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copied!<p>I once wrote a state machine in C++, where I needed the same transition for a lot of state pairs (source → target pairs). I want to illustrate an example:</p> <pre><code>4 -> 8 \ 5 -> 9 \_ action1() 6 -> 10 / 7 -> 11 / 8 -> 4 \ 9 -> 5 \_ action2() 10 -> 6 / 11 -> 7 / </code></pre> <p>What I came up with was a set of (transition criteria + next state + "action" function to be called). To keep things general, both the transition criteria and the next state were written as functors (lambda functions):</p> <pre><code>typedef std::function<bool(int)> TransitionCriteria; typedef std::function<int(int)> TransitionNewState; typedef std::function<void(int)> TransitionAction; // gets passed the old state </code></pre> <p>This solution is nice if you have a lot of transitions which apply for a lot of different states as in the example above. However, for each "step", this method requires to linearly scan the list of all different transitions.</p> <p>For the examples above, there would be two such transitions:</p> <pre><code>struct Transition { TransitionCriteria criteria; TransitionNewState newState; TransitionAction action; Transition(TransitionCriteria c, TransitionNewState n, TransitionAction a) : criteria(c), newState(n), action(a) {} }; std::vector<Transition> transitions; transitions.push_back(Transition( [](int oldState){ return oldState >= 4 && oldState < 8; }, [](int oldState){ return oldState + 4; }, [](int oldState){ std::cout << "action1" << std::endl; } )); transitions.push_back(Transition( [](int oldState){ return oldState >= 8 && oldState < 12; }, [](int oldState){ return oldState - 4; }, [](int oldState){ std::cout << "action2" << std::endl; } )); </code></pre>

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