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SML (State Machine Language)

Your scalable C++14 one header only State Machine Library with no dependencies

https://www.youtube.com/watch?v=Zb6xcd2as6o

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Let's release a TCP connection!

Quick start

Download

[Boost::ext].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 {
  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 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::ext].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

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Examples

• Arduino

https://godbolt.org/z/Y983h4

https://www.tinkercad.com/things/9epUrFrzKP3

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• AVR

https://godbolt.org/z/qhx8Md

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• Match3

https://github.com/modern-cpp-examples/match3

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Documentation

• Introduction
  • UML State Machine
  • Do I need a State Machine?
  • Real Life examples?
  • Why [Boost].SML?
  • Problems with Boost.MSM - eUML
  • [Boost].SML design goals
  • What 'lite' implies?
  • Supported UML features
  • Additional features
  • Related materials
  • Acknowledgements
• Overview
  • Quick Start
  • Dependencies
  • Supported/Tested compilers
  • Configuration
  • Exception Safety
  • Thread Safety
  • Design
  • Error messages
• Features/Benchmarks
  • Features
  • Benchmarks
• Tutorial/Workshop
  • 0. Read Boost.MSM - eUML documentation
  • 1. Create events and states
  • 2. Create guards and actions
  • 3. Create a transition table
  • 4. Set initial states
  • 5. Create a state machine
  • 6. Process events
  • 8. Handle errors
  • 9. Test it
  • 10. Debug it
• UML vs SML
  • Unified Modeling Language™ (UML®) Version 2.5
  • Initial Pseudostate
  • Terminate Pseudostate
  • External transition
  • Anonymous transition
• User Guide
  • transitional [concept]
  • configurable [concept]
  • callable [concept]
  • dispatchable [concept]
  • state [core]
  • event [core]
  • make_transition_table [state machine]
  • sm [state machine]
  • policies [state machine]
  • testing::sm [testing]
  • make_dispatch_table [utility]
• Examples
  • Hello World
  • Events
  • States
  • Actions Guards
  • Transitions
  • Defer/Process
  • Orthogonal Regions
  • Composite
  • History
  • Error handling
  • Logging
  • Nested
  • Testing
  • Runtime Dispatcher
  • eUML Emulation
  • Dependencies
  • Data
  • In-Place
  • Dependency Injection
  • Arduino Integration
  • SDL2 Integration
  • Plant UML Integration
• FAQ
• CHANGELOG
  • [1.1.11] - 2024-03-09
  • [1.1.10] - 2024-03-09
  • [1.1.9] - 2023-09-13
  • [1.1.6] - 2022-09-07
  • [1.1.5] - 2022-03-23
  • [1.1.4] - 2021-02-16
  • [1.1.3] - 2020-08-02
  • [1.1.2] - 2020-06-14
  • [1.1.1] - 2020-05-17
  • [1.1.0] - 2019-01-08
  • [1.0.1] - 2016-05-06
  • [1.0.0] - 2016-01-28

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Disclaimer [Boost::ext].SML is not an official Boost library.