 2> m_reader_registries;
+
+    std::atomic_size_t m_registry_index{0};
+
+    std::atomic<lr> m_leftright{read_left};
+
+    T m_left alignas(MPM_LEFTRIGHT_CACHE_LINE_SIZE);
+    T m_right alignas(MPM_LEFTRIGHT_CACHE_LINE_SIZE);
+    std::mutex m_writemutex;
+};
+
+//! Simple implementation of ReaderRegistry concept
+//!
+//! This implemtation is wait-free but readers will contend
+//! on a single cache line due to the use of a shared counter.
+//!
+//! \concept{ReaderRegistry}
+class alignas(MPM_LEFTRIGHT_CACHE_LINE_SIZE) atomic_reader_registry {
+public:
+    void arrive() noexcept;
+    void depart() noexcept;
+    bool empty() const noexcept;
+
+private:
+    std::atomic_uint_fast32_t m_count{0};
+};
+
+//! Distributed implementation of ReaderRegistry
+//!
+//! Uses an array of N counters (suitably padded) and hashes reader
+//! thread ids to indices into said array so that concurrent reader
+//! registration can be made unlikely to contend. The likelihood
+//! of a collision is dependent on the number of concurrent readers
+//! relative to N.
+//!
+//! arrive() and depart() will perform better if N is a power of two.
+//!
+//! \concept{ReaderRegistry}
+template<std::size_t N, typename Hasher = std::hash<std::thread::id>>
+class alignas(MPM_LEFTRIGHT_CACHE_LINE_SIZE)
+    distributed_atomic_reader_registry {
+public:
+    void arrive() noexcept;
+    void depart() noexcept;
+    bool empty() const noexcept;
+
+private:
+    class alignas(MPM_LEFTRIGHT_CACHE_LINE_SIZE) counter {
+    public:
+        void incr() noexcept;
+        void decr() noexcept;
+        std::uint_fast32_t relaxed_read() const noexcept;
+
+    private:
+        std::atomic_uint_fast32_t m_value{0};
+    };
+
+    std::array<counter, N> m_counters{};
+};
+
+//! Default leftright uses the simpler reader registry; prefer
+//! distributed_atomic_reader_registry when reads are highly contended
+template<typename T>
+using leftright = basic_leftright<T, atomic_reader_registry>;
+
+#ifndef DOCS
+
+template<typename T, typename R>
+basic_leftright<T, R>::basic_leftright(T &&seed) noexcept(
+    std::is_nothrow_copy_constructible<T>::value
+    && std::is_nothrow_move_constructible<T>::value)
+    : m_left(std::move(seed)),
+      m_right(m_left)
+{}
+
+template<typename T, typename R>
+basic_leftright<T, R>::basic_leftright(const T &seed) noexcept(
+    std::is_nothrow_copy_constructible<T>::value)
+    : m_left(seed),
+      m_right(seed)
+{}
+
+template<typename T, typename R>
+template<typename... Args>
+basic_leftright<T, R>::basic_leftright(in_place_t, Args &&...args) noexcept(
+    std::is_nothrow_copy_constructible<T>::value
+    && std::is_nothrow_constructible<T, Args...>::value)
+    : m_left(std::forward<Args>(args)...),
+      m_right(m_left)
+{}
+
+template<typename T, typename R>
+template<typename F>
+typename std::result_of<F(const T &)>::type
+basic_leftright<T, R>::observe(F f) const
+    noexcept(noexcept(f(std::declval<const T &>())))
+{
+    std::size_t idx = m_registry_index.load(std::memory_order_acquire);
+    scoped_read_indication sri(m_reader_registries[idx]);
+    return read_left == m_leftright.load(std::memory_order_acquire)
+        ? f(m_left)
+        : f(m_right);
+}
+
+template<typename T, typename R>
+template<typename F>
+typename std::result_of<F(T &)>::type
+basic_leftright<T, R>::modify(F f)
+{
+    static_assert(noexcept(f(std::declval<T &>())),
+                  "Modify functor must be noexcept");
+    std::unique_lock<std::mutex> xlock(m_writemutex);
+    if (read_left == m_leftright.load(std::memory_order_relaxed)) {
+        f(m_right);
+        m_leftright.store(read_right, std::memory_order_release);
+        toggle_reader_registry(xlock);
+        return f(m_left);
+    } else {
+        f(m_left);
+        m_leftright.store(read_left, std::memory_order_release);
+        toggle_reader_registry(xlock);
+        return f(m_right);
+    }
+}
+
+template<typename T, typename R>
+template<typename Lock>
+void
+basic_leftright<T, R>::toggle_reader_registry(Lock &l) noexcept
+{
+    assert(l);
+    const std::size_t current =
+        m_registry_index.load(std::memory_order_acquire);
+    const std::size_t next = (current + 1) & 0x1;
+
+    while (!m_reader_registries[next].empty()) { std::this_thread::yield(); }
+
+    m_registry_index.store(next, std::memory_order_release);
+
+    while (!m_reader_registries[current].empty()) { std::this_thread::yield(); }
+}
+
+template<typename T, typename R>
+basic_leftright<T, R>::scoped_read_indication::scoped_read_indication(
+    R &r) noexcept
+    : m_reg(r)
+{
+    m_reg.arrive();
+}
+
+template<typename T, typename R>
+basic_leftright<T,
+                R>::scoped_read_indication::~scoped_read_indication() noexcept
+{
+    m_reg.depart();
+}
+
+inline void
+atomic_reader_registry::arrive() noexcept
+{
+    m_count.fetch_add(1, std::memory_order_acq_rel);
+}
+
+inline void
+atomic_reader_registry::depart() noexcept
+{
+    m_count.fetch_sub(1, std::memory_order_acq_rel);
+}
+
+inline bool
+atomic_reader_registry::empty() const noexcept
+{
+    return 0 == m_count.load(std::memory_order_acquire);
+}
+
+template<std::size_t N, typename Hasher>
+void
+distributed_atomic_reader_registry<N, Hasher>::arrive() noexcept
+{
+    std::size_t index = Hasher()(std::this_thread::get_id()) % N;
+    m_counters[index].incr();
+}
+
+template<std::size_t N, typename Hasher>
+void
+distributed_atomic_reader_registry<N, Hasher>::depart() noexcept
+{
+    std::size_t index = Hasher()(std::this_thread::get_id()) % N;
+    m_counters[index].decr();
+}
+
+template<std::size_t N, typename Hasher>
+bool
+distributed_atomic_reader_registry<N, Hasher>::empty() const noexcept
+{
+    bool retval =
+        std::none_of(begin(m_counters), end(m_counters),
+                     [](const counter &ctr) { return ctr.relaxed_read(); });
+    std::atomic_thread_fence(std::memory_order_acquire);
+    return retval;
+}
+
+template<typename std::size_t N, typename H>
+void
+distributed_atomic_reader_registry<N, H>::counter::incr() noexcept
+{
+    (void) m_value.fetch_add(1, std::memory_order_acq_rel);
+}
+
+template<typename std::size_t N, typename H>
+void
+distributed_atomic_reader_registry<N, H>::counter::decr() noexcept
+{
+    (void) m_value.fetch_sub(1, std::memory_order_acq_rel);
+}
+
+template<typename std::size_t N, typename H>
+std::uint_fast32_t
+distributed_atomic_reader_registry<N, H>::counter::relaxed_read() const noexcept
+{
+    return m_value.load(std::memory_order_relaxed);
+}
+
+#endif
+
+}// namespace mpm

