関連ワード:  c++ weak_ptr shared_ptr    c++ weak_ptr vs shared_ptr    c++ weak pointer to shared pointer    c++ get weak_ptr from shared_ptr    c++ create weak_ptr from shared_ptr    c++ convert shared_ptr to weak_ptr    c++ unique_ptr shared_ptr weak_ptr    c++ cast shared_ptr to weak_ptr    c++ assign shared_ptr to weak_ptr