gistfile1.cpp

#include <iostream>
#include <vector>
#include <memory>

class Instance {
    std::shared_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;
    }
};

class InstanceMgr {
    std::vector<std::shared_ptr<const Instance>> repos;
public:
    void create() { 
        auto i1 = std::make_shared<Instance>(nullptr, 0);
        auto i2 = std::make_shared<const Instance>(i1, 0);

        i1->setRef(i2); // creating a cycle 

        repos.emplace_back(i1);
        repos.emplace_back(i2);
    }
};