Returning and using a pointer to a multidimensional array of another class

Question

I'm trying to access the 'room' array via functions in the room class itself, i.e:

 class ship{

    room * rooms[3][4];
 public:

   room ** getrooms(){

       return *rooms;
   }

 }


class room{

  //variables...
  ship * ship_ptr;
  public:
     getShipPtr(); //assume it returns the pointer to the class ship
     void function_x(){
    this->getShipPtr()->getrooms()->rooms[x][y]->doThat();
      }
  }

I'm doing something wrong pointer-wise but I'm not really sure, what could correct the code so I can access the room array out of the ship class? Note: Assume that the rooms are already initiated

What part of the code is supposed to check to make sure the access to rooms is not out of bounds? — crashmstr
– crashmstr, Commented Jan 5, 2016 at 20:27
I didn't put it here, for simplicity's sake, and because that's not a problem (at least for now) @crashmstr — Zetsuno
– Zetsuno, Commented Jan 5, 2016 at 20:29
It is tagged C++, hence get used to std::array and std::vector and do not allocate a single value for a member variable (avoid new/delete) — user2249683
– user2249683, Commented Jan 5, 2016 at 20:30
@DieterLücking I'm not allowed to use either std::vector or std::array, my professors insist that I take 'this way' — Zetsuno
– Zetsuno, Commented Jan 5, 2016 at 20:32
Why do the rooms need to know about the ship? Looks like either a flaw in the design or a circular reference: ship->room->ship->room... It's like having a vector of integers and each integer refers to the vector. — Thomas Matthews
– Thomas Matthews, Commented Jan 5, 2016 at 20:36

R Sahu · Accepted Answer · 2016-01-05 20:40:46Z

3

I'm doing something wrong pointer-wise but I'm not really sure

You are making the false assumption that a 2D array can decay to a pointer to a pointer just like a 1D array can decay to a pointer.

int arr1[10];
int* ptr1 = arr1; // OK. 1D array decays to a pointer.

int arr2[10][20];
int** ptr2 = arr2; // Not OK. 2D array does not decay to a pointer to pointer.

int (*ptr2)[20] = arr2; // OK. ptr2 is a pointer to "an array of 20" objects.

My suggestion:

Simplify your code and change the interface to:

room* getRoom(size_t i, size_t j){
   // Add checks to make sure that i and j are within bounds.
   return rooms[i][j];
}

answered Jan 5, 2016 at 20:40

R Sahu

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1 Comment

Zetsuno Over a year ago

Thanks a lot, worked like a charm. And you were right, I was assuming that 2D arrays would decay into a pointer like 1D arrays do.

vsoftco · Accepted Answer · 2016-01-05 20:50:56Z

1

I am not going to insist on using std::vectors etc, although your teacher should teach you to use them from the very beginning. So let's stick to writing C in C++. You cannot return a 2D array as a double pointer, as the former does not decay to the latter. If you insist in returning a pointer to your array, what you need is

room* (*getrooms())[4] // define a function returning a pointer to array-4 of room*
{
    return rooms;
}

That is because a 2D array of type e.g. T arr[A][B] decays to a pointer to array-B of T.

edited Jan 5, 2016 at 20:50

answered Jan 5, 2016 at 20:43

vsoftco

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Comments

Thomas Matthews · Accepted Answer · 2016-01-05 20:55:32Z

I believe the root cause of your issue is the cyclic dependency of having rooms know about the ship and the ship know about the rooms.

I suggest a different design:

class Ship
{
  Room ship_rooms[3][4];  // Note, not dynamically allocated.
  public:
    void move_unit(Room& from_room, Room& destination)
    {
      destination.set_unit(from_room.get_unit());
    }
    void move_unit(unsigned int from_row, unsigned int from_column,
                   unsigned int dest_row, unsigned int dest_column)
    {
      ship_rooms[dest_row][dest_column].set_unit(ship_rooms[from_row][from_column].get_unit());
    }
    Room& get_room(unsigned int row, unsigned int column)
    { return ship_rooms[row][column]; }
};

In the above design, the Ship is in charge of moving units between rooms. Rooms will get (cut) units or set units (receive them). This eliminates the need for a room to know anything about the Ship.

Another possible design is for a room to move something to another room:

class Room
{
  Item unit;
  public:
    void receive_unit(const Room& other)
    {
      unit = other.unit;
    }
    void transfer_unit(Room& other)
    {
      other.unit = unit;
    }
};

The above structure allow rooms to communicate with each other, without knowing anything about the ship or container.

Note: These designs resolve the issue of pointer by not requiring pointers. References are used instead.

Downvoters: Please add explanation in the comment.

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