From 0ed34d2c24135e6037695cd35eccfb06e24f2ef3 Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 01:39:38 -0400
Subject: [PATCH 1/9] remove code related to euml and proto

---
 include/boost/msm/backmp11/state_machine.hpp | 135 -------------------
 1 file changed, 135 deletions(-)

diff --git a/include/boost/msm/backmp11/state_machine.hpp b/include/boost/msm/backmp11/state_machine.hpp
index 132c77dd..151f5054 100644
--- a/include/boost/msm/backmp11/state_machine.hpp
+++ b/include/boost/msm/backmp11/state_machine.hpp
@@ -57,7 +57,6 @@
 
 #include <boost/msm/active_state_switching_policies.hpp>
 #include <boost/msm/row_tags.hpp>
-#include <boost/msm/msm_grammar.hpp>
 #include <boost/msm/back/traits.hpp>
 #include <boost/msm/back/fold_to_list.hpp>
 #include <boost/msm/backmp11/favor_compile_time.hpp>
@@ -116,17 +115,6 @@ typedef ::boost::parameter::parameters<
     >
 > state_machine_signature;
 
-// just here to disable use of proto when not needed
-template <class T, class F,class Enable=void>
-struct make_euml_terminal;
-template <class T,class F>
-struct make_euml_terminal<T,F,typename ::boost::disable_if<has_using_declared_table<F> >::type>
-{};
-template <class T,class F>
-struct make_euml_terminal<T,F,typename ::boost::enable_if<has_using_declared_table<F> >::type>
-    : public proto::extends<typename proto::terminal< boost::msm::state_tag>::type, T, boost::msm::state_domain>
-{};
-
 // library-containing class for state machines.  Pass the actual FSM class as
 // the Concrete parameter.
 // A0=Derived,A1=NoHistory,A2=CompilePolicy,A3=FsmCheckPolicy >
@@ -141,11 +129,6 @@ class state_machine : //public Derived
     public ::boost::parameter::binding<
             typename state_machine_signature::bind<A0,A1,A2,A3,A4>::type, tag::front_end
     >::type
-    , public make_euml_terminal<state_machine<A0,A1,A2,A3,A4>,
-                         typename ::boost::parameter::binding<
-                                    typename state_machine_signature::bind<A0,A1,A2,A3,A4>::type, tag::front_end
-                         >::type
-      >
 {
 public:
     // Create ArgumentPack
@@ -1691,124 +1674,6 @@ class state_machine : //public Derived
          fill_states(this);
     }
 
-     // Construct with the default initial states and some default argument(s)
-#if defined (BOOST_NO_CXX11_RVALUE_REFERENCES)                                      \
-    || defined (BOOST_NO_CXX11_VARIADIC_TEMPLATES)                                  \
-    || defined (BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS)
-     template <class Expr>
-     state_machine
-     (Expr const& expr, typename ::boost::enable_if<typename ::boost::proto::is_expr<Expr>::type >::type* = 0)
-         :Derived()
-         , m_events_queue()
-         , m_deferred_events_queue()
-         , m_history()
-         , m_event_processing(false)
-         , m_is_included(false)
-         , m_visitors()
-         , m_substate_list()
-     {
-         BOOST_MPL_ASSERT_MSG(
-             (::boost::proto::matches<Expr, FoldToList>::value),
-             THE_STATES_EXPRESSION_PASSED_DOES_NOT_MATCH_GRAMMAR,
-             (FoldToList));
-
-         // initialize our list of states with the ones defined in Derived::initial_state
-         ::boost::mpl::for_each< seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> >
-             (init_states(m_states));
-         m_history.set_initial_states(m_states);
-         // create states
-         set_states(expr);
-         fill_states(this);
-     }
-#define MSM_CONSTRUCTOR_HELPER_EXECUTE_SUB(z, n, unused) ARG ## n t ## n
-#define MSM_CONSTRUCTOR_HELPER_EXECUTE(z, n, unused)                                \
-        template <BOOST_PP_ENUM_PARAMS(n, class ARG)>                               \
-        state_machine<A0,A1,A2,A3,A4                                                \
-        >(BOOST_PP_ENUM(n, MSM_CONSTRUCTOR_HELPER_EXECUTE_SUB, ~ ),                 \
-        typename ::boost::disable_if<typename ::boost::proto::is_expr<ARG0>::type >::type* =0 )                \
-        :Derived(BOOST_PP_ENUM_PARAMS(n,t))                                         \
-         ,m_events_queue()                                                          \
-         ,m_deferred_events_queue()                                                 \
-         ,m_history()                                                               \
-         ,m_event_processing(false)                                                 \
-         ,m_is_included(false)                                                      \
-         ,m_visitors()                                                              \
-         ,m_substate_list()                                                         \
-     {                                                                              \
-         ::boost::mpl::for_each< seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> > \
-                        (init_states(m_states));                                    \
-         m_history.set_initial_states(m_states);                                    \
-         fill_states(this);                                                         \
-     }                                                                              \
-        template <class Expr,BOOST_PP_ENUM_PARAMS(n, class ARG)>                    \
-        state_machine<A0,A1,A2,A3,A4                                                \
-        >(Expr const& expr,BOOST_PP_ENUM(n, MSM_CONSTRUCTOR_HELPER_EXECUTE_SUB, ~ ), \
-        typename ::boost::enable_if<typename ::boost::proto::is_expr<Expr>::type >::type* =0 ) \
-        :Derived(BOOST_PP_ENUM_PARAMS(n,t))                                         \
-         ,m_events_queue()                                                          \
-         ,m_deferred_events_queue()                                                 \
-         ,m_history()                                                               \
-         ,m_event_processing(false)                                                 \
-         ,m_is_included(false)                                                      \
-         ,m_visitors()                                                              \
-         ,m_substate_list()                                                         \
-     {                                                                              \
-         BOOST_MPL_ASSERT_MSG(                                                      \
-         ( ::boost::proto::matches<Expr, FoldToList>::value),                       \
-             THE_STATES_EXPRESSION_PASSED_DOES_NOT_MATCH_GRAMMAR,                   \
-             (FoldToList));                                                         \
-         ::boost::mpl::for_each< seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> > \
-                        (init_states(m_states));                                    \
-         m_history.set_initial_states(m_states);                                    \
-         set_states(expr);                                                          \
-         fill_states(this);                                                         \
-     }
-
-     BOOST_PP_REPEAT_FROM_TO(1,BOOST_PP_ADD(BOOST_MSM_CONSTRUCTOR_ARG_SIZE,1), MSM_CONSTRUCTOR_HELPER_EXECUTE, ~)
-#undef MSM_CONSTRUCTOR_HELPER_EXECUTE
-#undef MSM_CONSTRUCTOR_HELPER_EXECUTE_SUB
-
-#else
-    template <class ARG0,class... ARG,class=typename ::boost::disable_if<typename ::boost::proto::is_expr<ARG0>::type >::type>
-    state_machine(ARG0&& t0,ARG&&... t)
-    :Derived(std::forward<ARG0>(t0), std::forward<ARG>(t)...)
-     ,m_events_queue()
-     ,m_deferred_events_queue()
-     ,m_history()
-     ,m_event_processing(false)
-     ,m_is_included(false)
-     ,m_visitors()
-     ,m_substate_list()
-     {
-         ::boost::mpl::for_each< seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> >
-                        (init_states(m_states));
-         m_history.set_initial_states(m_states);
-         fill_states(this);
-     }
-    template <class Expr,class... ARG,class=typename ::boost::enable_if<typename ::boost::proto::is_expr<Expr>::type >::type>
-    state_machine(Expr const& expr,ARG&&... t)
-    :Derived(std::forward<ARG>(t)...)
-     ,m_events_queue()
-     ,m_deferred_events_queue()
-     ,m_history()
-     ,m_event_processing(false)
-     ,m_is_included(false)
-     ,m_visitors()
-     ,m_substate_list()
-     {
-         BOOST_MPL_ASSERT_MSG(
-         ( ::boost::proto::matches<Expr, FoldToList>::value),
-             THE_STATES_EXPRESSION_PASSED_DOES_NOT_MATCH_GRAMMAR,
-             (FoldToList));
-         ::boost::mpl::for_each< seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> >
-                        (init_states(m_states));
-         m_history.set_initial_states(m_states);
-         set_states(expr);
-         fill_states(this);
-     }
-#endif
-
-
      // assignment operator using the copy policy to decide if non_copyable, shallow or deep copying is necessary
      library_sm& operator= (library_sm const& rhs)
      {

