C++ returning an array of objects from a class method

Question

I am trying to return a two dimensional array of 'room' objects as a kind of print/display of what's in the array.

Room* m_map[10][10];

generated like so:

//Initialise array to NULL
for(int x=0;x<10;x++)
{
    for(int y=0;y<10;y++)
        m_map[x][y] = NULL;
}

for(int n=0; n<10; n++)
{       
    for(int ran=0; ran<3; ran++)
    {
        int r_n = rand() % 10 ;

        Room* r = new Room(n, "Normal", true, false, false, true);
        m_map[r_n][n] = r;
    }       

}

So what this gives is a scatter of Rooms in the array.

I'd then like to display/print for the user where these rooms are, in reference to the NULL.

So I could for example if NULL display '#', if it's a Room Leave a ' '.

I'm unsure of the bit I should return in the method.

Any help or pointing in the right direction would really be appreciated

You simply cannot return arrays from functions or pass arrays to functions. Arrays will degrade to a pointer to the first element in these situations. — Ed Swangren
– Ed Swangren, Commented Mar 1, 2013 at 18:48
It isn't clear how the function is related to this print/display you mention. Could you clarify that? Also, is this function a member function of some class that contains the array? — juanchopanza
– juanchopanza, Commented Mar 1, 2013 at 18:51
Using C-style arrays in C++ is kind of missing the point of C++. — tadman
– tadman, Commented Mar 1, 2013 at 18:55
@juanchopanza I have appended the question to clarify what I am trying to do — Springfox
– Springfox, Commented Mar 1, 2013 at 19:02

juanchopanza · Accepted Answer · 2013-03-01 19:12:42Z

7

It would be easier to use an std::array<Room, N> if the array is fixed size (and the size is known at compile time), or an std::vector<Room> if it isn't.

#include <array> // or <tr1/array> if no C++11

std::array<Room, TheSize> returnArray() const
{
    return m_map;
}

Be careful though, depending on the use-case, you might want to return a reference, not a copy:

const std::array<Room, TheSize>& returnArray() const
{
    return m_map;
}

That said, if all you want to do is print some objects, then don't expose implementation details such as the array. You can either provide a print function ot, better still, override std::ostream& operator<< for your type.

// inside your class declaration:
friend 
std::ostream& operator<<(std::ostream& o, const MyClassWithRooms& r)
{
  // loop over r.m_map and print to o
}

Edit If the array is 2D (as mentioned in comments) then there are options: a nested array structure (std::array<std::array<T,N>,M>, or a "flat" N*M array, and some careful playing with indices).

Edit 2: If you need to store things that can be either set or not, you could use arrays or vectors of std::unique_ptr<Room> instead of plain old Rooms of raw Room*. Another option is simply to use a 2D map, an std::map<int, std::map<int, Room>>.

edited Mar 1, 2013 at 19:12

answered Mar 1, 2013 at 18:46

juanchopanza

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3 Comments

Loki Astari Over a year ago

It would be better not to expose it at all. But provide a print method.

Springfox Over a year ago

It's a two dimensional array that I'm returning, as a 10 x 10 grid... how do I use that? Sorry I didn't say that earlier I will edit the post.

juanchopanza Over a year ago

@Springfox the same applies, you can nest the structures (array or arrays or vector of vectors, or use a flat 1D array, see edit).

default · Accepted Answer · 2013-03-01 18:47:12Z

0

Easiest way to do it is to use a std::vector< Room > for your array. That way you don't have to worry about memory management, and the size is built into the array.

answered Mar 1, 2013 at 18:47

default

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