C++ How to return dynamically allocated 2D array from function

Question

I get a memory error every time I run this. Am I even close?

int **createField(int N, int ** ary) {
    ary = new int*[N];
    for(int i = 0; i < N; ++i)
        ary[i] = new int[N];

    for(int i = 0; i < N; ++i)
        for(int j = 0; j < N+1; ++j)
            ary[i][j] = 0;

    return ary;
}

int playGame (int N, int ** ary) {
    cout<<"Enter the coordinates of the " << N << " shots:"<<endl;
    for(int i = 0; i < N; i++)
    {
        for(int j = 0; j < N; j++)
        {
            cout<<"Enter element ["<<i<<"]["<<j<<"]: ";
            cin>>ary[i][j];
        }
    }
    return 0;
}

int main() {
    int N;
    cout << "Enter the number of Redshirts: ";
    cin >> N;
    int** ary;
    createField(N, ary);
    playGame(N, ary);
 }

You're using cout - which means you're using c++. Please don't tag C else people watching C tags will launch tomatoes in your general direction — UKMonkey, Nov 30 '17 at 12:59
this question is asked at least 3 times a week on SO. Read the duplicate link, then CTRL+A / suppr on your code and use `std::vector>` — Jean-François Fabre, Nov 30 '17 at 13:00
Welcome on `StackOverflow`, please read the [How to Ask](https://stackoverflow.com/help/how-to-ask) post! — Hille, Nov 30 '17 at 13:00
@Jean-FrançoisFabre: Do you have a possible duplicate for the C++ tag? — Cheers and hth. - Alf, Nov 30 '17 at 13:02
`j < N+1` and reading the final value of `j` ain't going to end well. — Bathsheba, Nov 30 '17 at 13:03
what the hell, my close vote has gone away. Did you reopen? because it's the same issue. why reopening? — Jean-François Fabre, Nov 30 '17 at 13:03
@Jean-FrançoisFabre: I closed it with the duplicate you had. Then realized it was wrong because that was for C (sorry I didn't notice first). This is a C++ question, where the answer involves C++ references. — Cheers and hth. - Alf, Nov 30 '17 at 13:04
ok, but this is a C problem. I'm sure there's a canonical somewhere but cannot find it. — Jean-François Fabre, Nov 30 '17 at 13:07
`int **createField(int N, int ** ary)` should be `int **createField(int N)` (or returning class as `vector`). — Jarod42, Nov 30 '17 at 13:09
@Cheersandhth.-Alf ok, now you have your original question ... & answer.... Sob... — Jean-François Fabre, Nov 30 '17 at 13:15

score 2 · Answer 1 · answered Nov 30 '17 at 13:13

2

The following don't initialize ary

int** ary;
createField(N, ary);

it should be

int** ary;
ary = createField(N, ary);

but in fact second argument of createField is unneeded, so it would be

int **createField(int N) {
    int** ary = new int*[N];
    for(int i = 0; i < N; ++i)
        ary[i] = new int[N];

    for(int i = 0; i < N; ++i)
        for(int j = 0; j < N+1; ++j)
            ary[i][j] = 0;

    return ary;
}

and then

int** ary = createField(N);

But better would be to use std::vector to not handle memory manually.

answered Nov 30 '17 at 13:13

Jarod42

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`ary = createField(N, ary);` => `ary = createField(N);` – Jean-François Fabre Nov 30 '17 at 13:15
@Jean-FrançoisFabre: It is what I said, no ? `ary = createField(N, ary);` is the minimal change to make the code "works". Additional changes are required to make the code readable/safe. – Jarod42 Nov 30 '17 at 13:37
yes, OP is returning a value without assigning it. – Jean-François Fabre Nov 30 '17 at 13:38

C++ How to return dynamically allocated 2D array from function

1 Answers1