From 096413acd19fd19b396057f64e963e21dd7b40f5 Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Mon, 15 Jun 2026 21:28:29 +0200
Subject: [PATCH 1/2] feat(backmp11): event pool configuration

---
 .../ROOT/attachments/backmp11/BackAdapter.cpp |  1 +
 .../backmp11/HeaplessStateMachine.cpp         | 29 +++++++++-------
 doc/modules/ROOT/pages/backmp11-back-end.adoc | 34 ++++++++++++++-----
 .../pages/backmp11-back-end/examples.adoc     |  2 +-
 doc/modules/ROOT/pages/version-history.adoc   |  3 +-
 .../backmp11/detail/event_pool_processor.hpp  | 26 ++++++++------
 .../backmp11/detail/state_machine_base.hpp    |  6 ++--
 .../msm/backmp11/state_machine_config.hpp     | 33 +++++++++++++-----
 test/Backmp11Transitions.cpp                  |  3 +-
 9 files changed, 89 insertions(+), 48 deletions(-)

diff --git a/doc/modules/ROOT/attachments/backmp11/BackAdapter.cpp b/doc/modules/ROOT/attachments/backmp11/BackAdapter.cpp
index 4dfbc55e..e9dcc5ec 100644
--- a/doc/modules/ROOT/attachments/backmp11/BackAdapter.cpp
+++ b/doc/modules/ROOT/attachments/backmp11/BackAdapter.cpp
@@ -81,6 +81,7 @@ struct back_config_adapter : boost::msm::backmp11::state_machine_config
     template <typename T>
     using event_pool_container =
         typename QueueContainerPolicy::template In<T>::type;
+    using event_pool = boost::msm::backmp11::event_pool<event_pool_container>;
 };
 
 template <class A1, class A2, class A3, class A4>
diff --git a/doc/modules/ROOT/attachments/backmp11/HeaplessStateMachine.cpp b/doc/modules/ROOT/attachments/backmp11/HeaplessStateMachine.cpp
index cd1d6b6b..a0c7df6c 100644
--- a/doc/modules/ROOT/attachments/backmp11/HeaplessStateMachine.cpp
+++ b/doc/modules/ROOT/attachments/backmp11/HeaplessStateMachine.cpp
@@ -26,13 +26,26 @@ namespace mp11 = boost::mp11;
 namespace
 {
 
+static constexpr size_t max_event_size = 32;
+
+struct MyConfig : back::state_machine_config
+{
+    // Use a static vector as event pool container.
+    // This container is sufficient for enqueueing and deferring events,
+    // but it does not support completion transitions.
+    // If needed, use a heapless deque implementation (e.g. etl::deque).
+    template <typename T>
+    using static_vector = boost::container::static_vector<T, 10>;
+    using event_pool =
+        back::event_pool</*Container=*/static_vector,
+                         /*InlineCapacity=*/max_event_size>;
+};
+
 // Events.
 struct Greet
 {
     // A heapless event must be copy constructible and nothrow move constructible.
-    // The maximum size depends on alignment requirements and the target system,
-    // 32 bytes should always work.
-    alignas(void*) std::array<char, 32> message{};
+    std::array<char, max_event_size> message{};
 };
 
 // Actions.
@@ -65,16 +78,6 @@ struct MyStateMachine_ : front::state_machine_def<MyStateMachine_>
     >;
 };
 
-struct MyConfig : back::state_machine_config
-{
-    // Use a static vector as event pool container.
-    // This container is sufficient for enqueueing and deferring events,
-    // but it does not support completion transitions.
-    // If needed, use a heapless deque implementation (e.g. etl::deque).
-    template <typename T>
-    using event_pool_container = boost::container::static_vector<T, 10>;
-};
-
 using MyStateMachine = back::state_machine<MyStateMachine_, MyConfig>;
 
 [[maybe_unused]] void heapless_state_machine_example()
diff --git a/doc/modules/ROOT/pages/backmp11-back-end.adoc b/doc/modules/ROOT/pages/backmp11-back-end.adoc
index 574c9559..41259976 100644
--- a/doc/modules/ROOT/pages/backmp11-back-end.adoc
+++ b/doc/modules/ROOT/pages/backmp11-back-end.adoc
@@ -63,9 +63,8 @@ struct default_state_machine_config
     using context = no_context;
     // Optimizes for runtime speed or compile time.
     using compile_policy = favor_runtime_speed;
-    // Configures the container type of the event pool.
-    template <typename T>
-    using event_pool_container = std::deque<T>;
+    // Configures the event pool.
+    using event_pool = backmp11::event_pool<std::deque>;
     // Type of the Fsm parameter passed in actions and guards.
     using fsm_parameter = local_transition_owner;
     // Sets up an observer for monitoring state machine activities.
@@ -174,12 +173,22 @@ When a transition defined in SM1 causes SM2 and SM3 to exit:
 
 === Event pool
 
-The setting `event_pool_container` defines the container type of the state machine's event pool. It requires an event pool to handle event enqueueing, deferral, and completion transitions. The default event pool container is a `std::deque<T>`.
+The setting `event_pool` configures the container and inline storage the back-end uses to hold work that cannot be processed immediately: enqueued events, deferred events, and completion transitions.
 
