Question

I’m new to C++.

How can I implement a State Machine in C++?

I’m getting only the messages and should know the next state.

What is the proper structure I need to use?

Answer 1

typedef std::pair<State,Message> StateMessagePair;
typedef std::map<StateMessagePair,State> StateDiagram;
StateDiagram sd;
// add logic to diagram
...
State currentState = getInitialState();
...
// process message
Message m = getMessage();
StateDiagram::iterator it=sd.find(std::make_pair(currentState,m)));
if (it==sd.end()) exit("incorrect message");
currentState = it->second;

EDIT:
Building up the state diagram is done like this (example is for a cola-vending machine):

StateDiagram.insert(std::make_pair(State::Idle           ,Message::MakeChoice   ),State::WaitingForMoney);
StateDiagram.insert(std::make_pair(State::WaitingForMoney,Message::Cancel       ),State::Idle);
StateDiagram.insert(std::make_pair(State::WaitingForMoney,Message::MoneyEntered ),State::FindCan);
StateDiagram.insert(std::make_pair(State::FindCan        ,Message::CanSentToUser),State::Idle);

Default actions can be implemented using a second map, where the key is only the State, like this:

typedef std::map<State,State> StateDiagramForDefaults;

Instead of printing “incorrect message”, the logic can perform a lookup in the StateDiagramForDefaults.

If actions needs to be added to the state diagram, the value of the map should be a pair consisting of an action, and a new state, like this:

typedef std::pair<State,Message> StateMessagePair;
typedef std::pair<State,IAction *> StateActionPair;
typedef std::map<StateMessagePair,StateActionPair> StateDiagram;

The logic that builds up the diagram should then “new” an instance of a class that implements IAction, and put that in the StateDiagram.

The executing logic then just executes the IAction implementation via a virtual method (e.g. execute() or the ()-operator).