CODEWITHSUNDEEP
c++c++17constexpr
v010dya
v010dya

Reputation: 5848

How to initialise one array using another constexpr array in a static manner

I have a constexpr array like this:

constexpr std::array<int, 4> BASES_TO_CHECK = { 8, 16, 32, 64 };

I would like to do something akin to:

std::array<std::thread, BASES_TO_CHECK.size()> calc;
for(size_t i = 0; i<BASES_TO_CHECK.size(); ++i)
{
    calc[i]=std::thread(calculate<BASES_TO_CHECK[i]>,  std::ref(recordMap[BASES_TO_CHECK[i]]),  std::ref(counterMap.at(BASES_TO_CHECK[i])),  std::ref(done));
}

However, as the variable is used as a template parameter, so that won't work. I have ended up doing this:

std::array<std::thread, BASES_TO_CHECK.size()> calc = {
    std::thread(calculate<BASES_TO_CHECK[0]>,  std::ref(recordMap[BASES_TO_CHECK[0]]),  std::ref(counterMap.at(BASES_TO_CHECK[0])),  std::ref(done)),
    std::thread(calculate<BASES_TO_CHECK[1]>,  std::ref(recordMap[BASES_TO_CHECK[1]]),  std::ref(counterMap.at(BASES_TO_CHECK[1])),  std::ref(done)),
    std::thread(calculate<BASES_TO_CHECK[2]>,  std::ref(recordMap[BASES_TO_CHECK[2]]),  std::ref(counterMap.at(BASES_TO_CHECK[2])),  std::ref(done)),
    std::thread(calculate<BASES_TO_CHECK[3]>,  std::ref(recordMap[BASES_TO_CHECK[3]]),  std::ref(counterMap.at(BASES_TO_CHECK[3])),  std::ref(done))
};

And this works, but is relying on me not changing the number of elements in the BASES_TO_CHECK without also manually updating the part of the code where calc array is initialised.

Upvotes: 2

Views: 89

Answers (1)

cpplearner
cpplearner

Reputation: 15878

template<std::size_t... i>
std::array<std::thread, BASES_TO_CHECK.size()> gen_impl(std::index_sequence<i...>) {
    return {
                std::thread(calculate<BASES_TO_CHECK[i]>,
                            std::ref(recordMap[BASES_TO_CHECK[i]]),
                            std::ref(counterMap.at(BASES_TO_CHECK[i])),
                            std::ref(done)
                           )...
           };
}

auto calc = gen_impl(std::make_index_sequence<BASES_TO_CHECK.size()>{});

Upvotes: 3

Related Questions