document operator, limitation for two raw MFP actions and show wrap() fix (issue #389)#678
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…oost-ext#389) Two raw member-function-pointer actions combined with comma, / (&C::a, &C::b) silently run only the last action. Raw MFPs are pointer types, not class types, so the C++ standard requires the built-in comma operator to be selected regardless of any user-defined operator, overloads. This is a language limitation that cannot be fixed in the library. The workaround is to wrap the first MFP with sml::wrap(): / (sml::wrap(&C::a), &C::b) wrap() returns a zero_wrapper class type, enabling the user-defined operator, overload that creates a seq_ action sequence. Subsequent actions (raw MFPs, lambdas, or functors) chain correctly from there. Changes: - Add an explanatory comment above operator, in sml.hpp describing the language constraint and the wrap() workaround. - Update example/actions_guards.cpp: replace the single-MFP action on the s5 transition with (wrap(&self::action4), &self::action5) to demonstrate two-MFP sequencing. - Add regression test member_functions_two_mfp_actions_with_wrap in test/ft/transition_table.cpp that verifies both actions execute.
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Fixes #389.
Root cause
/ (&C::action_a, &C::action_b)uses the C++ built-in comma operator instead of the library's action-sequencingoperator,. Raw member-function pointers are pointer types, not class types. The C++ standard only allows user-definedoperator,to be selected when at least one operand is a class or enum type, so the built-in comma always wins for two raw MFPs — silently discarding the first action.This is a C++ language constraint that cannot be removed at the library level.
Workaround:
sml::wrap()the first MFPsml::wrap()returns azero_wrapper<MFP>, which is a class type. That satisfies the requirement and the user-definedoperator,is selected:Subsequent actions in the list (additional MFPs, lambdas, functors) chain correctly from there without extra
wrap()calls, because eachoperator,already produces a class-typedseq_result.Changes
include/boost/sml.hpp: add a comment above the globaloperator,overload explaining the language constraint and thewrap()workaround.example/actions_guards.cpp: replace the single-MFP action on thes5transition with(wrap(&self::action4), &self::action5)to demonstrate two-MFP sequencing.test/ft/transition_table.cpp: addmember_functions_two_mfp_actions_with_wrap— verifies that both actions run when the first MFP is wrapped.