3party/eventbus/include/detail/typelist.h

@@ -0,0 +1,49 @@
+#pragma once
+
+namespace mpm {
+namespace detail {
+//! A type used to indicate an empty typelist tail. I.e. the end marker
+//! of a typelist.
+struct null_t {};
+
+//! Typelist a la [Alexandrescu](http://www.drdobbs.com/generic-programmingtypelists-and-applica/184403813)
+template<typename Head, typename Tail = null_t>
+struct typelist {
+    //! The first element, a non-typelist by convention
+    using head = Head;
+
+    //! The second element, can be another typelist
+    using tail = Tail;
+};
+
+template<typename F, typename TL>
+struct for_each_helper {
+    void operator()(F f) const
+    {
+        f.template operator()<typename TL::head>();
+        for_each_helper<F, typename TL::tail>()(f);
+    }
+};
+
+template<typename F>
+struct for_each_helper<F, null_t> {
+    void operator()(const F &) {}
+};
+
+//! Iterate over a typelist at runtime.
+//! The supplied functor, f, is invoked for each element in the typlist TL
+//!
+//! \tparam TL A typelist
+//! \tparam F A FunctionObject
+//! \param f A functor that will be called as f.template operator()\<T>()
+//!          where T is an element in TL. This functor will be invoked once
+//!          for each element of TL
+//! \relates typelist
+template<typename TL, typename F>
+void
+for_each_type(const F &f)
+{
+    for_each_helper<F, TL>()(f);
+}
+}// namespace detail
+}// namespace mpm

