Creating a C array wrapper (a better std::array?)

Question

Here's a simple 2-dimensional C-style array example:

int c[2][2] = {
    { 1, 2 },
    { 3, 4 }
};

If I want this to be std::array, I have to use this:

std::array<std::array<int, 2>, 2> c = {
    { { 1, 2 } },
    { { 3, 4 } }
};

The declaration is more involved, and I have to use extra { } for initialization.

Is it possible to create a C-style array wrapper, with which I can do this?

my_array<int, 2, 2> c = {
    { 1, 2 },
    { 3, 4 }
};

So the declaration is simpler, and there is no need of the extra { }.

If creating something like this is possible, would this solution have some drawbacks compared to std::array?

I've managed to do this:

template <typename T, std::size_t ...S>
struct ArrayTypeHelper;

template <typename T, std::size_t HEAD, std::size_t ...TAIL>
struct ArrayTypeHelper<T, HEAD, TAIL...> {
    using Type = typename ArrayTypeHelper<T, TAIL...>::Type[HEAD];
};
template <typename T, std::size_t HEAD>
struct ArrayTypeHelper<T, HEAD> {
    using Type = T[HEAD];
};

template <typename T, std::size_t ...S>
struct my_array {
    typename ArrayTypeHelper<T, S...>::Type data;
};

But this still needs extra { }:

my_array<int, 2, 2> b = { {
    { 1, 2 },
    { 3, 4 }
} };

How about std::array<std::array<int, 2>, 2> c{1, 2, 3, 4};? — user7860670
– user7860670, Commented Dec 2, 2018 at 16:22
With C++17 you can do it simpler since it can deduce the template arguments from the initialization. — Jesper Juhl
– Jesper Juhl, Commented Dec 2, 2018 at 16:27
@VTT: I don't really like this style. And there are compilers which warn for this. — geza
– geza, Commented Dec 2, 2018 at 16:28
@geza I guess by "some compilers" you mean clang? Then it is a known bug. You can still keep each row on separate line to make it look like matrix — user7860670
– user7860670, Commented Dec 2, 2018 at 16:33

Oliv · Accepted Answer · 2018-12-02 19:44:33Z

3

The problem is that a c-array member is always viewed by the brace elision algorithm as a single entity. If the inner initializer begins with a brace , the algorithm expects an initializer list with one element. So the solution is to define the carray wrapper in such a way that the brace elision algorithm knows how many elements are aggregated by the wrapper.

To do this, the only solution I see is to simulate an array through multiple inheritance:

#include <utility>
using namespace std;

template<class T,size_t DeDuplicate>
struct holder{
  T val;
  };

template<class T,class IndSeq>
struct carray_wrapper_b;

template<class T,size_t...Is>
struct carray_wrapper_b<T,index_sequence<Is...>>
  :holder<T,Is>...
  {   };

template<class T,size_t I,size_t...Is>
struct make_carray_{
  using type = carray_wrapper_b<typename make_carray_<T,Is...>::type
                               ,make_index_sequence<I>>;
  };
template<class T,size_t I>
struct make_carray_<T,I>{
  using type = carray_wrapper_b<T,make_index_sequence<I>>;
  };

template<class T,size_t...Is>
using carray_wrapper = typename make_carray_<T,Is...>::type;

carray_wrapper<int,2,2>  x = { {1,2},{3,4}};
carray_wrapper<int,2,2>  y = { 1,2,3,4};

This array wrapper can also be used for initialization purpose only:

template<class T,size_t I,size_t...Is>

struct carray{
  carray(initializer_list<T>);
  carray(initializer_list<carray_wrapper<T,Is...>>);
  };

carray<int,2,2,3> arr{1,2,3,4,5,6,7};
carray<int,2,2,3> arr2{{1,2},{3,4}};
carray<int,2,2,3> arr3{{{1,2},{3,4}},{1,2}};

edited Dec 2, 2018 at 19:44

answered Dec 2, 2018 at 17:33

Oliv

18.2k1 gold badge34 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Oliv Over a year ago

@geza Finaly I found a solution, you can make a c-style array member of your wrapper class and use initializer_list constructors that takes as elements the carray_wrapper I proposed.

geza Over a year ago

Nice trick, thanks Oliv! I'd upvote this answer more if I could, it deserves more than 2.

Oliv Over a year ago

@geza Thank you :)

Collectives™ on Stack Overflow

Creating a C array wrapper (a better std::array?)

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related