What is the safe way to fill multidimensional array using std::fill?

What is the safe way to fill multidimensional array using std::fill?

The easy default initialization would be using braced inilization.

char flags[26][80]{};

The above will initialize all the elements in the flags to default char.

2-D Array filling using std::fill or std::fill_n

However, in order to provide different value to initialize the above is not enough. The options are std::fill and std::fill_n. (Assuming that the array flags is public in your class)

std::fill(
   &a.flags[0][0],
   &a.flags[0][0] + sizeof(a.flags) / sizeof(a.flags[0][0]),
   '0');

// or using `std::fill_n`
// std::fill_n(&a.flags[0][0], sizeof(a.flags) / sizeof(a.flags[0][0]), '1');

To generalize this for any 2d-array of any type with any initializing value, I would suggest a templated function as follows. This will also avoid the sizeof calculation of the total elements in the array.

#include <algorithm> // std::fill_n, std::fill
#include <cstddef>   // std::size_t

template<typename Type, std::size_t M, std::size_t N>
constexpr void fill_2D_array(Type(&arr2D)[M][N], const Type val = Type{}) noexcept
{
   std::fill_n(&arr2D[0][0], M * N, val);
   // or using std::fill
   // std::fill(&arr2D[0][0], &arr2D[0][0] + (M * N ), val);
}

Now you can initialize your flags like

fill_2D_array(a.flags, '0'); // flags should be `public` in your class!

(See Live Online)

3-D Array filling using std::fill or std::fill_n

Adding one more non-template size parameter to the above template function, this can be brought to 3d-arrays as well

#include <algorithm> // std::fill_n
#include <cstddef>   // std::size_t

template<typename Type, std::size_t M, std::size_t N, std::size_t O>
constexpr void fill_3D_array(Type(&arr3D)[M][N][O], const Type val = Type{}) noexcept
{
   std::fill_n(&arr3D[0][0][0], M * N * O, val);
}

(See Live Online)

char flags[26][80];
std::fill((char*)flags, (char*)flags + sizeof(flags)/sizeof(char), 0);