3party/eventbus/include/enable_polymorphic_dispatch.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "detail/typelist.h"
+
+namespace mpm {
+namespace detail {
+/// root class for polymorphic event hierarchies
+class event {
+protected:
+    //don't dispatch as this type - it's an internal
+    //implementation detail
+    using dispatch_as = null_t;
+
+    virtual ~event() {}
+};
+
+/// forward decl from eventbus.h
+template<typename E>
+struct dispatch_typelist;
+}// namespace detail
+
+//! Publicly inherit from this class to obtain polymorphic
+//! event delivery.
+template<typename T, typename Base = detail::event>
+class enable_polymorphic_dispatch : public Base {
+protected:
+    // \internal
+    // Couldn't use a typelist based on std::tuple here because
+    // std::tuple_cat won't work with an incomplete type, and the
+    // derived event types are incomplete when decltype(tuple_cat)
+    // would be used
+
+#ifdef DOCS
+    using dispatch_as = implementation - defined;
+#else
+    template<typename E>
+    friend class detail::dispatch_typelist;
+    using dispatch_as = detail::typelist<T, typename Base::dispatch_as>;
+#endif
+};
+}// namespace mpm

3party/eventbus/include/eventbus.h

@@ -0,0 +1,433 @@
+#pragma once
+
+#include "enable_polymorphic_dispatch.h"
+#include "detail/typelist.h"
+#include <memory>
+#include "detail/leftright.h"
+#include <typeindex>
+#include <unordered_map>
+#include <utility>
+
+//! The mpm namespace
+namespace mpm {
+//! \defgroup Concepts
+//! Concept is a term that describes a named set of requirements for a type
+
+//! \defgroup Event
+//! \ingroup Concepts
+//! \{
+//!
+//! An instance of any C++ object type. That is, anything but a function,
+//! reference, or void type
+//!
+//! \par Requirements
+//! Given:\n
+//! E, an implementation of the Event concept
+//!
+//! |Expression                       | Requirements             | Return type        |
+//! |:--------------------------------|:-------------------------|:-------------------|
+//! |std::is_object<E>::value == true | E must be an object type | bool, must be true |
+//! \}
+
+//! \defgroup EventHandler
+//! \ingroup Concepts
+//! \{
+//!
+//! A Callable that can be invoked to handle an instance of Event.
+//! Callable's INVOKE operation must be noexcept.
+//!
+//! \par Extends
+//! Callable
+//!
+//! \par Requirements
+//! Given:\n
+//! E an implementation of the Event concept,
+//! e and instance of E,
+//! H an implementation of the EventHandler concept handling events of type E,
+//! h an instance of H
+//!
+//! |Expression                      | Requirements           |
+//! |:-------------------------------|:-----------------------|
+//! |h(e)                            | well-formed            |
+//! |noexcept(h(e)) == true          | h(e) must be noexcept  |
+//! \}
+
+namespace detail {
+//! Adapts an instance of the Event concept into the
+//! class hierarchy rooted at mpm::detail::event
+template<typename Event>
+class adapted_event : public detail::event {
+public:
+    using dispatch_as = detail::typelist<Event>;
+
+    adapted_event(const Event &event) : m_event{event} {}
+
+    operator const Event &() const { return m_event; }
+
+private:
+    const Event &m_event;
+};
+
+//! Holds the subscriber event type and the handler instance
+//! in a type-erased manner so that they can be put into a
+//! homogeneous container (e.g. the std::unordered_multimap as below)
+class subscription {
+public:
+    using id_t = std::intptr_t;
+
+    template<typename E, typename H, typename Alloc>
+    subscription(const H &handler, const Alloc &alloc, E *)
+        : m_self{std::allocate_shared<model<E, H>>(alloc, handler)}
+    {}
+
+    void deliver(const event &e) { m_self->deliver(e); }
+
+    id_t id() const { return reinterpret_cast<std::intptr_t>(m_self.get()); }
+
+private:
+    struct concept
+    {
+        virtual ~concept(){};
+        virtual void deliver(const event &e) = 0;
+    };
+
+    // base template for events that extend detail::event (i.e. for
+    // polymorphic dispatchable events). These can be safely
+    // static_cast to the subscribed event type
+    template<typename E, typename H, typename Enable = void>
+    struct model : concept {
+        explicit model(H h) : handler(std::move(h)) {}
+
+        void deliver(const event &e) final
+        {
+            handler(static_cast<const E &>(e));
+        }
+
+        H handler;
+    };
+
+    // Specialization for events that do not use
+    // enable_polymorphic_dispatch. The diffence is that we static_cast
+    // to adapted_event<E>.
+    template<typename E, typename H>
+    struct model<E,
+                 H,
+                 typename std::enable_if<
+                     !std::is_base_of<detail::event, E>::value>::type>
+        : concept {
+        explicit model(H h) : handler(std::move(h)) {}
+
+        void deliver(const event &e) final
+        {
+            handler(static_cast<const adapted_event<E> &>(e));
+        }
+
+        H handler;
+    };
+
+    std::shared_ptr<concept> m_self;