From b32663e039a8892930ee9616261285b67f25c0b2 Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 01:49:40 -0400
Subject: [PATCH 2/9] Remove other code related to proto

---
 include/boost/msm/backmp11/state_machine.hpp | 19 +------------------
 1 file changed, 1 insertion(+), 18 deletions(-)

diff --git a/include/boost/msm/backmp11/state_machine.hpp b/include/boost/msm/backmp11/state_machine.hpp
index 151f5054..ff7e4ae0 100644
--- a/include/boost/msm/backmp11/state_machine.hpp
+++ b/include/boost/msm/backmp11/state_machine.hpp
@@ -58,7 +58,6 @@
 #include <boost/msm/active_state_switching_policies.hpp>
 #include <boost/msm/row_tags.hpp>
 #include <boost/msm/back/traits.hpp>
-#include <boost/msm/back/fold_to_list.hpp>
 #include <boost/msm/backmp11/favor_compile_time.hpp>
 #include <boost/msm/backmp11/metafunctions.hpp>
 #include <boost/msm/backmp11/history_policies.hpp>
@@ -77,7 +76,6 @@ namespace boost { namespace msm { namespace backmp11
 
 using back::no_fsm_check;
 using back::queue_container_deque;
-using back::FoldToList;
 
 
 // event used internally for wrapping a direct entry
@@ -1637,23 +1635,8 @@ class state_machine : //public Derived
          int* const m_initial_states;
          int m_index;
      };
+
  public:
-     struct update_state
-     {
-         update_state(substate_list& to_overwrite_):to_overwrite(&to_overwrite_){}
-         template<typename StateType>
-         void operator()(StateType const& astate) const
-         {
-             std::get<get_state_id<stt, StateType>::value>(*to_overwrite)=astate;
-         }
-         substate_list* to_overwrite;
-     };
-     template <class Expr>
-     void set_states(Expr const& expr)
-     {
-         ::boost::fusion::for_each(
-             ::boost::fusion::as_vector(FoldToList()(expr, boost::fusion::nil_())),update_state(this->m_substate_list));
-     }
 
     // Construct with the default initial states
     state_machine()

From 3e476ea6fe683673e78704de5c27c0ef60dad1ad Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 02:02:40 -0400
Subject: [PATCH 3/9] exclude backmp11 from eUML tests

---
 test/AnonymousEuml.cpp          | 4 +---
 test/BackCommon.hpp             | 6 ++++++
 test/CompositeEuml.cpp          | 6 +-----
 test/OrthogonalDeferredEuml.cpp | 9 +--------
 test/SerializeSimpleEuml.cpp    | 4 +---
 test/SimpleEuml.cpp             | 4 +---
 test/SimpleEuml2.cpp            | 4 +---
 test/SimpleInternalEuml.cpp     | 4 +---
 8 files changed, 13 insertions(+), 28 deletions(-)

diff --git a/test/AnonymousEuml.cpp b/test/AnonymousEuml.cpp
index cca60a74..240b025c 100644
--- a/test/AnonymousEuml.cpp
+++ b/test/AnonymousEuml.cpp
@@ -9,6 +9,7 @@
 // http://www.boost.org/LICENSE_1_0.txt)
 
 // back-end
+#define BOOST_MSM_TEST_EUML
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
@@ -163,6 +164,3 @@ namespace
 
     }
 }
-
-using backmp11_fsm = boost::msm::backmp11::state_machine<my_machine_, boost::msm::backmp11::favor_compile_time>;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_fsm);
\ No newline at end of file
diff --git a/test/BackCommon.hpp b/test/BackCommon.hpp
index 482e4238..27044b7a 100644
--- a/test/BackCommon.hpp
+++ b/test/BackCommon.hpp
@@ -19,8 +19,11 @@ template<typename Front>
 using get_test_machines = boost::mpl::vector<
     boost::msm::back::state_machine<Front>,
     boost::msm::back::state_machine<Front, boost::msm::back::favor_compile_time>,
+// EUML is not supported by backmp11.
+#ifndef BOOST_MSM_TEST_EUML
     boost::msm::backmp11::state_machine<Front>,
     boost::msm::backmp11::state_machine<Front, boost::msm::backmp11::favor_compile_time>,
+#endif // BOOST_MSM_TEST_EUML
     boost::msm::back11::state_machine<Front>
     >;
 
