如何重置 boost::sml::sm 实例

Question

boost::sml::sm 似乎没有任何特定的重置方法，并且它缺少移动赋值运算符，因此我无法执行诸如分配新实例之类的操作（例如 machine = boost::sml::sm<MyMachine>{}）。我目前的方法是调用 dtor 和 ctor 的丑陋技巧：

machine.~sm();
new (&machine) std::decay_t<decltype(machine)>();

有没有更好的办法？

Answer 1

像往常一样做（即永远不要写你实际写的东西）：使用包装器来管理生命周期，例如使用optional 或unique_ptr。

例子：

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//
// Copyright (c) 2016-2018 Kris Jusiak (kris at jusiak dot net)
//
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
//
#include <boost/sml.hpp>
#include <cassert>

namespace sml = boost::sml;

namespace {
// events
struct release {};
struct ack {};
struct fin {};
struct timeout {};

// guards
const auto is_ack_valid = [](const ack&) { return true; };
const auto is_fin_valid = [](const fin&) { return true; };

// actions
const auto send_fin = [] {};
const auto send_ack = [] {};

struct hello_world {
  auto operator()() const {
    using namespace sml;
    // clang-format off
    return make_transition_table(
      *"established"_s + event<release> / send_fin = "fin wait 1"_s,
       "fin wait 1"_s + event<ack> [ is_ack_valid ] = "fin wait 2"_s,
       "fin wait 2"_s + event<fin> [ is_fin_valid ] / send_ack = "timed wait"_s,
       "timed wait"_s + event<timeout> / send_ack = X
    );
    // clang-format on
  }
};
}

#include <boost/optional.hpp>
#include <iostream>
int main() {
  using namespace sml;

  boost::optional<sm<hello_world> > m;

  static_assert(2 == sizeof(m), "sizeof(m) != 2b");

  auto run = [&] {
      assert(m->is("established"_s));

      m->process_event(release{});
      assert(m->is("fin wait 1"_s));

      m->process_event(ack{});
      assert(m->is("fin wait 2"_s));

      m->process_event(fin{});
      assert(m->is("timed wait"_s));

      m->process_event(timeout{});
      assert(m->is(X));  // released
  };

  for (auto i : {1,2,3}) {
      std::cout << "Run " << i << "\n";
      m.emplace(); // default constructs a sm in-place
      run();
  }
}

打印

Run 1
Run 2
Run 3