How to fill std::array from protobuf repeatedField?

Question

From a RepeatedField which is defined to have exactly N elements, I am filling up an std::array.

std::array<int, 6> entry;
size_t i = 0;
for (const auto& item: msg.items()){
  assert(i < 6);
  entry[i++] = item;
}

This works fine, but is there a better way to initialize it? filling up std::vector works fine with initialization lists, but std::array is really different.

Is it possible to use RepeatedFields in aggregate initialization of structs contaning std::array ?

What I tried so far does not even compile:

entry = {msg.items().begin(), msg.items().end()};

Which is understandable, because std::array doesn't have a constructor accepting an std::initializer_list; But is what I am trying to do even possible?

Answer 1

the most simple way is use aggregate initialization directly

assert(items.size() >= 6)
std::array entry = {
   items[0],items[1],items[2],
   items[3],items[4],items[5],
};

---

or you can write a function to help you

template<std::size_t N, typename C>
auto get_array(C&& c){
    assert(c.size() >= N); // or ==, depend on the need
    return [&]<std::size_t ...Index>(std::index_sequence<Index...>){
        return std::array{c[Index]...};
    }(std::make_index_sequence<N>{});
}

void foo()
{
    std::array entry = get_array<6>(items);
}