@@ -28,8 +31,11 @@ template <template <template <typename...> class, typename = void> class hierarc
 using get_hierarchical_test_machines = boost::mpl::vector<
     hierarchical<boost::msm::back::state_machine>,
     hierarchical<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>,
+// EUML is not supported by backmp11.
+#ifndef BOOST_MSM_TEST_EUML
     hierarchical<boost::msm::backmp11::state_machine>,
     hierarchical<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>,
+#endif // BOOST_MSM_TEST_EUML
     hierarchical<boost::msm::back11::state_machine>
 >;
 
diff --git a/test/CompositeEuml.cpp b/test/CompositeEuml.cpp
index 343e33d1..dfca324a 100644
--- a/test/CompositeEuml.cpp
+++ b/test/CompositeEuml.cpp
@@ -8,6 +8,7 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#define BOOST_MSM_TEST_EUML
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
@@ -289,8 +290,3 @@ namespace
 using back0 = hierarchical_state_machine<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>::player;
 using back1 = hierarchical_state_machine<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>::Playing_type;
 BOOST_MSM_BACK_GENERATE_PROCESS_EVENT(back1);
-
-using backmp11_0 = hierarchical_state_machine<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>::player;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_0);
-using backmp11_1 = hierarchical_state_machine<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>::Playing_type;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_1);
\ No newline at end of file
diff --git a/test/OrthogonalDeferredEuml.cpp b/test/OrthogonalDeferredEuml.cpp
index d51d2c8a..5ed0cded 100644
--- a/test/OrthogonalDeferredEuml.cpp
+++ b/test/OrthogonalDeferredEuml.cpp
@@ -155,9 +155,7 @@ namespace
     // Pick a back-end
     typedef boost::mpl::vector<
         hierarchical_state_machine<boost::msm::back::state_machine>,
-        hierarchical_state_machine<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>,
-        hierarchical_state_machine<boost::msm::backmp11::state_machine>,
-        hierarchical_state_machine<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>
+        hierarchical_state_machine<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>
         // TODO:
         // Investigate reason for test failure in back11.
         // hierarchical_state_machine<boost::msm::back11::state_machine>
@@ -375,8 +373,3 @@ namespace
 using back0 = hierarchical_state_machine<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>::player;
 using back1 = hierarchical_state_machine<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>::Playing_type;
 BOOST_MSM_BACK_GENERATE_PROCESS_EVENT(back1);
-
-using backmp11_0 = hierarchical_state_machine<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>::player;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_0);
-using backmp11_1 = hierarchical_state_machine<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>::Playing_type;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_1);
\ No newline at end of file
diff --git a/test/SerializeSimpleEuml.cpp b/test/SerializeSimpleEuml.cpp
index 537fff64..4ee57cc4 100644
--- a/test/SerializeSimpleEuml.cpp
+++ b/test/SerializeSimpleEuml.cpp
@@ -8,6 +8,7 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#define BOOST_MSM_TEST_EUML
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
@@ -213,6 +214,3 @@ namespace
 // at the risk of a programming error creating duplicate objects.
 // this is to get rid of warning because p is not const
 // BOOST_CLASS_TRACKING(player, boost::serialization::track_never)
-
-using backmp11_fsm = boost::msm::backmp11::state_machine<player_, boost::msm::backmp11::favor_compile_time>;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_fsm);
diff --git a/test/SimpleEuml.cpp b/test/SimpleEuml.cpp
index 3f728a67..e2097e07 100644
--- a/test/SimpleEuml.cpp
+++ b/test/SimpleEuml.cpp
@@ -8,6 +8,7 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#define BOOST_MSM_TEST_EUML
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
@@ -193,6 +194,3 @@ namespace
                             "Stopped entry not called correctly");
     }
 }
-
-using backmp11_fsm = boost::msm::backmp11::state_machine<player_, boost::msm::backmp11::favor_compile_time>;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_fsm);
\ No newline at end of file
diff --git a/test/SimpleEuml2.cpp b/test/SimpleEuml2.cpp
index a378a26e..614d2308 100644
--- a/test/SimpleEuml2.cpp
+++ b/test/SimpleEuml2.cpp
@@ -8,6 +8,7 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#define BOOST_MSM_TEST_EUML
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
@@ -193,6 +194,3 @@ namespace
                             "Stopped entry not called correctly");
     }
 }
-
-using backmp11_fsm = boost::msm::backmp11::state_machine<player_, boost::msm::backmp11::favor_compile_time>;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_fsm);
\ No newline at end of file
diff --git a/test/SimpleInternalEuml.cpp b/test/SimpleInternalEuml.cpp
index 17b48d8b..459a1797 100644
--- a/test/SimpleInternalEuml.cpp
+++ b/test/SimpleInternalEuml.cpp
@@ -8,6 +8,7 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#define BOOST_MSM_TEST_EUML
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 #ifndef BOOST_MSM_NONSTANDALONE_TEST
@@ -251,6 +252,3 @@ namespace
                             "Stopped entry not called correctly");
     }
 }
-
-using backmp11_fsm = boost::msm::backmp11::state_machine<player_, boost::msm::backmp11::favor_compile_time>;
-BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(backmp11_fsm);
\ No newline at end of file

From ed52c1089ee7ab3fee14bab5fe2f913546e17a72 Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 02:10:16 -0400
Subject: [PATCH 4/9] update README

---
 include/boost/msm/backmp11/README.md | 41 ++++++++++++++++++----------
 1 file changed, 27 insertions(+), 14 deletions(-)

diff --git a/include/boost/msm/backmp11/README.md b/include/boost/msm/backmp11/README.md
index 03ca5146..09359f26 100644
--- a/include/boost/msm/backmp11/README.md
+++ b/include/boost/msm/backmp11/README.md
@@ -1,13 +1,27 @@
 # Boost MSM backmp11 backend
 
-The new backend `backmp11` is a new backwards-compatible backend that has the following goals:
+This README file is temporary and contains information about `backmp11`, a new backend that is mostly backwards-compatible with `back`. It is currently in **experimental** stage, thus some details about the compatibility might change.
+
+This file's contents should eventually move into the MSM documentation.
+
+The new backend has the following goals:
 
 - reduce compilation runtime and RAM usage
 - reduce state machine runtime
 
-It is named after the metaprogramming library Boost Mp11, the main contributor to the optimizations:
-It replaces usages of MPL with Mp11 to get rid of the C++03 emulation of variadic templates.
-This backend contains additional optimizations that are further described below.
+It is named after the metaprogramming library Boost Mp11, the main contributor to the optimizations. Usages of MPL are replaced with Mp11 to get rid of the costly C++03 emulation of variadic templates.
+
+
+## Breaking changes
+
+### The targeted minimum C++ version is C++17
+
+For the sake of compiler features such as `if constexpr`.
+
+
+### The eUML frontend is not supported
+
+The support of EUML induces longer compilation times by the need to include the Boost proto headers and applying C++03 variadic template emulation. If you want to use a UML-like syntax, please try out the new PUML frontend.
 
