gistfile1.cpp

class Algorithm {
    
public:
    void onEvent(Event x) {
        this->strategy->onEvent(x);
    }
protected:
    Strategy* strategy;
    std::vector<int> data;
}


class Strategy {

    friend class Algorithm;

public:

    Strategy(Algorithm& algo) : algo(algo) {};

    virtual void onEvent(Event x) = 0;
    
protected:
        
    Algorithm& algo; // Strategy needs to know Algorithm to modify its data
}


class ConcreteStrategy {

    ConcreteStrategy(Algorithm& algo) : Strategy(algo) {}

    virtual void onEvent(Event x) {
        this->algo->data[...] = ...
    }
}