I want to construct an array value at compile time and have seen multiple sources online suggesting using a struct with a constexpr constructor:
template<int N>
struct A {
constexpr A() : arr() {
for (auto i = 0; i != N; ++i)
arr[i] = i;
}
int arr[N];
};
int main() {
constexpr auto a = A<4>();
for (auto x : a.arr)
std::cout << x << '\n';
}
Is this just old advice (maybe pre-C++17 advice?) or am I missing something because it seems to me I can just do the following:
constexpr std::array<int, 4> get_ary() {
std::array<int, 4> ary = {};
for (int i = 0; i < 4; ++i) {
ary[i] = i;
}
return ary;
}
int main() {
constexpr auto ary = get_ary();
static_assert(ary.size() == 4, "The length should be 4!");
}
Can't you just make a constexpr array by making a constexpr function that returns one?
No, you cannot return an array from a function whether it's constexpr or not.
You can however return instance of a class that contains an array as a member. Your A
is an example of a such class template, and so is std::array
. Both examples are allowed.
The std::array
example won't work pre-C++17. There's no issue with returning, but there is an issue with using the non-constexpr operator[]
.