How to have zero or more arguments to a std::function used as value in a std::map?

Question

I have a Engine class which has a member of type std::map, that maps enum to member functions. I'm able to map the enum to all the functions, if the number of arguments of the functions are same.

enum STATE { OFF, ON, HIBERNATE, RESTART };
enum RETCODE { SUCCESS, FAIL, UNKNOWN, OUTOFBOUND };

class Engine
{
    Engine();
    RETCODE StateOn(double val);
    RETCODE StateOff(double val);
    RETCODE StateHibernate(double val);
    RETCODE StateRestart(double val);
private:
    const std::map<STATE, std::function<RETCODE(double)>> Map_State_FnPtr;
};

Engine::Engine() : Map_State_FnPtr {
        { STATE::ON, [=](double val) {return StateOn(val); } },
        { STATE::OFF, [=](double val) {return StateOff(val); } },
        { STATE::HIBERNATE, [=](double val) {return StateHibernate(val); } },
        { STATE::RESTART, [=](double val) {return StateRestart(val); } }
    }
{
    // c'tor body
}

But I have a scenario where some of the functions can have zero or multiple arguments. How do I declare & construct the map variable in such scenario?

class Engine
{
    Engine();
    RETCODE StateOn();                                              // No arguments
    RETCODE StateOff(double val);                                   // 1 argument
    RETCODE StateHibernate(double val, const std::string & data);   // Multiple arguments
    RETCODE StateRestart(double val);
private:
    const std::unordered_map<STATE, std::function<RETCODE()>> Map_State_FnPtr; // What should be the declaration?
};

Any other suggestions for this scenario? Kindly help.

Answer 1

You can always ignore parameters

{
    { STATE::ON, [=](double, const std::string &) {return StateOn(); } },
    { STATE::OFF, [=](double val, const std::string &) {return StateOff(val); } },
    { STATE::HIBERNATE, [=](double val, const std::string & data) {return StateHibernate(val, data); } },
    { STATE::RESTART, [=](double val, const std::string &) {return StateRestart(val); } }
}