You get cyclic references of std::shared_ptr if they refer to each other.

The Issue

If you have a cyclic reference of std::shared_ptr, the reference counter will never become 0. You can break this cycle if you embed an std::weak_ptr in the cycle. std::weak_ptr does not modify the reference counter.

Theoretically, you can use a raw pointer to break the cycle of std::shared_ptr's, but a raw pointer has two disadvantages. First, a raw pointer has no well-defined interface. Second, a raw pointer does not support an interface to create a std::shared_ptr out of it.