State Pattern

Originally published 2016-08-15 · Boris Ivanov

1. Usage

• Allow an object to change behavior when State changed.
• implements OOP Final State Machine(FSM)

2. UML class diagram

• Context is wrapper class - interface to state
• Define abstract state base class
• Define state specific behavior in derived state classes
• maintain pointer to current state in wrapper context class

3. Pros

• Easier to extend with new states by adding a new object
• Easier to assure that all signals are treated by the states, since the base class should define the signals as abstract functions.
• Easier to extend a particular states behavior by deriving from the state. The state pattern should put a particular states behavior in one object.

4. Cons

• More difficult to see all states and their relations by looking at code, since they are dispersed among several different classes.
• Could end up creating an unmanageable number of objects. But compare this to the corresponding if/else blocks needed with the corresponding ENUM solution.

5. Source code

/*
From https://sourcemaking.com/design_patterns/state/cpp/1
*/
#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])();
  }
}