无法使用模板参数编译 boost 元状态机

Question

无法确定 TSm_ 中违反了哪条规则。 Sm_ 已编译，TSm_ 未编译：

错误 C2974：'boost::mpl::vector'：'T0' 的模板参数无效， 预期类型

错误 C2974：“boost::mpl::vector”：“T3”的模板参数无效， 预期类型

区别在于 TSm_ 是模板，而 Sm_ 不是。

#include <boost/msm/back/state_machine.hpp>
#include <boost/msm/front/state_machine_def.hpp>

namespace msm = boost::msm;
namespace mpl = boost::mpl;

struct Event {};

class Sm_ : public msm::front::state_machine_def<Sm_>
{
public:
    using Me = Sm_;
    struct First : public msm::front::state<>
    {};
    using initial_state = First;
    struct Second : public msm::front::state<>
    {};
    void trans(const Event& /*e*/)
    {}
    struct transition_table : public mpl::vector<
        a_row<First, Event, Second, &Me::trans>
    >
    {};
};

using Sm = msm::back::state_machine<Sm_>;

// broken one

enum class Side : char
{
    Buy = 0, Sell = 1
};

template <Side side>
class TSm_ : public msm::front::state_machine_def<TSm_<side>>
{
public:
    using Me = TSm_<side>;
    using Base = msm::front::state_machine_def<Me>;
    using Base::a_row;
    struct First : public msm::front::state<>
    {};
    using initial_state = First;
    struct Second : public msm::front::state<>
    {};
    void trans(const Event& /*e*/)
    {}
    struct transition_table : public mpl::vector<
        a_row<First, Event, Second, &Me::trans> // compilation is failed here
    >
    {};
};

template <Side side>
using TSm = msm::back::state_machine<TSm_<side>>;

请帮忙

更新

我找到了编译的方法：a_row 不是类型而是模板，它的别名也应该是模板

struct Begin {};

template <Side side>
class TSm_ : public msm::front::state_machine_def<TSm_<side>>
{
public:
    //-----------------------------------------------------------------------------------------------------
    using Me = TSm_<side>;
    using Base = msm::front::state_machine_def<Me>;
    template<
        typename T1
        , class Event
        , typename T2
        , void (Me::*action)(Event const&)
    >
    using a_row = typename Base::a_row;
    //-----------------------------------------------------------------------------------------------------
    struct First : public msm::front::state<>
    {};
    //-----------------------------------------------------------------------------------------------------
    using initial_state = First;
    //-----------------------------------------------------------------------------------------------------
    struct Second : public msm::front::state<>
    {};
    //-----------------------------------------------------------------------------------------------------
    void trans(const Begin& /*e*/)
    {}
    //-----------------------------------------------------------------------------------------------------
    struct transition_table : public mpl::vector<
        a_row<First, Begin, Second, &Me::trans>
    >
    {};
};

是不是这个规则2) An alias template is a template which, when specialized, is equivalent to the result of substituting the template arguments of the alias template for the template parameters in the type-id？

Answer 1

当你用模板状态机类定义转换表时，你可以使用row、a_row和g_row家族的functor前端。

见https://www.boost.org/doc/libs/1_67_0/libs/msm/doc/HTML/ch03s03.html

如果你使用函子前端，你不需要关心msm::front::state_machine_def的子类是不是模板。

这是基于你的更新代码：

#include <iostream>

#include <boost/msm/back/state_machine.hpp>
#include <boost/msm/front/state_machine_def.hpp>

// include functor_row
#include <boost/msm/front/functor_row.hpp>

namespace msm = boost::msm;
namespace msmf = msm::front;
namespace mpl = boost::mpl;

struct Event {};

// broken one

enum class Side : char
{
    Buy = 0, Sell = 1
};

template <Side side>
class TSm_ : public msm::front::state_machine_def<TSm_<side>>
{
public:
    using Me = TSm_<side>;
    using Base = msm::front::state_machine_def<Me>;
    struct First : public msm::front::state<>
    {};
    using initial_state = First;
    struct Second : public msm::front::state<>
    {};

    // Replace the member function trans() with the functor trans.
    struct trans {
            template <class Fsm, class SourceState, class TargetState>
            void operator()(Event const&, Fsm&, SourceState&, TargetState&) const {
            std::cout << "called" << std::endl;
        }
    };

    struct transition_table : public mpl::vector<
                 //from     event    to      action,    guard
        msmf::Row <First,   Event,   Second, Me::trans, msmf::none >
        // This is functor front-end
    >
    {};
};

template <Side side>
using TSm = msm::back::state_machine<TSm_<side>>;

int main() {
    TSm<Side::Buy> sm;
    sm.start();
    sm.process_event(Event());
}

运行演示：https://wandbox.org/permlink/e9Qoc2xW3TTGhsmu