CODEWITHSUNDEEP
c++arraystemplatesinitializationrange
Piotr Siupa
Piotr Siupa

Reputation: 4838

Initialize array by the range

I need to create an array with all values in a range. I can't set values after creating it, because array must be constexpr.

template<int FIRST, int LAST>
struct Foo
{
    static constexpr int array[LAST - FIRST + 1] = ???;
};

For exemple Foo<3, 7>::array;

should be equivalent to static constexpr int array[5] = {3, 4, 5, 6, 7};.

Is it even possible?

Upvotes: 1

Views: 2653

Answers (2)

T.C.
T.C.

Reputation: 137330

Yes, it can be done in C++11. No, it's not pretty. You essentially need to reimplement C++14's compile-time integer sequences.

///////////////// Reimplement compile-time integer sequences ///////////////////////
// There's plenty of better implementations around.
// This one is similar to libstdc++'s implementation.
template<class T, T... Ints>
struct integer_seq {
    using next = integer_seq<T, Ints..., sizeof...(Ints)>;
    static constexpr std::size_t size() { return sizeof...(Ints); }
};

template<class T, int Len>
struct seq_builder{
    static_assert(Len > 0, "Length must be nonnegative");
    using type = typename seq_builder<T, Len-1>::type::next;
};

template<class T>
struct seq_builder<T, 0>{
    using type = integer_seq<T>;
};

template<class T, int length>
using make_int_sequence = typename seq_builder<T, length>::type;

/////////////////// Actual stuff starts here/////////////////////////////////

template<int FIRST, int LAST, class = make_int_sequence<int, LAST+1-FIRST>>
struct Foo;

template<int FIRST, int LAST, int... SEQ>
struct Foo<FIRST, LAST, integer_seq<int, SEQ...>>
{
    static constexpr int array[sizeof...(SEQ)] = {(FIRST+SEQ)...};
};

template<int FIRST, int LAST, int... SEQ>
constexpr int Foo<FIRST, LAST, integer_seq<int, SEQ...>>::array[sizeof...(SEQ)];

Demo.

Upvotes: 5

zmbq
zmbq

Reputation: 39013

It's possible in the proposed C++14 standard. I don't think it's possible with C++11.

If you really must do that, and don't just have a few such arrays that you can hard-code, you can resort to C macros. Apparently there are some libraries that help you perform loops in macros .

Upvotes: 0

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