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

Question

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.

Can you elaborate a little more on what you mean when you say the variable is used as a template parameter? — AndyG
– AndyG, Commented Mar 27, 2019 at 17:51
@AndyG There is a template parameter, and i needed to be able to change it sequentually (see example, that was already in the question) — v010dya
– v010dya, Commented Mar 28, 2019 at 14:33

cpplearner · Accepted Answer · 2019-03-27 06:22:00Z

3

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()>{});

answered Mar 27, 2019 at 6:22

cpplearner

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3 Comments

v010dya Over a year ago

I don't quite understand. Would i need to make everything global for this? BASES_TO_CHECK can be global, it's even better that way, but recordMap and counterMap are local variables.

v010dya Over a year ago

Ok I have made BASES_TO_CHECK global, and added passed by reference parameters recordMap, counterMap, and done (as they are suppose to be local variables). Thanks a lot. It would be nice to have explanation as to what is happening, but i'll try to read it up myself.

Rerito Over a year ago

Well your recordMap, counterMap and done can still be passed as parameters to the gen_impl function.

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