+};
+
+struct cookie {
+    cookie() = default;
+
+    cookie(subscription::id_t _id, std::type_index _ti) : id(_id), ti(_ti) {}
+
+    subscription::id_t id = 0;
+    std::type_index ti = typeid(std::nullptr_t);
+};
+
+//! Extract a dispatch typelist for an event supplying a dispatch_as
+//! typedef. If the supplied event type is not in the head position of
+//! the dispatch_as typelist then it is prepended to said list
+template<typename E>
+struct dispatch_typelist {
+    // todo C++14 can be std::conditional_t
+    // most derived type goes first
+    using type = typename std::conditional<
+        std::is_same<E, typename E::dispatch_as::head>::value,
+        typename E::dispatch_as,
+        detail::typelist<E, typename E::dispatch_as>>::type;
+};
+
+template<typename SubsMap>
+struct deliver {
+    deliver(const event &e, const SubsMap &subs) : m_subs{subs}, m_event{e} {}
+
+    template<typename T>
+    void operator()()
+    {
+        auto handlers = m_subs.equal_range(std::type_index(typeid(T)));
+        for (auto pos = handlers.first; pos != handlers.second; ++pos) {
+            const_cast<subscription &>(pos->second).deliver(m_event);
+        }
+    }
+
+    const SubsMap &m_subs;
+    const event &m_event;
+};
+
+// not meant to be used as a virtual base class - nothing should
+// destruct via this type
+class unsubscribable {
+public:
+    virtual void unsubscribe(const cookie &c) = 0;
+
+protected:
+    ~unsubscribable() {}
+};
+}// namespace detail
+
+//! A small POD type representing a subscription
+using cookie = detail::cookie;
+
+//! Accepts events from publishers and delivers them to subscribers
+template<typename Allocator>
+class basic_eventbus : public detail::unsubscribable {
+public:
+    //! The type of the Allocator used by this eventbus
+    using allocator_type = Allocator;
+
+    //! Delegates to basic_eventbus(allocator_type) using
+    //! default construction for the allocator_type instance.
+    basic_eventbus();
+
+    //! Constructs an instance using the supplied allocator_type.
+    explicit basic_eventbus(allocator_type alloc);
+
+#ifdef DOCS
+
+    //! Publish an instance of E.
+    //!
+    //! The event instance will be delivered either as only type E or
+    //! if E has been defined with mpm::enable_polymorphic_dispatch it
+    //! will be delivered as every type in its inheritance chain.
+    //!
+    //! \tparam E An instance of the Event concept
+    //! \param event The event to publish
+    //! \returns void
+    template<typename E>
+    void publish(const E &event) noexcept;
+
+#else
+
+    template<typename E>
+    typename std::enable_if<std::is_base_of<detail::event, E>::value>::type
+    publish(const E &event) noexcept;
+
+    template<typename E>
+    typename std::enable_if<!std::is_base_of<detail::event, E>::value>::type
+    publish(const E &event) noexcept;
+
+#endif
+
+    //! Subscribe to instances of Event
+    //!
+    //! The supplied handler will be invoked when events of type Event
+    //! are published or when classes derived from Event that have chosen
+    //! to enable polymorphic dispatch are published.
+    //!
+    //! \tparam Event The type for which to subscribe
+    //! \tparam EventHandler an instance of the EventHandler concept
+    //! \param handler An instance of EventHandler
+    //! \returns A cookie which will allow for this handler to be
+    //!         unsubscribed later via basic_eventbus::unsubscribe
+    template<typename Event, typename EventHandler>
+    cookie subscribe(const EventHandler &handler);
+
+    //! Unsubscribes an event handler
+    //!
+    //! \param c A cookie obtained when basic_eventbus::subscribe was called
+    //! \returns void
+    void unsubscribe(const cookie &c) override;
+
+private:
+    using subscriptions =
+        leftright<std::unordered_multimap<std::type_index,
+                                          detail::subscription,
+                                          std::hash<std::type_index>,
+                                          std::equal_to<std::type_index>,
+                                          allocator_type>>;
+    allocator_type m_alloc;
+    subscriptions m_subscriptions;
+};
+
+//! Default eventbus uses std::allocator<char>
+using eventbus = basic_eventbus<std::allocator<char>>;
+
+template<typename A>
+basic_eventbus<A>::basic_eventbus() : basic_eventbus(allocator_type())
+{}
+
+template<typename A>
+basic_eventbus<A>::basic_eventbus(allocator_type alloc)
+    : m_alloc{std::move(alloc)},
+      m_subscriptions{in_place, alloc}
+{}
+
+#ifndef DOCS
+
+template<typename A>
+template<typename Event>
+typename std::enable_if<std::is_base_of<detail::event, Event>::value>::type
+basic_eventbus<A>::publish(const Event &event) noexcept
+{
+    using types = typename detail::dispatch_typelist<Event>::type;
+    m_subscriptions.observe([&](typename subscriptions::const_reference subs) {
+        detail::for_each_type<types>(
+            detail::deliver<decltype(subs)>{event, subs});
+    });
+}
+
+template<typename A>