 
 ## How to use it
@@ -20,20 +34,20 @@ When using the `favor_compile_time` policy, a different macro to generate missin
 - use `BOOST_MSM_BACKMP11_GENERATE_DISPATCH_TABLE(<fsmname>)` in place of `BOOST_MSM_BACK_GENERATE_PROCESS_EVENT(<fsmname>)`
 
 
-## General optimizations
+## Applied optimizations
 
-- Replacement of CPU-intensive calls due to C++03 recursion from MPL to Mp11
+- Replacement of CPU-intensive calls (due to C++03 recursion from MPL) with Mp11
 - Applied type punning where useful (to reduce template instantiations, e.g. std::deque & other things around the dispatch table)
 
 
-## Optimizations applied to the `favor_runtime_speed` policy
+### `favor_runtime_speed` policy
 
 Summary:
 - Optimized cell initialization with initializer arrays (to reduce template instantiations)
-- Default-initialized everything and afterwards only defer transition cells
+- Default-initialized everything and afterwards only the defer transition cells
 
 
-## Optimizations applied to the `favor_compile_time` policy
+### `favor_compile_time` policy
 
 Once an event is given to the FSM for processing, it is immediately converted to `any` and processing continues with this `any` event.
 The structure of the dispatch table has been reworked, one dispatch table is created per state as a hash map.
@@ -44,14 +58,13 @@ The new mechanism renders the `process_any_event` function obsolete and enables
 
 Summary:
 - Use one dispatch table per state to reduce compiler processing time
-  - The algorithms for procesing the STT and states are optimized to go through rows and states only once
+  - The algorithms for processing the STT and states are optimized to go through rows and states only once
   - These dispatch tables are hash tables with type_id as key
-- Apply type erasure with boost::any as early as possible and do further processing only with any events
+- Apply type erasure with `any` as early as possible and do further processing only with any events
   - each dispatch table only has to cover the events it's handling, no template instantiations required for forwarding events to sub-SMs
-- Use `std::any` if C++17 is available (up to 30% runtime impact because of small value optimization in `std::any`)
 
 
-## Learnings:
+### Learnings:
 
 - If only a subset needs to be processed, prefer copy_if & transform over fold
-- Selecting a template-based function overload in Mp11 seems more efficient than using enable_if/disable_if
+- Selecting a template-based function overload in Mp11 seems more efficient than using enable_if/disable_if
\ No newline at end of file

From a63c292bb3d772f07860b27cbc7b12232ed48d89 Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 02:24:06 -0400
Subject: [PATCH 5/9] add breaking change about default-construction to README
 and remove BOOST_MSM_CONSTRUCTOR_ARG_SIZE

---
 include/boost/msm/back11/state_machine.hpp | 4 ----
 include/boost/msm/backmp11/README.md       | 7 +++++++
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/boost/msm/back11/state_machine.hpp b/include/boost/msm/back11/state_machine.hpp
index 58b8a137..0cef08ee 100644
--- a/include/boost/msm/back11/state_machine.hpp
+++ b/include/boost/msm/back11/state_machine.hpp
@@ -70,10 +70,6 @@
 #include <boost/msm/back/queue_container_deque.hpp>
 #include <boost/msm/back11/dispatch_table.hpp>
 
-#ifndef BOOST_MSM_CONSTRUCTOR_ARG_SIZE
-#define BOOST_MSM_CONSTRUCTOR_ARG_SIZE 5 // default max number of arguments for constructors
-#endif
-
 namespace boost { namespace msm { namespace back11
 {
 // event used internally for wrapping a direct entry
diff --git a/include/boost/msm/backmp11/README.md b/include/boost/msm/backmp11/README.md
index 09359f26..9b92827c 100644
--- a/include/boost/msm/backmp11/README.md
+++ b/include/boost/msm/backmp11/README.md
@@ -24,6 +24,13 @@ For the sake of compiler features such as `if constexpr`.
 The support of EUML induces longer compilation times by the need to include the Boost proto headers and applying C++03 variadic template emulation. If you want to use a UML-like syntax, please try out the new PUML frontend.
 
 
+### A state machine's frontend must be default-constructible
+
+TODO:
+- Double-check that removal is not an issue, check available alternatives post-construction time
+- Provide rationale (proto headers, limitations for hierarchical state machines(?))
+
+
 ## How to use it
 
 The backend and both its policies `favor_runtime_speed` and `favor_compile_time` should be compatible with existing code. Required replacements to try it out:

From 6f1da3908f1c6e37ff5b4e098c0a3e8c14fb882d Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 17:05:46 -0400
Subject: [PATCH 6/9] keep a constructor with forwarding to the frontend,
 deactivate set_states tests for backmp11 and add separate test case for the
 constructor

---
 include/boost/msm/backmp11/README.md         |  25 +-
 include/boost/msm/backmp11/state_machine.hpp |  25 +-
 test/AnonymousEuml.cpp                       |   3 +-
 test/BackCommon.hpp                          |  12 +-
 test/Backmp11TestConstructor.cpp             | 295 +++++++++++++++++++
 test/CompositeEuml.cpp                       |   3 +-
 test/SerializeSimpleEuml.cpp                 |   3 +-
 test/SetStates.cpp                           |   2 +
 test/SimpleEuml.cpp                          |   3 +-
 test/SimpleEuml2.cpp                         |   3 +-
 test/SimpleInternalEuml.cpp                  |   3 +-
 test/TestConstructor.cpp                     |   2 +
 12 files changed, 346 insertions(+), 33 deletions(-)
 create mode 100644 test/Backmp11TestConstructor.cpp

diff --git a/include/boost/msm/backmp11/README.md b/include/boost/msm/backmp11/README.md
index 9b92827c..acf28cca 100644
--- a/include/boost/msm/backmp11/README.md
+++ b/include/boost/msm/backmp11/README.md
@@ -1,8 +1,6 @@
 # Boost MSM backmp11 backend
 
-This README file is temporary and contains information about `backmp11`, a new backend that is mostly backwards-compatible with `back`. It is currently in **experimental** stage, thus some details about the compatibility might change.
-
-This file's contents should eventually move into the MSM documentation.
+This README file is temporary and contains information about `backmp11`, a new backend that is mostly backwards-compatible with `back`. It is currently in **experimental** stage, thus some details about the compatibility might change (feedback welcome!). This file's contents should eventually move into the MSM documentation.
 
