C++: Can a class has an object of it's own type?

Question

I am trying to solve the Conway's game of life in c++. So according to my design, I have a cell class which contains a list of 8 neighbours, which itself is an object of type cell. Is it ok to do that. The code looks like this

class Cell {
private:
  int position_row_;
  int position_colm_;
  std::shared_ptr<State> state_;

  std::vector<Cell> neighbours_;
};

Now, the other question that is bugging me is, what type of relationship is it. While designing i thought of it to be 'Aggregation'; but now i don't think so. Can someone please clarify?

Answer 1

A class cannot include itself as a data member class C { C c; }; for two reasons:

1. When you try to define the data member the class is still an incomplete type.
2. You are creating an endless chain of objects which would require infinite memory.

A class can contain a vector of itself class C { std::vector<C> vec; }; because, like a pointer, a vector declaration only requires that data type has been declared - not defined.

Your code will compile and run but you said your intent was for aggregation which isn't the case:

• Your code std::vector<Cell> neighbours; is a case of composition because the objects in the vector are owned by the Cell object.
• Using pointers to other Cell objects std::vector<Cell*> neighbours; would be a case of aggregation because the Cell object would use the objects in the vector but not own them.