+template<typename Event>
+typename std::enable_if<!std::is_base_of<detail::event, Event>::value>::type
+basic_eventbus<A>::publish(const Event &event) noexcept
+{
+    publish(detail::adapted_event<Event>{event});
+}
+
+#endif
+
+template<typename A>
+template<typename Event, typename EventHandler>
+cookie
+basic_eventbus<A>::subscribe(const EventHandler &handler)
+{
+    static_assert(std::is_object<Event>::value, "Events must be object types");
+    static_assert(noexcept(handler(std::declval<const Event &>())),
+                  "Need noexcept handler for Event");
+
+    Event *ptr{};
+    detail::subscription sub{handler, m_alloc, ptr};
+    return m_subscriptions.modify(
+        [&](typename subscriptions::reference subs) noexcept {
+            auto idx = std::type_index(typeid(Event));
+            subs.emplace(idx, sub);
+            return cookie{sub.id(), idx};
+        });
+}
+
+template<typename A>
+void
+basic_eventbus<A>::unsubscribe(const cookie &c)
+{
+    m_subscriptions.modify(
+        [=](typename subscriptions::reference subs) noexcept {
+            auto range = subs.equal_range(c.ti);
+            for (auto pos = range.first; pos != range.second; ++pos) {
+                if (pos->second.id() == c.id) {
+                    subs.erase(pos);
+                    return;
+                }
+            }
+        });
+}
+
+//! An RAII type for eventbus subscriptions
+//!
+//! This type will ensure that the encapsulated eventbus subscription
+//! is released when it goes out of scope.
+//!
+//! \tparam Event the type of event for which this subscription subscribes
+template<typename Event>
+class scoped_subscription {
+public:
+    using event_type = Event;
+
+    //! Construct a scoped_subscription not managing any subscription
+    scoped_subscription() = default;
+
+    //! Move from another scoped_subscription.
+    scoped_subscription(scoped_subscription &&other) noexcept
+        : m_ebus{other.m_ebus},
+          m_cookie{other.m_cookie}
+    {
+        other.m_ebus = nullptr;
+    }
+
+    //! Copying a scoped_subscription is prohibited
+    scoped_subscription(const scoped_subscription &) = delete;
+
+    //! Move-assign from another scoped_subscription
+    scoped_subscription &operator=(scoped_subscription &&other) noexcept
+    {
+        reset();
+        m_ebus = other.m_ebus;
+        m_cookie = other.m_cookie;
+        other.m_ebus = nullptr;
+    }
+
+    void swap(scoped_subscription &other) noexcept
+    {
+        using std::swap;
+        swap(m_ebus, other.m_ebus);
+        swap(m_cookie, other.m_cookie);
+    }
+
+    //! Initializes a subscription that will be usubscribed when
+    //! this object goes out of scope.
+    //!
+    //! Subscription is constructed with ebus.subscribe<Event>(h)
+    //!
+    //! \tparam Alloc The allocator type of the event bus supplied
+    //! \tparam Handler the type of the handler supplied
+    //! \param ebus An instance of basic_eventbus to use for subscribing
+    //! \param h An instance of an event handler
+    template<typename Alloc, typename Handler>
+    scoped_subscription(basic_eventbus<Alloc> &ebus, const Handler &h)
+        : m_ebus{&ebus},
+          m_cookie{ebus.template subscribe<event_type>(h)}
+    {}
+
+    //! Initializes a subscription that will be usubscribed when
+    //! this object goes out of scope.
+    //!
+    //! Subscription is constructed with ebus.subscribe<Event>(h). If
+    //! this object currently manages a subscription, that subscription
+    //! will be cleared as though reset() had been called.
+    //!
+    //! \tparam Alloc The allocator type of the event bus supplied
+    //! \tparam Handler the type of the handler supplied
+    //! \param ebus An instance of basic_eventbus to use for subscribing
+    //! \param h An instance of an event handler
+    template<typename Alloc, typename Handler>
+    void assign(basic_eventbus<Alloc> &ebus, const Handler &h)
+    {
+        reset();
+        m_ebus = &ebus;
+        m_cookie = ebus.template subscribe<event_type>(h);
+    }
+
+    //! Unsubscribes this objects managed subscription if one exists
+    ~scoped_subscription() { reset(); }
+
+    //! Unsubscribes this objects managed subscription if one exists
+    //! \returns void
+    void reset()
+    {
+        if (m_ebus) { m_ebus->unsubscribe(m_cookie); }
+    }
+
+private:
+    friend void swap(scoped_subscription &lhs, scoped_subscription &rhs)
+    {
+        lhs.swap(rhs);
+    }
+
+    detail::unsubscribable *m_ebus = nullptr;//non-owning
+    cookie m_cookie{};
+};
+}// namespace mpm
+
+namespace ulib {
+template<typename T, typename Base = mpm::detail::event>
+using enable_polymorphic_dispatch = mpm::enable_polymorphic_dispatch<T, Base>;
+
+// using eventbus = mpm::basic_eventbus<std::allocator<char>>;
+using eventbus = mpm::basic_eventbus<std::allocator<
+    std::pair<const std::type_index, mpm::detail::subscription>>>;
+
+template<typename EventType>
+using scoped_subscription = mpm::scoped_subscription<EventType>;
+}// namespace ulib