 The new backend has the following goals:
 
@@ -12,11 +10,21 @@ The new backend has the following goals:
 It is named after the metaprogramming library Boost Mp11, the main contributor to the optimizations. Usages of MPL are replaced with Mp11 to get rid of the costly C++03 emulation of variadic templates.
 
 
+## New features
+
+
+## Resolved limitations
+
+### Forwarding constructor arguments to the frontend 
+
+The number of max. constructor arguments is not limited anymore to 'BOOST_MSM_CONSTRUCTOR_ARG_SIZE' and references can be passed directly, there is no need to wrap them with 'boost::ref'.
+
+
 ## Breaking changes
 
 ### The targeted minimum C++ version is C++17
 
-For the sake of compiler features such as `if constexpr`.
+C++11 brings the strongly needed variadic template support for MSM, but later C++ versions provide other important features - for example C++17's `if constexpr`.
 
 
 ### The eUML frontend is not supported
@@ -24,11 +32,12 @@ For the sake of compiler features such as `if constexpr`.
 The support of EUML induces longer compilation times by the need to include the Boost proto headers and applying C++03 variadic template emulation. If you want to use a UML-like syntax, please try out the new PUML frontend.
 
 
-### A state machine's frontend must be default-constructible
+### The backend's constructor does not allow initialization of states and `set_states` is not available
+
+There were some caveats with one constructor that was used for different use cases: On the one hand some arguments were immediately forwarded to the frontend's constructor, on the other hand the stream operator was used to identify other arguments in the constructor as states, to copy them into the state machine. Besides the syntax of the later being rather unusual, when doing both at once the syntax becomes too difficult to understand; even more so if states within hierarchical sub state machines were initialized in this fashion.
 
-TODO:
-- Double-check that removal is not an issue, check available alternatives post-construction time
-- Provide rationale (proto headers, limitations for hierarchical state machines(?))
+In order to keep API of the constructor simpler and less ambiguous, it only supports forwarding arguments to the frontend and no more.
+Also the `set_states` API is removed. If setting a state is required, this can still be done (in a little more verbose, but also more direct & explicit fashion) by getting a reference to the desired state via `get_state` and then assigning the desired new state to it.
 
 
 ## How to use it
diff --git a/include/boost/msm/backmp11/state_machine.hpp b/include/boost/msm/backmp11/state_machine.hpp
index ff7e4ae0..3123f26c 100644
--- a/include/boost/msm/backmp11/state_machine.hpp
+++ b/include/boost/msm/backmp11/state_machine.hpp
@@ -1638,24 +1638,25 @@ class state_machine : //public Derived
 
  public:
 
-    // Construct with the default initial states
-    state_machine()
-         :Derived()
-         ,m_events_queue()
-         ,m_deferred_events_queue()
-         ,m_history()
-         ,m_event_processing(false)
-         ,m_is_included(false)
-         ,m_visitors()
-         ,m_substate_list()
-    {
+    // Construct with the default initial states and forward constructor arguments to the frontend.
+    template <typename... Args>
+    state_machine(Args&&... args)
+    : Derived(std::forward<Args>(args)...)
+      ,m_events_queue()
+      ,m_deferred_events_queue()
+      ,m_history()
+      ,m_event_processing(false)
+      ,m_is_included(false)
+      ,m_visitors()
+      ,m_substate_list()
+     {
          // initialize our list of states with the ones defined in Derived::initial_state
          ::boost::mpl::for_each< seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> >
                         (init_states(m_states));
          m_history.set_initial_states(m_states);
          // create states
          fill_states(this);
-    }
+     }
 
      // assignment operator using the copy policy to decide if non_copyable, shallow or deep copying is necessary
      library_sm& operator= (library_sm const& rhs)
diff --git a/test/AnonymousEuml.cpp b/test/AnonymousEuml.cpp
index 240b025c..3d738049 100644
--- a/test/AnonymousEuml.cpp
+++ b/test/AnonymousEuml.cpp
@@ -9,7 +9,8 @@
 // http://www.boost.org/LICENSE_1_0.txt)
 
 // back-end
-#define BOOST_MSM_TEST_EUML
+// EUML is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
diff --git a/test/BackCommon.hpp b/test/BackCommon.hpp
index 27044b7a..74fc5d90 100644
--- a/test/BackCommon.hpp
+++ b/test/BackCommon.hpp
@@ -19,23 +19,21 @@ template<typename Front>
 using get_test_machines = boost::mpl::vector<
     boost::msm::back::state_machine<Front>,
     boost::msm::back::state_machine<Front, boost::msm::back::favor_compile_time>,
-// EUML is not supported by backmp11.
-#ifndef BOOST_MSM_TEST_EUML
+#if !defined(BOOST_MSM_TEST_SKIP_BACKMP11)
     boost::msm::backmp11::state_machine<Front>,
     boost::msm::backmp11::state_machine<Front, boost::msm::backmp11::favor_compile_time>,
-#endif // BOOST_MSM_TEST_EUML
+#endif // BOOST_MSM_TEST_SKIP_BACKMP11
     boost::msm::back11::state_machine<Front>
     >;
 
 template <template <template <typename...> class, typename = void> class hierarchical>
 using get_hierarchical_test_machines = boost::mpl::vector<
-    hierarchical<boost::msm::back::state_machine>,
+hierarchical<boost::msm::back::state_machine>,
     hierarchical<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>,
-// EUML is not supported by backmp11.
-#ifndef BOOST_MSM_TEST_EUML
+#if !defined(BOOST_MSM_TEST_SKIP_BACKMP11)
     hierarchical<boost::msm::backmp11::state_machine>,
     hierarchical<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>,
-#endif // BOOST_MSM_TEST_EUML
+#endif // BOOST_MSM_TEST_SKIP_BACKMP11
     hierarchical<boost::msm::back11::state_machine>
 >;
 
