The observer pattern allows objects to listen for state changes in a certain Subject.
This implementation makes no assumptions and is completely decoupled from the observers. This is done by setting the state as a template typename of the class and attaching only the function that handles the changes.
This function must be of format void f(const StateType&) where StateType is the same as the one specified to the Subject class.
An example:
struct State { int data; };
void handleStateChange(const State& newState)
{
cout << "State changed to: " << newState.data << endl;
}
int main()
{
// Create a new subject with a certain state.
Subject<State> someSubject;
// Attach the observer, this can be any function.
someSubject.attach(handleStateChange);
// Get the state and change it.
State state = someSubject.getState();
state.data = 1337;
someSubject.setState(state);
return 0;
}