Someone

template <typename T>
class shared_ptr {
  T* p_;
  mutable const shared_ptr<T>* next_;
  mutable const shared_ptr<T>* prev_;
  
public:
  shared_ptr() : p_(0), next_(0), prev_(0) { }
  explicit shared_ptr(T* p) : p_(p), next_(0), prev_(0) { }
  shared_ptr(const shared_ptr<T>& p) : p_(p.p_), next_(&p), prev_(p.prev_) {
    if (p.prev_) {
      p.prev_->next_ = this;
    }
    p.prev_ = this;
  }
  ~shared_ptr() {
    if (prev_) { prev_->next_ = next_; }
    if (next_) { next_->prev_ = prev_; }
    if (!(next_ || prev_)) {
      delete p_;
    }
  }

  void swap(shared_ptr<T>& other) {
    std::swap(p_, other.p_);
    std::swap(next_, other.next_);
    std::swap(prev_, other.prev_);
  }

  shared_ptr<T>& operator=(const shared_ptr<T>& other) {
    if (p_ == other.p_) {
      return *this;
    } else {
      shared_ptr<T> temp(other);
      temp.swap(*this);
      return *this;
    }
  }

  T* get() const { return p_; }
  T* operator->() const { return p_; }
  T& operator*() const { return *p_; }
};