diff --git a/test/Backmp11TestConstructor.cpp b/test/Backmp11TestConstructor.cpp
new file mode 100644
index 00000000..aa2353e7
--- /dev/null
+++ b/test/Backmp11TestConstructor.cpp
@@ -0,0 +1,295 @@
+// Copyright 2010 Christophe Henry
+// henry UNDERSCORE christophe AT hotmail DOT com
+// This is an extended version of the state machine available in the boost::mpl library
+// Distributed under the same license as the original.
+// Copyright for the original version:
+// Copyright 2005 David Abrahams and Aleksey Gurtovoy. Distributed
+// under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+// back-end
+#include "BackCommon.hpp"
+//front-end
+#include <boost/msm/front/state_machine_def.hpp>
+#ifndef BOOST_MSM_NONSTANDALONE_TEST
+#define BOOST_TEST_MODULE test_constructor
+#endif
+#include <boost/test/unit_test.hpp>
+#include <boost/config.hpp>
+
+namespace msm = boost::msm;
+namespace mpl = boost::mpl;
+
+
+namespace
+{
+    struct SomeExternalContext
+    {
+        SomeExternalContext(int b):bla(b){}
+        int bla;
+    };
+    // events
+    struct play {};
+    struct end_pause {};
+    struct stop {};
+    struct pause {};
+    struct open_close {};
+    struct NextSong {};
+    struct PreviousSong {};
+
+    // A "complicated" event type that carries some data.
+    enum DiskTypeEnum
+    {
+        DISK_CD=0,
+        DISK_DVD=1
+    };
+    struct cd_detected
+    {
+        cd_detected(std::string name, DiskTypeEnum diskType)
+            : name(name),
+            disc_type(diskType)
+        {}
+
+        std::string name;
+        DiskTypeEnum disc_type;
+    };
+
+    template<template <typename...> class Back, typename Policy = void>
+    struct hierarchical_state_machine
+    {
+    // front-end: define the FSM structure 
+    struct player_ : public msm::front::state_machine_def<player_>
+    {
+        BOOST_MSM_TEST_DEFINE_DEPENDENT_TEMPLATES(player_)
+
+        player_(SomeExternalContext& context,int someint)
+            :context_(context)
+        {
+            BOOST_CHECK_MESSAGE(context_.bla == 3,"Wrong context value");
+            BOOST_CHECK_MESSAGE(someint == 5,"Wrong int value");
+            context.bla = 10;
+        }
+
+        SomeExternalContext& context_;
+
+        // The list of FSM states
+        struct Empty : public msm::front::state<> 
+        {
+            int data_;
+            Empty():data_(0){}
+            Empty(int i):data_(i){}
+            // every (optional) entry/exit methods get the event passed.
+            template <class Event,class FSM>
+            void on_entry(Event const&,FSM& ) {std::cout << "entering: Empty" << std::endl;}
+            template <class Event,class FSM>
+            void on_exit(Event const&,FSM& ) {std::cout << "leaving: Empty" << std::endl;}
+        };
+        struct Open : public msm::front::state<> 
+        {	 
+            int data_;
+            Open():data_(0){}
+            Open(int i):data_(i){}
+
+            template <class Event,class FSM>
+            void on_entry(Event const& ,FSM&) {std::cout << "entering: Open" << std::endl;}
+            template <class Event,class FSM>
+            void on_exit(Event const&,FSM& ) {std::cout << "leaving: Open" << std::endl;}
+        };
+
+        // sm_ptr still supported but deprecated as functors are a much better way to do the same thing
+        struct Stopped : public msm::front::state<msm::front::default_base_state,msm::front::sm_ptr> 
+        {	 
+            template <class Event,class FSM>
+            void on_entry(Event const& ,FSM&) {std::cout << "entering: Stopped" << std::endl;}
+            template <class Event,class FSM>
+            void on_exit(Event const&,FSM& ) {std::cout << "leaving: Stopped" << std::endl;}
+            void set_sm_ptr(player_* pl)
+            {
+                m_player=pl;
+            }
+            player_* m_player;
+        };
+
+        struct Playing_ : public msm::front::state_machine_def<Playing_>
+        {
+            BOOST_MSM_TEST_DEFINE_DEPENDENT_TEMPLATES(Playing_)
+
+            // when playing, the CD is loaded and we are in either pause or playing (duh)
+            template <class Event,class FSM>
+            void on_entry(Event const&,FSM& ) {std::cout << "entering: Playing" << std::endl;}
+            template <class Event,class FSM>
+            void on_exit(Event const&,FSM& ) {std::cout << "leaving: Playing" << std::endl;}
+
+            // The list of FSM states
+            struct Song1 : public msm::front::state<>
+            {
+                int data_;
+                Song1():data_(0){}
+                Song1(int i):data_(i){}
+
+                template <class Event,class FSM>
+                void on_entry(Event const&,FSM& ) {std::cout << "starting: First song" << std::endl;}
+                template <class Event,class FSM>
+                void on_exit(Event const&,FSM& ) {std::cout << "finishing: First Song" << std::endl;}
+
+            };
+            struct Song2 : public msm::front::state<>
+            { 
+                template <class Event,class FSM>
+                void on_entry(Event const&,FSM& ) {std::cout << "starting: Second song" << std::endl;}
+                template <class Event,class FSM>
+                void on_exit(Event const&,FSM& ) {std::cout << "finishing: Second Song" << std::endl;}
+            };
+            struct Song3 : public msm::front::state<>
+            { 
+                template <class Event,class FSM>
+                void on_entry(Event const&,FSM& ) {std::cout << "starting: Third song" << std::endl;}
+                template <class Event,class FSM>
+                void on_exit(Event const&,FSM& ) {std::cout << "finishing: Third Song" << std::endl;}
+            };
+            // the initial state. Must be defined
+            typedef Song1 initial_state;
+            // transition actions
+            void start_next_song(NextSong const&)       { std::cout << "Playing::start_next_song\n"; }
+            void start_prev_song(PreviousSong const&)       { std::cout << "Playing::start_prev_song\n"; }
+            // guard conditions
+
+            typedef Playing_ pl; // makes transition table cleaner
+            // Transition table for Playing
+            struct transition_table : mpl::vector4<
+                //      Start     Event         Next      Action               Guard
+                //    +---------+-------------+---------+---------------------+----------------------+
+                a_row < Song1   , NextSong    , Song2   , &pl::start_next_song                       >,
+                a_row < Song2   , PreviousSong, Song1   , &pl::start_prev_song                       >,
+                a_row < Song2   , NextSong    , Song3   , &pl::start_next_song                       >,
+                a_row < Song3   , PreviousSong, Song2   , &pl::start_prev_song                       >
+                //    +---------+-------------+---------+---------------------+----------------------+
+            > {};
+            // Replaces the default no-transition response.
+            template <class FSM,class Event>
+            void no_transition(Event const& e, FSM&,int state)
+            {
+                std::cout << "no transition from state " << state
+                    << " on event " << typeid(e).name() << std::endl;
+            }
+        };
+        // back-end
+        typedef Back<Playing_, Policy> Playing;
+
+        // state not defining any entry or exit
+        struct Paused : public msm::front::state<>
+        {
+        };
+
+        // the initial state of the player SM. Must be defined
+        typedef Empty initial_state;
+
+        // transition actions
+        void start_playback(play const&)       { std::cout << "player::start_playback\n"; }
+        void open_drawer(open_close const&)    { std::cout << "player::open_drawer\n"; }
+        void close_drawer(open_close const&)   { std::cout << "player::close_drawer\n"; }
+        void store_cd_info(cd_detected const&) { std::cout << "player::store_cd_info\n"; }
+        void stop_playback(stop const&)        { std::cout << "player::stop_playback\n"; }
+        void pause_playback(pause const&)      { std::cout << "player::pause_playback\n"; }
+        void resume_playback(end_pause const&)      { std::cout << "player::resume_playback\n"; }
+        void stop_and_open(open_close const&)  { std::cout << "player::stop_and_open\n"; }
+        void stopped_again(stop const&)	{std::cout << "player::stopped_again\n";}
+        // guard conditions
+        bool good_disk_format(cd_detected const& evt)
+        {
+            // to test a guard condition, let's say we understand only CDs, not DVD
+            if (evt.disc_type != DISK_CD)
+            {
+                std::cout << "wrong disk, sorry" << std::endl;
+                return false;
+            }
+            return true;
+        }
+        // used to show a transition conflict. This guard will simply deactivate one transition and thus
+        // solve the conflict
+        bool auto_start(cd_detected const&)
+        {
+            return false;
+        }
+
+        typedef player_ p; // makes transition table cleaner
+
+        // Transition table for player
+        struct transition_table : mpl::vector<
+            //    Start     Event         Next      Action				 Guard
+            //  +---------+-------------+---------+---------------------+----------------------+
+          a_row < Stopped , play        , Playing , &p::start_playback                         >,
+          a_row < Stopped , open_close  , Open    , &p::open_drawer                            >,
+           _row < Stopped , stop        , Stopped                                              >,
+            //  +---------+-------------+---------+---------------------+----------------------+
+          a_row < Open    , open_close  , Empty   , &p::close_drawer                           >,
+            //  +---------+-------------+---------+---------------------+----------------------+
+          a_row < Empty   , open_close  , Open    , &p::open_drawer                            >,
+            row < Empty   , cd_detected , Stopped , &p::store_cd_info   ,&p::good_disk_format  >,
+            row < Empty   , cd_detected , Playing , &p::store_cd_info   ,&p::auto_start        >,
+            //  +---------+-------------+---------+---------------------+----------------------+
+          a_row < Playing , stop        , Stopped , &p::stop_playback                          >,
+          a_row < Playing , pause       , Paused  , &p::pause_playback                         >,
+          a_row < Playing , open_close  , Open    , &p::stop_and_open                          >,
+            //  +---------+-------------+---------+---------------------+----------------------+
+          a_row < Paused  , end_pause   , Playing , &p::resume_playback                        >,
+          a_row < Paused  , stop        , Stopped , &p::stop_playback                          >,
+          a_row < Paused  , open_close  , Open    , &p::stop_and_open                          >
+            //  +---------+-------------+---------+---------------------+----------------------+
+        > {};
+        // Replaces the default no-transition response.
+        template <class FSM,class Event>
+        void no_transition(Event const& , FSM&,int)
+        {
+            BOOST_FAIL("no_transition called!");
+        }
+    };
+    typedef Back<player_, Policy> player;
+    };
+    // Pick a back-end
+    typedef boost::mpl::vector<
+        hierarchical_state_machine<boost::msm::backmp11::state_machine>,
+        hierarchical_state_machine<boost::msm::backmp11::state_machine, boost::msm::backmp11::favor_compile_time>
+        > test_machines; 
+
+    BOOST_AUTO_TEST_CASE_TEMPLATE( test_constructor, test_machine, test_machines )
+    {
+        typedef typename test_machine::player player;
+        typedef typename test_machine::player_ player_;
+
+        SomeExternalContext ctx(3);
+        player p1(ctx, 5);
+        BOOST_CHECK_MESSAGE(p1.context_.bla == 10,"Wrong returned context value");
+
+        p1.template get_state<typename player_::Empty&>() = typename player_::Empty(1);
+        BOOST_CHECK_MESSAGE(p1.template get_state<typename player_::Empty&>().data_ == 1,"Wrong Empty value");
+
+        p1.template get_state<typename player_::Empty&>() = typename player_::Empty(5);
+        BOOST_CHECK_MESSAGE(p1.template get_state<typename player_::Empty&>().data_ == 5,"Wrong Empty value");
+
+        p1.template get_state<typename player_::Empty&>() = typename player_::Empty(7);
+        p1.template get_state<typename player_::Open&>() = typename player_::Open(2);
+        BOOST_CHECK_MESSAGE(p1.template get_state<typename player_::Empty&>().data_ == 7,"Wrong Empty value");
+        BOOST_CHECK_MESSAGE(p1.template get_state<typename player_::Open&>().data_ == 2,"Wrong Open value");
+
+#if defined(BOOST_MSVC) && BOOST_MSVC >= 1910 && BOOST_MSVC < 1930
+
+// error C2440: '<function-style-cast>': cannot convert from 'const boost::msm::msm_terminal<Expr>' to '`anonymous-namespace'::player_::Playing'
+
+#elif defined(BOOST_CLANG_VERSION) && BOOST_CLANG_VERSION >= 160000 && BOOST_CLANG_VERSION < 170000
+
+// error: ambiguous conversion for functional-style cast from 'const typename boost::proto::detail::enable_binary<deduce_domain, deduce_domain::proto_grammar, boost::mpl::or_<is_extension<const define_states_creation<is_proto_expr> &>, is_extension<Song1>>, boost::proto::tag::shift_left, const define_states_creation<> &, const Song1 &>::type'
+
+#else
+
+        ctx.bla = 3;
+        player p(ctx, 5);
+        p.template get_state<typename player_::Empty&>() = typename player_::Empty(1);
+        p.template get_state<typename player_::Playing&>().template get_state<typename player_::Playing_::Song1&>() = typename player_::Playing_::Song1(8);
+        BOOST_CHECK_MESSAGE(p.template get_state<typename player_::Playing&>().template get_state<typename player_::Playing_::Song1&>().data_ == 8,"Wrong Open value");
+
+#endif
+    }
+}
+
diff --git a/test/CompositeEuml.cpp b/test/CompositeEuml.cpp
index dfca324a..fa9dbb45 100644
--- a/test/CompositeEuml.cpp
+++ b/test/CompositeEuml.cpp
@@ -8,7 +8,8 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
-#define BOOST_MSM_TEST_EUML
+// EUML is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
diff --git a/test/SerializeSimpleEuml.cpp b/test/SerializeSimpleEuml.cpp
index 4ee57cc4..cf6e3140 100644
--- a/test/SerializeSimpleEuml.cpp
+++ b/test/SerializeSimpleEuml.cpp
@@ -8,7 +8,8 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
-#define BOOST_MSM_TEST_EUML
+// EUML is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
diff --git a/test/SetStates.cpp b/test/SetStates.cpp
index 06cd3ea2..a39294a0 100644
--- a/test/SetStates.cpp
+++ b/test/SetStates.cpp
@@ -9,6 +9,8 @@
 // http://www.boost.org/LICENSE_1_0.txt)
 
