I was reading this question on SO.
The question itself is not so interesting, but I was wondering whether it exists and how to implement a compile time solution.
Regard to the first sequence:
All numbers except the ones which can be divided by 3.
The sequence should be something like:
[1, 2, 4, 5, 7, 8, 10, 11, 13, 14, ...]
By induction, I've found the math formula for that sequence:
f(0) = 0;
f(x > 0) = floor[(3x - 1) / 2];
So I've implemented a C++ constexpr
function which generates the i-th number in the sequence:
#include <type_traits>
template <typename T = std::size_t>
constexpr T generate_ith_number(const std::size_t index) {
static_assert(std::is_integral<T>::value, "T must to be an integral type");
if (index == 0) return 0;
return (3 * index - 1) / 2;
}
Now I'd like to generate a "compile-time array/sequence" which stores the first N-th numbers of the sequence.
The structure should be something like:
template <typename T, T... values>
struct sequence {};
template <typename T, std::size_t SIZE>
struct generate_sequence {}; // TODO: implement
Questions (more than one, but related among them):
1) How to implement that kind of integer_sequence
?
2) Is it possible to build an std::array
from that integer_sequence
at compile time?
constexpr functions Consuming code requires the return value at compile time to initialize a constexpr variable, or to provide a non-type template argument. When its arguments are constexpr values, a constexpr function produces a compile-time constant.
constexpr functions will be evaluated at compile time when all its arguments are constant expressions and the result is used in a constant expression as well.
1) How to implement that kind of
integer_sequence
?
template <std::size_t... Is>
constexpr auto make_sequence_impl(std::index_sequence<Is...>)
{
return std::index_sequence<generate_ith_number(Is)...>{};
}
template <std::size_t N>
constexpr auto make_sequence()
{
return make_sequence_impl(std::make_index_sequence<N>{});
}
2) Is it possible to build an
std::array
from thatinteger_sequence
at compile time?
template <std::size_t... Is>
constexpr auto make_array_from_sequence_impl(std::index_sequence<Is...>)
{
return std::array<std::size_t, sizeof...(Is)>{Is...};
}
template <typename Seq>
constexpr auto make_array_from_sequence(Seq)
{
return make_array_from_sequence_impl(Seq{});
}
Usage:
int main()
{
constexpr auto arr = make_array_from_sequence(make_sequence<6>());
static_assert(arr[0] == 0);
static_assert(arr[1] == 1);
static_assert(arr[2] == 2);
static_assert(arr[3] == 4);
static_assert(arr[4] == 5);
static_assert(arr[5] == 7);
}
live example on wandbox.org
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With