Boost::msm How to initialize state_machine_def and msm::front::state with non-default constructor

Question

I have a state machine look like this:

class FsmDef : public boost::msm::front::state_machine_def<FsmDef> {
private:
    Args args;
    using State = boost::msm::front::state<>;
public:
    FsmDef(Args args) : args{args}
    {}


    struct InitState {};
    struct State1 {
        Args1 args1;
        State1(Args1 args1) : args1(args1)
         {}
    };

    struct transition_table : boost::mpl::vector<
        boost::msm::front::Row<Init, boost::msm::front::none, State1>
    > { };

    using initial_state = InitState;
};

using Fsm = boost::msm::back::state_machine<FsmDef>;

Fsm fsm;

How can I construct fsm and initialize private data for FsmDef. The same thing with State1.

Answer 1

FsmDef can be non default constructible. But State1 need to be default constructible.

Here is a way to pass arguments to FsmDef.

#include <iostream>
#include <boost/msm/back/state_machine.hpp>

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

struct Args {
    int val;
};

class FsmDef : public boost::msm::front::state_machine_def<FsmDef> {
private:
    Args args_;
    using State = boost::msm::front::state<>;
public:
    FsmDef(Args args) : args_{args}
    {
        std::cout << args_.val << std::endl;
    }


    struct InitState : boost::msm::front::state<> {};
    struct State1 : boost::msm::front::state<> {
    // states must be default constructible
    //    Args1 args1;
    //    State1(Args1 args1) : args1(args1)
    //    {}
    };

    struct transition_table : boost::mpl::vector<
        boost::msm::front::Row<InitState, boost::msm::front::none, State1>
    > { };

    using initial_state = InitState;
};

using Fsm = boost::msm::back::state_machine<FsmDef>;

int main() {
    Args a {42};
    Fsm fsm(a);
}

Running demo https://wandbox.org/permlink/ZhhblHFKYWd3ieDK

Fsm, boost::msm::back::state_machine<FsmDef> has the constructor that has the same parameters as FsmDef. AFAIK, it is not explicitly documented.

Here is the code that defining the constructor.

https://github.com/boostorg/msm/blob/boost-1.64.0/include/boost/msm/back/state_machine.hpp#L1625