How to initialize std::array elegantly if T is not default constructible?

Question

How do I initialize std::array if T is not default constructible?

I know it\'s possible to initialize it like that:

Answer 1

Following will solve your issue:

#if 1 // Not in C++11, but in C++1y (with a non linear better version)

template  struct index_sequence {};

template 
struct make_index_sequence : make_index_sequence {};

template 
struct make_index_sequence<0, Is...> : index_sequence {};

#endif

namespace detail
{
    template 
    constexpr std::array
    create_array(T value, index_sequence)
    {
        // cast Is to void to remove the warning: unused value
        return {{(static_cast(Is), value)...}};
    }
}

template 
constexpr std::array create_array(const T& value)
{
    return detail::create_array(value, make_index_sequence());
}

So test it:

struct NoDefaultConstructible {
    constexpr NoDefaultConstructible(int i) : m_i(i) { }
    int m_i;
};

int main()
{
    constexpr auto ar1 = create_array<10>(NoDefaultConstructible(42));
    constexpr std::array ar2 = create_array<10>(NoDefaultConstructible(42));

    return 0;
}