How to access 1D arrays with multiple brackets for readability?

Question

I have a huge code using a 3D array managed with pointers. Something like:

int *** tab;
tab = malloc(m*sizeof(int**));
for(i= 1..n) tab[i] = malloc(n*sizeof(int*));  
... etc...

and later the elements are accessed with:

tab[i][j][k] = ...

But because of specific issues with this structure, I would like to declare tab as a contiguous array instead but still use the syntax with 3 brackets in the code. The compiler will internally replace them like this:

tab[i][j][k] = ...  =>  tab[i*m*n+j*m+k] = ...

So the array is accessed with only one pointer dereference. I'd like not to change the source code (no sed).

For example I could do this by declaring tab in stack:

int tab[n][m][l];

but unfortunately this doesn't work if m and n are runtime variables.

Answer 1

A C++ way is to enclose the 3D array in a class to have a natural accessor:

struct Arr3D
{
    int *arr;
    const int n, m, p; //size of the tab in 3 dimensions

public:
    Arr3D(int n, int m, int l): n(n), m(m), p(l) {
        arr = new int[n * m * p];
    }

    ~Arr3D() {
        delete[] arr;
    }

    int& val(int i, int j, int k) {   // read-write accessor
        // optionaly test for 0<=i<n...
        return arr[k + p * (j + i * m)];
    }
};

You create and use an array simply with:

Arr3D * parr = new Arr3D(3,4,5); // dynamic allocation
Arr3D arr(3, 4, 5);              // automatic allocation
...
arr(1,2,3) = 5;
int i = arr(2,0,1);

---

Alternatively, if you want to keep the syntax tab[i][j][k] you can if you use an auxilliary Arr2D class able to provide a view on a 2D array:

struct Arr2D
{
    int *arr;
    const int n, m; //size of the tab in 2 dimensions
    const bool is_view;

public:
    Arr2D(int n, int m): n(n), m(m), is_view(false) {
        arr = new int[n * m];
    }
    Arr2D(int *arr, int n, int m): arr(arr), n(n), m(m), is_view(true) {}

    ~Arr2D() {
        if (! is_view) delete[] arr;
    }

    int * operator[] (int i) {
        return arr + i * m;
    }
};

struct Arr3D
{
    int *arr;
    const int n, m, p; //size of the tab in 3 dimensions

public:
    Arr3D(int n, int m, int l): n(n), m(m), p(l) {
        arr = new int[n * m * p];
    }

    ~Arr3D() {
        delete[] arr;
    }

    Arr2D operator[](int i) {
        return Arr2D(arr + i * p * m, m, p);
    }
};

You can now simply use arr[i][j][k] ...