-The event pool container can be customized with `using event_pool_container = MyEventPoolContainer<T>;`. It has to support push without iterator invalidation, specifically the following API calls:
+```cpp
+template <template <typename> typename Container,
+          size_t InlineCapacity = 32,
+          size_t InlineAlign = alignof(std::max_align_t)>
+struct event_pool;
+```
+
+The default configuration uses `std::deque`. Each pooled event occupies a type-erased slot in the container.
+To avoid a heap allocation per slot, the back-end uses a small-object optimization: events that fit within `InlineCapacity` bytes and `InlineAlign` alignment are stored directly in the slot. Events exceeding either limit fall back to heap allocation.
 
-- `push_back(const T&)` (for event enqueueing and deferral)
-- `push_front(const T&)` (for completion transitions)
+The container must not invalidate existing iterators when `push_back` or `push_front` is called. The required interface is:
+
+- `push_back(const T&)` — for event enqueueing and deferral
+- `push_front(const T&)` — for completion transitions
 - `begin()`
 - `end()`
 - `erase(iterator)`
@@ -187,11 +196,18 @@ The event pool container can be customized with `using event_pool_container = My
 
 [NOTE]
 ====
-By replacing the default `std::deque` with a fixed-capacity container, the state machine can operate entirely without heap allocation.
+By replacing `std::deque` with a fixed-capacity container and setting `InlineCapacity` and `InlineAlign` to cover all event types, the state machine can operate entirely without heap allocation.
 See the xref:backmp11-back-end/examples.adoc#_heapless_state_machine[heapless state machine example] for a demo using `boost::container::static_vector`.
 ====
 
-You can deactivate the event pool with `using event_pool_container = no_event_pool_container<T>;`.
+Use `no_event_pool` to disable the event pool if the state machine does not require event enqueueing, deferral, and completion transitions:
+
+```cpp
+struct MyConfig : state_machine_config
+{
+    using event_pool = no_event_pool;
+};
+```
 
 
 === Compile policy
diff --git a/doc/modules/ROOT/pages/backmp11-back-end/examples.adoc b/doc/modules/ROOT/pages/backmp11-back-end/examples.adoc
index 6b5cfc2e..ef1b9b7f 100644
--- a/doc/modules/ROOT/pages/backmp11-back-end/examples.adoc
+++ b/doc/modules/ROOT/pages/backmp11-back-end/examples.adoc
@@ -10,7 +10,7 @@ It is helpful for utilizing `backmp11` in a legacy code base.
 
 This example demonstrates a state machine configured for heapless execution.
 
-It uses `boost::container::static_vector` as the event pool container — a fixed-capacity replacement for the default `std::deque` container. Note that this container type does not support completion events; use a heapless deque (e.g. `etl::deque`) if those are required.
+It uses `boost::container::static_vector` as the event pool container — a fixed-capacity replacement for the default `std::deque` container. Note that this container type does not support completion transitions; use a heapless deque (e.g. `etl::deque`) if those are required.
 
