Base down_casted = (Base)iExt;
	//cout << "(x, y, z) = " << down_casted.x_ << ", " << down_casted.y_ << ", " << down_casted.z_ << endl;
	//↑int zがなくなるから、コンパイルエラーになるよ
	cout << "(x, y) = " << down_casted.x_ << ", " << down_casted.y_ << endl;