fix: clear sub-SM defer queue when composite state is re-entered (#253)#705
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kris-jusiak merged 2 commits intoMay 30, 2026
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boost-ext#253) Deferred events enqueued inside a sub-SM's lifetime were silently retained across exit. On re-entry the sub-SM's defer_ queue was not cleared by update_composite_states, so any event deferred in a prior session would fire spuriously on the next internal transition that triggers process_defer_events. Root cause: each sm_impl holds its own defer_ (same policy as the root, propagated through TPolicies). update_composite_states resets current_state_[] via initialize() but never touched defer_. Fix: add clear_deferred_events() to sm_impl, tag-dispatched on is_same<defer_t, no_policy> so it compiles for SMs without a defer_queue policy. Call it in both overloads of update_composite_states (history and non-history) so the defer queue is always emptied when a composite state is (re-)entered.
…ransition (boost-ext#643) Users working with sub-SMs sometimes need explicit control over the defer queue — for example, to discard stale events when exiting a sub-SM via a specific transition rather than relying on automatic cleanup. Adds front::actions::clear_defer (exposed as the sml::clear_defer global action object, alongside sml::defer and sml::process), which calls sm_impl::clear_deferred_events() on the current SM when the action fires. Usage: state<Nested> + event<EXIT> / clear_defer = "Init"_s Verified GCC/Clang C++14/17/20 and MSVC 19.51 C++20.
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Commit 1 — fix: clear sub-SM defer queue on composite re-entry (#253)
Root cause: Each
sm_implholds its owndefer_queue (same policy as the root, propagated viaTPolicies).process_defer_eventsreplays events from this queue on the next internal transition inside the sub-SM.update_composite_statesresetscurrent_state_[]viainitialize()when a composite is entered, but never cleareddefer_. Events deferred in a prior session fired spuriously on the first internal transition of the next session.Fix: Added
clear_deferred_events()tosm_impl, tag-dispatched onis_same<defer_t, no_policy>for C++14 compatibility. Called in bothupdate_composite_statesoverloads (history and non-history) so the defer queue is always emptied when a composite state is (re-)entered.Tests:
test/ft/issue_253_defer_clear_on_exit.cpp— 3 cases: stale event must not fire after exit + re-entry + round-trip; normal deferral still works within a session; no spurious fires without prior deferral.Commit 2 — feat: add
sml::clear_deferaction (#643)Exposes the new
clear_deferred_events()infrastructure as a user-accessible action object (sml::clear_defer, alongsidesml::deferandsml::process). Lets users explicitly discard pending deferred events on a specific outgoing transition:state<Nested> + event<EXIT> / clear_defer = "Init"_sTests:
test/ft/issue_643_clear_defer_action.cpp— user-controlled clear discards stale deferred events; no-op on empty queue.Verified: GCC 14 + Clang 18, C++14/17/20 (39–40/40 tests pass). MSVC 19.51 C++20 clean.