How to declare a dynamic 2D const array in C ?-CodePudding

I thought that the following code would work. However, it is printing the first row and then throwing a segmentation fault.

const unsigned **grid = new const unsigned*[10]{new const unsigned[10]{0}};

for (int r = 0; r < 10; r  )
{
    for (int c = 0; c < 10; c  )
    {
        std::cout << grid[r][c] << " ";
    }
    std::cout << std::endl;
}

Does anyone know what I am (probably naively) doing incorrectly?

CodePudding user response：

Here are some options using dynamic allocation (of course it is simpler to use automatic allocation):

// should be delete[]'d at some point in future
auto* arr1 = new const unsigned[10][10]{};

// #include <memory>, C  14
auto arr2 = std::make_unique<const unsigned[][10]>(10);

// #include <vector>
auto const arr3 = std::vector<unsigned>(100);  // use index math

// #include <array>
using A10 = std::array<unsigned, 10>;
auto const arr4 = std::vector< A10 >(10, A10{});

CodePudding user response：

So here is a solution that should work with every compiler:

const unsigned **grid = new const unsigned*[10]{new const unsigned[10]{0},
    new const unsigned[10]{0}, new const unsigned[10]{0}, new const unsigned[10]{0}, 
    new const unsigned[10]{0}, new const unsigned[10]{0}, new const unsigned[10]{0}, 
    new const unsigned[10]{0}, new const unsigned[10]{0}, new const unsigned[10]{0} };
    for (int r = 0; r < 10; r  ){
        for (int c = 0; c < 10; c  )
        {
            std::cout << grid[r][c] << " ";
            
        }
        std::cout << std::endl;
    }

To be honest I am not understanding why the grid should be const.

CodePudding user response：

Here's how to properly handle 2D dynamic arrays. I'll remove const for now since it's most important that you handle dynamic memory properly:

#include <iostream>

int main() {
    unsigned** grid = new unsigned* [10];
    for (int i = 0; i < 10;   i) {
        grid[i] = new unsigned[10];
    }

    for (int r = 0; r < 10; r  )
    {
        for (int c = 0; c < 10; c  )
        {
            std::cout << grid[r][c] << " ";
        }
        std::cout << std::endl;
    }

    for (int i = 0; i < 10;   i) {
        delete[] grid[i];
    }
    delete[] grid;

    return 0;
}

Don't forget to delete[] your arrays allocated with new, and make sure you know the difference between "delete" and "delete[]"

As for the people who always decry writing code like this, this is a necessary step for anyone learning C and memory management. A plain example with raw pointers can only help.