owner가 아니라면 raw pointer를 참조해도 괜찮다 - when to use smart pointers

• https://stackoverflow.com/questions/64443556/modern-c-approach-for-providing-optional-arguments
• Smart Pointers - What, Why, Which?
• https://stackoverflow.com/questions/36673391/c-linked-list-using-smart-pointers#36675943

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Never use owning raw pointers and delete, except in rare cases when implementing your own low-level data structure (and even then keep that well encapsulated inside a class boundary).

리스트나 벡터와 같이 low-level 구조에서는 raw pointer가 더 낫다고 한다.