c++ 3d array function returns

Question

So i've got this 3 dimensional array that looks like this:

int map[100][100][100];

and I want to use it as a return type for a function, whether as a pointer or whatever like so:

int* World::CreateCell() {

    int map[100][100][100];

    return map;

}

However I cannot find the appropriate return type for a 3d array, it will not let me use an int* like you can for a 2D array.

Even things like this don't work:

int a[100][100][100];
int* b = a;

VS seems to think the data type is int*(100)(100) but that makes no sense and doesn't work.

For what it's worth, I've googled this and seen no solutions. Thank you

Answer 1

Since you want 3D C-style array, you need to have a pointer to a pointer to a pointer, i.e., int***. Also, you need to allocate the memory if you use a creation function like this. Otherwise, you return the statically allocated memory from that function.

Here is a quick example of how to do it:

#include <iostream>

static int*** create_cell() {    
    constexpr std::size_t n = 100;
    int*** map = new int**[n];
    for (std::size_t i = 0u; i < n; ++i) {
        map[i] = new int*[n];
        for (std::size_t j = 0u; j < n; ++j) {    
            map[i][j] = new int[n];
        }
    }   
    return map;
}

static void delete_cell(int***& map) { 
    constexpr std::size_t n = 100;
    for (std::size_t i = 0u; i < n; ++i) {
        for (std::size_t j = 0u; j < n; ++j) {    
            delete[] map[i][j];
        }
        delete[] map[i];
    }
    delete[] map;    
}

int main()
{
    int*** a = create_cell();

    a[0][0][0] = 1;
    std::cout << "a[0][0][0] = " << a[0][0][0] << std::endl;
    
    delete_cell(a);

    return 0;
}

It depends on your use case: BUT for modern c++ you can ease your life using containers from the stl such as std::vector and std::array. Check here for reference: std::array and std::vector

For example, you can define your 3D types:

#include <array>
#include <vector>
using vector_3d = std::vector<std::vector<std::vector<int>>>;
using array_3d = std::array<std::array<std::array<int, 100>, 100>, 100>;

and then use them as:

array_3d b;
b[0][0][0] = 1;
std::cout << "b[0][0][0] = " << b[0][0][0] << std::endl;