 // back-end
+// set_states API is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 //front-end
 #include <boost/msm/front/state_machine_def.hpp>
diff --git a/test/SimpleEuml.cpp b/test/SimpleEuml.cpp
index e2097e07..a47fab1d 100644
--- a/test/SimpleEuml.cpp
+++ b/test/SimpleEuml.cpp
@@ -8,7 +8,8 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
-#define BOOST_MSM_TEST_EUML
+// EUML is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
diff --git a/test/SimpleEuml2.cpp b/test/SimpleEuml2.cpp
index 614d2308..64897291 100644
--- a/test/SimpleEuml2.cpp
+++ b/test/SimpleEuml2.cpp
@@ -8,7 +8,8 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
-#define BOOST_MSM_TEST_EUML
+// EUML is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 
diff --git a/test/SimpleInternalEuml.cpp b/test/SimpleInternalEuml.cpp
index 459a1797..ebeaa263 100644
--- a/test/SimpleInternalEuml.cpp
+++ b/test/SimpleInternalEuml.cpp
@@ -8,7 +8,8 @@
 // file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
-#define BOOST_MSM_TEST_EUML
+// EUML is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 #include <boost/msm/front/euml/euml.hpp>
 #ifndef BOOST_MSM_NONSTANDALONE_TEST
diff --git a/test/TestConstructor.cpp b/test/TestConstructor.cpp
index 528196ab..88baf0bc 100644
--- a/test/TestConstructor.cpp
+++ b/test/TestConstructor.cpp
@@ -9,6 +9,8 @@
 // http://www.boost.org/LICENSE_1_0.txt)
 
