gistfile1.cpp

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

/// /////////////////////////////////

class ConcreteStrategy {
	friend class Algorithm<ConcreteStrategy>;
	
	void set_algorithm(Algorithm<ConcreteStrategy>* algo){this->algo = algo;}
public:
	ConcreteStrategy() : algo{nullptr} {}
	
	void onEvent(Event x) {
		this->algo->data[...] = ...
	}
	
private:
	Algorithm<ConcreteStrategy> * algo;
};