Question

This is an example of State machine design pattern .. i am facing some issue in it please explain and give solution for it .

This is the code:

#include <iostream>
using namespace std;
class Machine
{
  class State *current;
  public:
    Machine();
    void setCurrent(State *s)
    {
        current = s;
    }
    void on();
    void off();
};

class State
{
  public:
    virtual void on(Machine *m)
    {
        cout << "   already ON\n";
    }
    virtual void off(Machine *m)
    {
        cout << "   already OFF\n";
    }
};

void Machine::on()
{
  current->on(this);
}

void Machine::off()
{
  current->off(this);
}

class ON: public State
{
  public:
    ON()
    {
        cout << "   ON-ctor ";
    };
    ~ON()
    {
        cout << "   dtor-ON\n";
    };
    void off(Machine *m);
};

class OFF: public State
{
  public:
    OFF()
    {
        cout << "   OFF-ctor ";
    };
    ~OFF()
    {
        cout << "   dtor-OFF\n";
    };
    void on(Machine *m)
    {
        cout << "   going from OFF to ON";
        m->setCurrent(new ON());
        delete this;
    }
};

void ON::off(Machine *m)
{
  cout << "   going from ON to OFF";
  m->setCurrent(new OFF());
  delete this;
}

Machine::Machine()
{
  current = new OFF();
  cout << '\n';
}

int main()
{
  void(Machine:: *ptrs[])() = 
  {
    Machine::off, Machine::on
  };
  Machine fsm;
  int num;
  while (1)
  {
    cout << "Enter 0/1: ";
    cin >> num;
    (fsm. *ptrs[num])();
  }
}

This is the error:

prog.cpp: In function ‘int main()’:
prog.cpp:89: error: invalid use of non-static member function ‘void Machine::off()’
prog.cpp:89: error: invalid use of non-static member function ‘void Machine::on()’
prog.cpp:97: error: expected unqualified-id before ‘*’ token

Answer 1

There are two errors:

One, the address-of operator is mandatory for creating pointers to members. So they array initialisation should be:

void(Machine:: *ptrs[])() = 
{
  &Machine::off, &Machine::on
};

Two, the operator to dereference a pointer to member is .*. It’s a single token, so no whitespace is allowed:

(fsm.*ptrs[num])();