C++ static const array initializer

Question

I have the following class:

const unsigned E = 256;

class A {
    public:
        static const unsigned x[E];
    ...
}

and I want to initialize x as follows:

const unsigned A::x[E] = { 1, 2, 3, ..., E };

The above assignment seems to be trivial for now. But, the point is to initialize the value of array x based on the index. A quick try seems to tell me that even with c++11 this isn't possible.

Any input?

Thanks.

It wouldn't technically be initialization, but std::iota may suit your needs... — ildjarn
– ildjarn, Commented Apr 19, 2014 at 19:04
It sounds like std::integer_sequence, but it will not be available until C++14. en.cppreference.com/w/cpp/utility/integer_sequence — imreal
– imreal, Commented Apr 19, 2014 at 19:16
The sequential initialization is a first use case. It may gets complicated later such as: { f(0), f(1), f(2), ... }. — Wen Siang Lin
– Wen Siang Lin, Commented Apr 19, 2014 at 19:22

Casey · Accepted Answer · 2014-04-19 19:24:41Z

2

If you don't mind storing a std::array instead of a C array, this is pretty simple with an integer sequence:

template <int...I>
struct indices {};
template <int N, int...I>
struct make_indices : make_indices<N-1, N-1, I...> {};
template <int...I>
struct make_indices<0, I...> : indices<I...> {};

template <typename T, int...I>
constexpr std::array<T, sizeof...(I)>
iota_array_helper(indices<I...>) {
  return {I...};
}

template <typename T, std::size_t N>
constexpr std::array<T, N>
iota_array() {
  return iota_array_helper<T>(make_indices<N>());
}

which you can use as:

const unsigned E = 256;

class A {
    public:
        static const std::array<unsigned, E> x;
    ...
};

std::array<unsigned, E> A::x = iota_array<unsigned, E>();

Here it is live.

answered Apr 19, 2014 at 19:24

Casey

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2 Comments

Wen Siang Lin Over a year ago

This works but still needs a bit of modification because I want the array to be A::x[0]=1, [1]=2, etc. These level of template is beyond me. What is a good reference to read up on it?

ildjarn Over a year ago

@WenSiangLin : FYI, the only modification needed to offset the integers so that A::x[0]=1, [1]=2 holds true is to change return {I...}; to return {(I+1)...};. This can be generalized by adding an additional non-type template parameter to iota_array and iota_array_helper to hold the offset.

6502 · Accepted Answer · 2014-04-19 19:43:56Z

0

You can with some recursion trick

template<int... values>
struct myclass {
    static const unsigned char x[sizeof...(values)];
};

template<int... values>
const unsigned char myclass<values...>::x[sizeof...(values)] = { values... };

template<int count, int... values>
struct iota_array {
    typedef typename iota_array<count-1, count-1, values...>::value value;
};

template<int... values>
struct iota_array<0, values...> {
    typedef myclass<values...> value;
};

typename iota_array<E>::value myinstance;

The fact that you CAN of course doesn't mean that you should.

answered Apr 19, 2014 at 19:43

6502

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