 // back-end
+// set_states API is not supported by backmp11
+#define BOOST_MSM_TEST_SKIP_BACKMP11
 #include "BackCommon.hpp"
 //front-end
 #include <boost/msm/front/state_machine_def.hpp>

From 523b49a54f42d0441bb13a3a88e0ffce56f0c0cb Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Fri, 19 Sep 2025 17:09:36 -0400
Subject: [PATCH 7/9] revert accidental change in back11

---
 include/boost/msm/back11/state_machine.hpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/boost/msm/back11/state_machine.hpp b/include/boost/msm/back11/state_machine.hpp
index 0cef08ee..58b8a137 100644
--- a/include/boost/msm/back11/state_machine.hpp
+++ b/include/boost/msm/back11/state_machine.hpp
@@ -70,6 +70,10 @@
 #include <boost/msm/back/queue_container_deque.hpp>
 #include <boost/msm/back11/dispatch_table.hpp>
 
+#ifndef BOOST_MSM_CONSTRUCTOR_ARG_SIZE
+#define BOOST_MSM_CONSTRUCTOR_ARG_SIZE 5 // default max number of arguments for constructors
+#endif
+
 namespace boost { namespace msm { namespace back11
 {
 // event used internally for wrapping a direct entry

From 9e94618949ae421df4916d543631f0808187d47c Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Sat, 20 Sep 2025 03:17:57 -0400
Subject: [PATCH 8/9] fix missing indent

---
 test/BackCommon.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/BackCommon.hpp b/test/BackCommon.hpp
index 74fc5d90..621e8411 100644
--- a/test/BackCommon.hpp
+++ b/test/BackCommon.hpp
@@ -28,7 +28,7 @@ using get_test_machines = boost::mpl::vector<
 
 template <template <template <typename...> class, typename = void> class hierarchical>
 using get_hierarchical_test_machines = boost::mpl::vector<
-hierarchical<boost::msm::back::state_machine>,
+    hierarchical<boost::msm::back::state_machine>,
     hierarchical<boost::msm::back::state_machine, boost::msm::back::favor_compile_time>,
 #if !defined(BOOST_MSM_TEST_SKIP_BACKMP11)
     hierarchical<boost::msm::backmp11::state_machine>,

From 500c0a8bd77946bb11331b51b25277f8a0b21d73 Mon Sep 17 00:00:00 2001
From: Christian Granzin <christian.granzin@gmail.com>
Date: Sat, 20 Sep 2025 04:15:19 -0400
Subject: [PATCH 9/9] Add Backmp11TestConstructor.cpp to test Jamfile

---
 test/Jamfile.v2 | 1 +
 1 file changed, 1 insertion(+)

diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 50dd008d..b897e125 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -35,6 +35,7 @@ test-suite msm-unit-tests
     [ run AnonymousAndGuard.cpp ]
     [ run AnonymousEuml.cpp ]
     [ run Back11CompositeMachine.cpp ]
+    [ run Backmp11TestConstructor.cpp ]
     [ run BigWithFunctors.cpp ]
     [ run CompositeEuml.cpp ]
     [ run CompositeMachine.cpp ]