[Boost].SML: C++14 State Machine Library
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[Boost].SML (State Machine Language)

Your scalable C++14 one header only State Machine Library with no dependencies (Try it online!)


Let's release a TCP connection!

tcp release

Quick start

Download

[Boost].SML requires only one file. Get the latest header here!

Include

#include <boost/sml.hpp>
namespace sml = boost::sml;

Dependencies

struct sender {
  template<class TMsg>
  constexpr void send(const TMsg& msg) { std::printf("send: %d\n", msg.id); }
};

Events

struct ack { bool valid{}; };
struct fin { int id{}; bool valid{}; };
struct release {};
struct timeout {};

Guards

constexpr auto is_valid = [](const auto& event) { return event.valid; };

Actions

constexpr auto send_fin = [](sender& s) { s.send(fin{0}); };
constexpr auto send_ack = [](const auto& event, sender& s) { s.send(event); };

State Machine

struct tcp_release final {
  auto operator()() const {
    using namespace sml;
    /**
     * Initial state: *initial_state
     * Transition DSL: src_state + event [ guard ] / action = dst_state
     */
    return make_transition_table(
      *"established"_s + event<release>          / send_fin  = "fin wait 1"_s,
       "fin wait 1"_s  + event<ack> [ is_valid ]             = "fin wait 2"_s,
       "fin wait 2"_s  + event<fin> [ is_valid ] / send_ack  = "timed wait"_s,
       "timed wait"_s  + event<timeout>                      = X
    );
  }
};

Usage

int main() {
  using namespace sml;

  sender s{};
  sm<tcp_release> sm{s}; // pass dependencies via ctor
  assert(sm.is("established"_s));

  sm.process_event(release{}); // complexity O(1)
  assert(sm.is("fin wait 1"_s));

  sm.process_event(ack{true}); // prints 'send: 0'
  assert(sm.is("fin wait 2"_s));

  sm.process_event(fin{42, true}); // prints 'send: 42'
  assert(sm.is("timed wait"_s));

  sm.process_event(timeout{});
  assert(sm.is(X));  // terminated
}

MSVC-2015 (Example)

  • use state<class state_name> instead of "state_name"_s
  • expliclty state a lambda's result type auto action = [] -> void {}

Compile

  • GCC/Clang
    $CXX -std=c++14 -O2 -fno-exceptions -Wall -Wextra -Werror -pedantic -pedantic-errors tcp_release.cpp
  • MSVC
    cl /std:c++14 /Ox /W3 tcp_release.cpp

tcp_release.cpp Clang-3.8 GCC-6.3 MSVC-2015
Compilation Time 0.102s 0.118s 0.296s
Binary size (stripped) 6.2kb 6.2kb 105kb
ASM x86-64 -
https://godbolt.org/z/y99L50

main: # @main
  pushq %rax
  movl $.L.str, %edi
  xorl %esi, %esi
  xorl %eax, %eax
  callq printf
  movl $.L.str, %edi
  movl $42, %esi
  xorl %eax, %eax
  callq printf
  xorl %eax, %eax
  popq %rcx
  retq
.L.str:
  .asciz "send: %d\n"
      

Run

Output (https://wandbox.org/permlink/WbvV9HsIyiPkCFw7)

send: 0
send: 42

Benchmark

Complex Test

Enum/Switch Variant [Boost].SML - 1.1.0 Boost-1.65.MSM-eUML Boost-1.65.Statechart
Compilation time 0.132s 15.321s 0.582s 1m15.935s 5.671s
Execution time 679ms 827ms 622ms 664ms 2282ms
Memory usage 1b 2b/8b 1b 120b 224b
Executable size 15K 187K 34K 611K 211K

Documentation


Disclaimer Boost.SML is not an official Boost library.