How to create std::initializer_list with integer_sequence?

Question

I have an integer_sequence and I need to create std::initializer_list for constructor of std::map. The idea is below:

template<uint8_t... Indices>
static constexpr auto GenerateRegMap(std::integer_sequence<uint8_t, Indices...>)
    -> std::initializer_list<std::pair<uint8_t const, uint8_t>>
{
    return {{Indices, 0}...};
}

constexpr auto s = std::integer_sequence<uint8_t, 2, 4, 5, 7, 8, 10, 11, 13, 15>{};
std::map<uint8_t, uint8_t> m (GenerateRegMap(s));
std::cout << m.size() << std::endl;
for (const auto &pair : m)
{
    std::cout << static_cast<int>(pair.first) << " has value " << static_cast<int>(pair.second) << std::endl;
}

There std::initializer_list is normally generated, but it cannot be returned: it uses local storage. I cannot imagine how to write the content of GenerateRegMap in place.

Really m is private field of some class and it would be nice to init it in the member initializer list.

Why don't I return std::map<uint8_t, uint8_t> from GenerateRegMap? Because I've got an error: a constexpr function cannot have a nonliteral return type.

Answer 1

Instead of returning a local initializer_list from the GenerateRegMap, you can use C++14 variable template to generate the initializer_list directly

template<uint8_t... Indices>
constexpr std::initializer_list<std::pair<uint8_t const, uint8_t>> my_list = 
  {{Indices, 0}...};

constexpr auto s = my_list<2, 4, 5, 7, 8, 10, 11, 13, 15>;

std::map<uint8_t, uint8_t> m(s);

If you want the variable template to take integer_sequence as the template type, you can use template partial specialization to get values of integer_sequence to initialize initializer_list, for example

using L = std::initializer_list<std::pair<uint8_t const, uint8_t>>;
template<class Seq>
constexpr L my_list = {};
template<class Seq>
constexpr L my_list<const Seq> = my_list<Seq>;
template<uint8_t... Indices>
constexpr L my_list<std::integer_sequence<uint8_t, Indices...>> = {{Indices, 0}...};

This enables you to generate initializer_list with pre-defined integer_sequence

constexpr auto s = std::integer_sequence<uint8_t, 2, 4, 5, 7, 8, 10, 11, 13, 15>{};
std::map<uint8_t, uint8_t> m(my_list<decltype(s)>);