 
 == xref:attachment$backmp11/Logger.cpp[Logger]
diff --git a/doc/modules/ROOT/pages/version-history.adoc b/doc/modules/ROOT/pages/version-history.adoc
index 79021124..edaad19b 100644
--- a/doc/modules/ROOT/pages/version-history.adoc
+++ b/doc/modules/ROOT/pages/version-history.adoc
@@ -9,12 +9,13 @@
 ** Forward Kleene events to actions and guards without conversion (https://github.com/boostorg/msm/issues/229[#229])
 ** Ensure basic exception guarantee and propagate exceptions to the caller (https://github.com/boostorg/msm/issues/221[#221])
 ** Provide a reflection API and serialization support for Boost.Serialization, Boost.JSON and nlohmann/json (https://github.com/boostorg/msm/issues/197[#197])
+** Configurable event pool and inline storage (https://github.com/boostorg/msm/issues/239[#239])
 * Bug fixes (`backmp11`):
 ** State machine processes events although it is not running (https://github.com/boostorg/msm/issues/198[#198])
 * **Breaking changes (`backmp11`)**:
 ** Events in `process_event(...)` are not enqueued automatically anymore to reduce memory footprint, use `enqueue_event(...)` in actions (https://github.com/boostorg/msm/issues/178[#178])
 ** The back-end does not forward constructor arguments to the front-end anymore, it must be default-constructible. To use custom constructors, define them in the back-end (https://github.com/boostorg/msm/issues/232[#232])
-** The APIs `process_queued_events()` and `process_single_queued_event()` are removed, use `process_event_pool(...)` instead
+** The deprecated APIs `process_queued_events()` and `process_single_queued_event()` are removed, use `process_event_pool(...)` instead
 ** The default active state switch policy is set to "after source exit" in compliance to the UML specification. The other options are not UML-compliant and will be removed in version 1.93 (https://github.com/boostorg/msm/issues/222[#222])
 * Documentation:
 ** API reference is now generated from source with MrDocs
diff --git a/include/boost/msm/backmp11/detail/event_pool_processor.hpp b/include/boost/msm/backmp11/detail/event_pool_processor.hpp
index b7b1393a..fc5f63e1 100644
--- a/include/boost/msm/backmp11/detail/event_pool_processor.hpp
+++ b/include/boost/msm/backmp11/detail/event_pool_processor.hpp
@@ -17,6 +17,7 @@
 
 #include <boost/config.hpp>
 
+#include <boost/msm/backmp11/detail/basic_polymorphic.hpp>
 #include <boost/msm/backmp11/detail/common.hpp>
 #include <boost/msm/backmp11/state_machine_config.hpp>
 
@@ -97,15 +98,23 @@ class deferred_event : public event_occurrence
     Event m_event;
 };
 
-template <template <typename> typename EventPoolContainer,
-          typename ProcessableEvent,
+template <typename Config,
           typename = void>
 class event_pool_processor
 {
+    struct alignas(Config::inline_align) dummy_event
+    {
+        char data[Config::inline_capacity];
+    };
+    using deferred_dummy = deferred_event<dummy_event>;
+
   protected:
     static constexpr bool has_event_pool = true;
-    using processable_event = ProcessableEvent;
-    using event_pool_container_t = EventPoolContainer<processable_event>;
+    using processable_event =
+        basic_polymorphic<event_occurrence, sizeof(deferred_dummy),
+                          alignof(deferred_dummy)>;
+    using event_pool_container_t =
+        typename Config::template container_t<processable_event>;
 
     struct event_pool_t
     {
@@ -178,13 +187,10 @@ class event_pool_processor
     event_pool_t m_event_pool;
 };
 
-template <template <typename> typename EventPoolContainer,
-          typename ProcessableEvent>
+template <typename Config>
 class event_pool_processor<
-    EventPoolContainer,
-    ProcessableEvent,
-    std::enable_if_t<std::is_same_v<EventPoolContainer<ProcessableEvent>,
-                                    no_event_pool_container<ProcessableEvent>>>>
+    Config,
+    std::enable_if_t<std::is_same_v<Config, no_event_pool>>>
 {
   protected:
     static constexpr bool has_event_pool = false;
diff --git a/include/boost/msm/backmp11/detail/state_machine_base.hpp b/include/boost/msm/backmp11/detail/state_machine_base.hpp
index 974e328a..ae25b909 100644
--- a/include/boost/msm/backmp11/detail/state_machine_base.hpp
+++ b/include/boost/msm/backmp11/detail/state_machine_base.hpp
@@ -187,8 +187,7 @@ class observer_member<no_observer, nesting_role::unknown>
 
 template <typename Config, nesting_role NestingRole>
 class state_machine_base
-    : public event_pool_processor<Config::template event_pool_container,
-                                  basic_polymorphic<event_occurrence>>,
+    : public event_pool_processor<typename Config::event_pool>,
       public context_member<typename Config::context, NestingRole>,
       public observer_member<typename Config::observer, NestingRole>
 {
@@ -331,8 +330,7 @@ class state_machine_base
     friend struct transition_table_impl;
 
     using event_pool_processor =
-        detail::event_pool_processor<Config::template event_pool_container,
-                                     basic_polymorphic<event_occurrence>>;
+        detail::event_pool_processor<typename Config::event_pool>;
     using root_sm_base =
         state_machine_base<Config, get_root_nesting_role(NestingRole)>;
 
diff --git a/include/boost/msm/backmp11/state_machine_config.hpp b/include/boost/msm/backmp11/state_machine_config.hpp
index 4ad0f4ea..d82c5232 100644
--- a/include/boost/msm/backmp11/state_machine_config.hpp
+++ b/include/boost/msm/backmp11/state_machine_config.hpp
@@ -87,9 +87,27 @@ struct no_root_sm {};
 /// (see @ref state_machine_config::fsm_parameter).
 struct local_transition_owner {};
 
