gistfile1.cpp

#include <memory>

class Instance {
    std::weak_ptr<const Instance> ref;
    int                           value;
public:
    Instance (const std::shared_ptr<const Instance>& other, int value) : 
              ref(other), value(value) { }

    void setRef(const std::shared_ptr<const Instance>& ref) {
        this->ref = ref;
    }

    std::shared_ptr<const Instance> getRef() const {
        return ref.lock();
    }
};