June 2017 - Boost-users - lists.preview.boost.org

[MSM] What happens to MSM if guard returns `false`, mutable events and error recovery?
by Ovanes Markarian 05 Jun '17

05 Jun '17

Hi *, I am currently reading through the MSM docs and have one question, what happens to MSM if guard returns `false`. Does the MSM enter an error state in orthogonal region (http://www.boost.org/doc/ libs/1_64_0/libs/msm/doc/HTML/ch03s02.html#d0e577)? Even internals page describes, what happens if guard returns `true`, but not a single word on `false` case. Regarding the const events: I am programming an MSM to proxy network traffic and once a message with data was received it'll be analysed and moved between different states. Message instances implement move semantics. Currently, the MSM has an initial state `ready` where the `message_in` event is handled. This event contains the message to be analysed. From my current understanding I either have to make message field in `message_in` event mutable or MSM needs to to support r-value references. Can somebody tell me what's going to happen if the action only accepts r-value reference to an event instance? Currently, it seems to work, but would like to understand the implications. struct some_action_impl { template<class Fsm, class Event, class SourceState, class TargetState> void operator()(Event&& ev, Fsm& fsm, SourceState& source, TargetState& target) { ... } } My last question relates to multi-threading. I understand that MSM manages internal states, would it be safe to create MSM as a thread-local storage and restart the thread-local MSM if the new request comes in? It's more about, what happens to MSM after an error state? Is calling stop(), start() members enough? Many thanks for help, Ovanes

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[MSM] exception_caught question
by Ovanes Markarian 05 Jun '17

05 Jun '17

Hi *, I found that answer for exceptions which can occur during the processing: https://lists.boost.org/boost-users/2012/03/73904.php Here is handled the case that exception is derived from std::exception. What happens if not? Can the parameter be changed to std::exception_ptr or just a template parameter? Thanks for help, Ovanes

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[Units] What is the correct/recommended way to handle different ratios in the same unit
by Carel Combrink 05 Jun '17

05 Jun '17

Hi All, I have recently started looking at Boost.Units to use in my applications where unit conversions happens regularly. The conversions are mostly within the same unit, but different ratios, like hertz to killohertz and back (also distances). I have asked for some advice on StackOverflow, but still waiting for an answer to either indicate the correct way or confirm that what I am doing is correct. The mentioned question can be seen here: https://stackoverflow.com/q/44300549/991000 Is there someone that can perhaps look at my question on SO and offer some assistance or advice please? Regards, Carel

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Boost.Parameter python and lazy evaluation of default arguments
by Philipp.Buettgenbach＠gmail.com 05 Jun '17

05 Jun '17

Hello all, I have a question concerning the Boost.Parameter library and the python binding provided by this library. When I try to compile the attached code with clang I get: ------------------------------- /tmp$ clang -I/usr/include/python2.7/ -std=c++14 -c python_test.cpp In file included from python_test.cpp:7: In file included from /usr/include/boost/parameter/preprocessor.hpp:8: In file included from /usr/include/boost/parameter/parameters.hpp:40: /usr/include/boost/parameter/aux_/arg_list.hpp:361:16: error: no viable conversion from 'boost::parameter::aux::maybe<std::basic_string<char> >' to 'const std::basic_string<char>' return arg.value; ^~~~~~~~~ ------------------------------- If I just compile main or use standard default arguments everything compiles fine. Can anybody explain to me what's wrong with using lazy default argument evaluation when exporting to python? Thanks a lot, Philipp

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Source incompatible changes in Boost.Test
by Dominique Devienne 03 Jun '17

03 Jun '17

What's the point of changing a public symbol's case like this, leading to a hard build error? Seems a bit gratuitous, no? And not up to Boost's standards I might add. --DD if (BOOST_VERSION < 106000) boost::unit_test::tut_suite #else boost::unit_test::TUT_SUITE #endif

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[MSM] What happens to MSM if guard returns `false`, mutable events and error recovery?
by Christophe Henry 02 Jun '17

02 Jun '17

Hi, >I am currently reading through the MSM docs and have one question, > what happens to MSM if guard returns `false`. Does the MSM enter an error state > in orthogonal region (http://www.boost.org/doc/libs/1_64_0/libs/msm/doc/HTML/ch03s02.html#d0e577)? >Even internals page describes, what happens if guard returns `true`, >but not a single word on `false` case. What happens is described in http://www.boost.org/doc/libs/1_64_0/libs/msm/doc/HTML/ch02s02.html#d0e226 and in http://www.boost.org/doc/libs/1_64_0/libs/msm/doc/HTML/ch06.html#d0e3013 To sum up, in this case the next guard is tried and the next until no more is left. If all return false, processing in this region is finished. Each region processes independently from others but if one guard rejected the event, this event is seen as processed. >Regarding the const events: I am programming an MSM to proxy network >traffic and once a message with data was received it'll be analysed and >moved between different states. Message instances implement move semantics. >Currently, the MSM has an initial state `ready` where the `message_in` >event is handled. This event contains the message to be analysed. From my >current understanding I either have to make message field in `message_in` >event mutable or MSM needs to to support r-value references. Can somebody >tell me what's going to happen if the action only accepts r-value reference >to an event instance? Currently, it seems to work, but would like to >understand the implications. MSM is still C++98. const& were necessary to pass temporary objects. You can simply const_cast. I'm unsure about the r-value reference action. >My last question relates to multi-threading. I understand that MSM >manages internal states, would it be safe to create MSM as a >thread-local storage and restart the thread-local MSM if the new >request comes in? It's more about, what happens to MSM after an error >state? Is calling stop(), start() members enough? I don't get where there is multithreading in your question. Using TLS it is thread-safe to restart whatever you want. start() calls the first on_entry, stop(), the last on_exit, nothing else. HTH, Christophe

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