3party/nonstd/optional.h

@@ -2500,7 +2500,7 @@ template<typename T>
 using hash = std::hash<tl::optional<T>>;
 
 // if <= C++14
-#if __cplusplus < 201703L
+// #if __cplusplus < 201703L
 template<typename T>
 using optional = tl::optional<T>;
 
@@ -2530,7 +2530,7 @@ make_optional(std::initializer_list<U> il, Args &&...args)
 }
 
 constexpr tl::nullopt_t nullopt{tl::nullopt};
-#endif
+// #endif
 
 }// namespace ulib
 #endif

CMakeLists.txt

@@ -97,6 +97,7 @@ target_compile_definitions(${PROJECT_NAME} PRIVATE ULIB_LIBRARY_IMPL)
 target_include_directories(
   ${PROJECT_NAME}
   PUBLIC 3party/bnflite
+         3party/eventbus/include
          3party/inja
          3party/mongoose
          3party/nlohmann

tests/3party/eventbus/eventbus_unittest.cpp

@@ -0,0 +1,20 @@
+#include <gtest/gtest.h>
+#include <eventbus.h>
+
+TEST(EventBus, BaseTest)
+{
+    struct Event {
+        int x;
+    };
+
+    ulib::eventbus bus;
+    int calls = 0;
+    ulib::scoped_subscription<Event> ss(bus, [&](const Event &e) noexcept {
+        EXPECT_EQ(calls, e.x);
+        ++calls;
+    });
+    bus.publish(Event{0});
+    EXPECT_EQ(calls, 1);
+    bus.publish(Event{1});
+    EXPECT_EQ(calls, 2);
+}

tests/CMakeLists.txt

@@ -13,6 +13,7 @@ add_executable(
   ulib/system/thread_unittest.cpp
   ulib/system/thread_pool_unittest.cpp
   ulib/system/timer_unittest.cpp
+  3party/eventbus/eventbus_unittest.cpp
   3party/inja/inja_unittest.cpp
   3party/optional/optional_unittest.cpp
   3party/sqlpp11/sqlpp11_unittest.cpp