State Machine Sample

#include <cassert>
#include <iostream>
#include <map>
#include <sstream>
#include <boost/shared_ptr.hpp>

using namespace std;

// Represents an abstract transition.
// StateMachine maintains the mapping of transitions
// to actual target states.
class Transition
{
public:
    typedef string TransitionID;
    Transition(const TransitionID& id_) : id(id_) {}
    TransitionID id;

    // Two common transitions for starting/stopping
    // the state process.
    static Transition Init;
    static Transition Exit;
};

Transition Transition::Init("_Init");
Transition Transition::Exit("_Exit");

bool operator<(const Transition& rhs, const Transition& lhs)
{ return rhs.id < lhs.id; }

bool operator==(const Transition& rhs, const Transition& lhs)
{ return rhs.id == lhs.id; }

// A base class for data containers passed around between states.
struct Context
{
    virtual ~Context() {}
};

// Represents a single state.
class IState
{
public:
    typedef string StateID;
    virtual ~IState() {}
    virtual StateID getID() const = 0;
    virtual Transition run(Context& context) = 0;
};

class StateMachineException : public exception
{
public:
    StateMachineException(const string& what) : mWhat(what) {}
    ~StateMachineException() throw() {}
    virtual const char* what() const throw() { return mWhat.c_str(); }
private:
    string mWhat;
};

// Represents a finite state machine.
class StateMachine
{
public:
    typedef IState::StateID StateID;
    ~StateMachine() { clear(); }
    void clear();
    // init() must be called before run().
    // Override this to register states and transitions.
    virtual void init();
    virtual void run(Context& context, StateID initial);

protected:
    typedef boost::shared_ptr<IState> StatePtr;
    // Use these methods to set up the state graph.
    // Every state in your machine must be registered.
    // Every transition in your machine except for Transition::Exit must be registered.
    void registerState(StatePtr state);
    void registerTransition(StatePtr from, Transition t, StatePtr to);

private:
    StatePtr getNextState(StatePtr fromState, Transition t) const;
    StatePtr getState(StateID stateID) const;

    typedef pair<StateID, Transition> StateTransitionPair;
    typedef map<StateTransitionPair, StateID> StateTransitionMap;
    typedef map<StateID, StatePtr> StateMap;
    StateMap stateRegistry;
    StateTransitionMap stateTransitionRegistry;
};

void StateMachine::init()
{
}

void StateMachine::clear()
{
    stateRegistry.clear();
    stateTransitionRegistry.clear();
}

void StateMachine::registerState(StatePtr state)
{
    if (! state)
        throw StateMachineException("Null pointer passed to registerState");
    stateRegistry[state->getID()] = state;
}

void StateMachine::registerTransition(StatePtr fromState, Transition t, StatePtr toState)
{
    if (! fromState || ! toState)
        throw StateMachineException("Null pointer passed to registerTransition");
    stateTransitionRegistry[StateTransitionPair(fromState->getID(), t)] = toState->getID();
}

StateMachine::StatePtr StateMachine::getNextState(StatePtr fromState, Transition t) const
{
    assert(fromState);
    StateTransitionMap::const_iterator pvStateID  = stateTransitionRegistry.find(
        StateTransitionPair(fromState->getID(), t));
    if (pvStateID == stateTransitionRegistry.end()) {
        ostringstream msg;
        msg << "transition \"" << t.id << "\"";
        msg << " is not in the transition registry for state \"" << fromState->getID() << "\"";
        throw StateMachineException(msg.str());
    }

    return getState(pvStateID->second);
}

StateMachine::StatePtr StateMachine::getState(IState::StateID stateID) const
{
    StateMap::const_iterator pvState = stateRegistry.find(stateID);
    if (pvState == stateRegistry.end()) {
        ostringstream msg;
        msg << "state \"" << stateID << "\" is not in the state registry";
        throw StateMachineException(msg.str());
    }

    return pvState->second;
}

void StateMachine::run(Context& context, IState::StateID initial)
{
    Transition t = Transition::Init;
    StatePtr nextState = getState(initial);
    while (true) {
        StatePtr currentState = nextState;
        t = currentState->run(context);
        if (t == Transition::Exit)
            break;
        nextState = getNextState(currentState, t);
    }
}

// A simple state machine implementation follows...

// The data for this state machine.
// Pass the selection from state A through the rest of the machine.
struct ProcessContext : public Context
{
    string selection;
};

// A sample state.
class StateA : public IState
{
public:
    static StateID ID; // every state has one of these
    static Transition Continue;     // the state's declared transitions
    static Transition Fail;
    virtual StateID getID() const { return StateA::ID; }
    virtual Transition run(Context& context) {
        // Doing this without the Context superclass and downcast
        // (i.e. via templates) is possible but requires a
        // very different design.
        ProcessContext& pc = dynamic_cast<ProcessContext&>(context);
        cout << "Which do you mean, an African or European swallow?" << endl;
        getline(cin, pc.selection);
        if (pc.selection == "African" || pc.selection == "European") {
            // the state machine maps this transition
            // to the appropriate next state
            return Continue;
        } else {
            return Fail;
        }
    }
};

// The string IDs specified here help greatly with debugging, at a cost of efficiency.
// They can be generated for convenience, or converted to symbols for better performance.
IState::StateID StateA::ID("StateA");
Transition StateA::Continue("Continue");
Transition StateA::Fail("Fail");

// Two other trivial states follow.

class StateB : public IState
{
public:
    static StateID ID;
    // No transitions declared; only Exit is used.
    virtual StateID getID() const { return StateB::ID; }
    virtual Transition run(Context& context) {
        ProcessContext& pc = dynamic_cast<ProcessContext&>(context);
        cout << "You said: \"" << pc.selection << "\"" << endl;
        return Transition::Exit; // this stops the machine
    }
};

IState::StateID StateB::ID("StateB");

class StateError : public IState
{
public:
    static StateID ID;
    virtual StateID getID() const { return StateError::ID; }
    virtual Transition run(Context& context) {
        cout << "Sorry, you have died." << endl;
        return Transition::Exit; // this stops the machine
    }
};

IState::StateID StateError::ID("StateError");

// Here's one state machine.
// You could define a Process2 that wires up the states a different way.
// The register methods overwrite previous work, so you could also have
// Process2 derive from Process1 and have its init() override certain
// transitions that Process1 set.
class Process1 : public StateMachine
{
public:
    virtual void init();
    using StateMachine::run;
    virtual void run() {
        ProcessContext ctx;
        run(ctx, StateA::ID);
    }

private:
    StatePtr initState;
};

void Process1::init()
{
    StatePtr stateA(new StateA);
    StatePtr stateB(new StateB);
    StatePtr stateError(new StateError);

    registerState(stateA);
    registerState(stateB);
    registerState(stateError);
    registerTransition(stateA, StateA::Continue, stateB);
    registerTransition(stateA, StateA::Fail, stateError);
}

int main()
{
    try {
        Process1 p;
        p.init();
        p.run();
        return 0;
    } catch (exception& e) {
        cerr << "Caught exception of type " << typeid(e).name() << ": " << e.what() << endl;
        return 1;
    }
}