-/// Deactivates the event pool (see @ref state_machine_config::event_pool_container).
-template <typename T>
-struct no_event_pool_container {};
+/**
+ * @brief Defines the container type and inline storage for the event pool (see
+ * @ref state_machine_config::event_pool).
+ *
+ * @tparam Container Type of the container to use.
+ * @tparam InlineCapacity Max inline storage per event.
+ * @tparam InlineAlign Inline storage alignment.
+ */
+template <template <typename...> typename Container,
+          size_t InlineCapacity = 32,
+          size_t InlineAlign = alignof(std::max_align_t)>
+struct event_pool
+{
+    template <typename T>
+    using container_t = Container<T>;
+    static constexpr size_t inline_capacity = InlineCapacity;
+    static constexpr size_t inline_align = InlineAlign;
+};
+
+/// Deactivates the event pool (see @ref state_machine_config::event_pool).
+struct no_event_pool {};
 
 /// No observer is configured (see @ref state_machine_config::observer).
 struct no_observer {};
@@ -111,14 +129,13 @@ struct default_state_machine_config
      */
     using compile_policy = favor_runtime_speed;
     /**
-     * @brief Configures the container type of the event pool. Defaults to
-     * `std::deque`.
+     * @brief Configures the event pool.
+     * Defaults to `std::deque` and 32 bytes inline storage capacity per event.
      *
-     * The event pool is required to handle deferred events, enqueued events,
+     * The event pool is required to handle enqueued events, deferred events,
      * and completion transitions.
      */
-    template <typename T>
-    using event_pool_container = std::deque<T>;
+    using event_pool = backmp11::event_pool<std::deque>;
     /// Type of the Fsm parameter passed in actions and guards.
     /// Defaults to @ref local_transition_owner.
     using fsm_parameter = local_transition_owner;
diff --git a/test/Backmp11Transitions.cpp b/test/Backmp11Transitions.cpp
index 22c0d0bd..56b6deba 100644
--- a/test/Backmp11Transitions.cpp
+++ b/test/Backmp11Transitions.cpp
@@ -92,8 +92,7 @@ struct StateMachine_;
 struct default_config : state_machine_config
 {
     // using root_sm = StateMachine;
-    template <typename T>
-    using event_pool_container = no_event_pool_container<T>;
+    using event_pool = no_event_pool;
 };
 struct favor_compile_time_config : default_config
 {

From 767a0fa2804329d164631a70068c15cbfb99bd8e Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Tue, 16 Jun 2026 19:08:42 +0200
Subject: [PATCH 2/2] doc(backmp11): fix JSON examples

---
 doc/modules/ROOT/attachments/backmp11/serializer/BoostJson.cpp   | 1 +
 .../ROOT/attachments/backmp11/serializer/NlohmannJson.cpp        | 1 +
 2 files changed, 2 insertions(+)

diff --git a/doc/modules/ROOT/attachments/backmp11/serializer/BoostJson.cpp b/doc/modules/ROOT/attachments/backmp11/serializer/BoostJson.cpp
index 1e9b7e59..c5207fd9 100644
--- a/doc/modules/ROOT/attachments/backmp11/serializer/BoostJson.cpp
+++ b/doc/modules/ROOT/attachments/backmp11/serializer/BoostJson.cpp
@@ -55,6 +55,7 @@ std::string to_boost_json_string(const DimSwitch& dim_switch)
 
     // Turn On (state id 1) and set brightness to 75.
     dim_switch.process_event(TurnOn{});
+    dim_switch.process_event(Dim{75});
     // Prints:
     // {"front_end":{"brightness":75},"states":{"1":{"times_pressed":1}},"active_state_ids":[1],"stopped":false}
     std::cout << to_boost_json_string(dim_switch) << std::endl;
diff --git a/doc/modules/ROOT/attachments/backmp11/serializer/NlohmannJson.cpp b/doc/modules/ROOT/attachments/backmp11/serializer/NlohmannJson.cpp
index a27e2125..e0ef722b 100644
--- a/doc/modules/ROOT/attachments/backmp11/serializer/NlohmannJson.cpp
+++ b/doc/modules/ROOT/attachments/backmp11/serializer/NlohmannJson.cpp
@@ -70,6 +70,7 @@ std::string to_nlohmann_json_string(const DimSwitch& dim_switch)
 
     // Turn On (state id 1) and set brightness to 75.
     dim_switch.process_event(TurnOn{});
+    dim_switch.process_event(Dim{75});
     // Prints:
     // {
     //     "active_state_ids": [