class Consumer {
    public:
        virtual void consume(int data) = 0;
    };
 
    class A {
    public:
        A() {}
        void attachConsumer(Consumer* consumer) { consumer_ = consumer; }
        void process(int data) {
            // must always check consumer_ here!
            consumer_->consume(data); }
    private:
        Consumer* consumer_;
    };
 
    class B : public Consumer {
    public:
        B() {}
        void consume(int data) { /* consume data */ }
    };
 
	int main() {
	    A a_ = A();
        // ... for reasons I won't tell you, B must be created later than A ...
	    B* b_ = new B();
	    a_.attachConsumer(b_);
	    a_.process(5);
	    return 0;
	}

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