statemachine.h

// struct Data may be any kind of data. it may be external or inside the context class.
// states shall have access to this data.
struct Data{
	Data(int d) : data1(d){}
	int data1;
};

// the context class may be anything, that has internal states
// and behaves differently depending on the internal state.
// this context class has only one signal for clarity.
// having a hole bunch of signals is common.
class Context{
private:
	struct State {//top-level state
		virtual void signalA(){}//put code here for signalA in superstate
		Data* data;// pointer to data, which physically resides inside the context class (contextdata)
	} *statePtr;   // a pointer to current state. Used for polymorphism.

	struct StateA : public State {
		virtual void signalA(){// put code here for transition from A with signalA
			data->data1 += 2; // manipulate extended state data
			new (this) StateB;// change to state B
		}
	};

	struct StateB : public State {
		virtual void signalA(){//put code here for state B and signalA. Data may be accessed via data->
			if ( data->data1 > 10 ) // choice point
				new (this) StateA;
			else
				new (this) StateB;
		}
	};

	StateA stateMember;//The memory for the state is part of context object
	Data contextdata;  //Data is also kept inside the context object

	public:
	Context()
	: statePtr(&stateMember) // assigning start state
	, contextdata(0)         // initializing data
	{
		statePtr->data = &contextdata; // connecting state->data with the context data
	}

	void signalA(){statePtr->signalA();} // context delegates signals to state 
};