How to use decltype to get vector element's type as template argument

Question

Here is part of my code:

int getLength(const vector<int> &arr) {
    auto  n=arr.size(),dis=n;
    unordered_map<int,decltype(dis)> S;
    //...
}

So far so good. Now instead of hardcoded "int" for my std::unordered_map, I tried to change it to something like: unordered_map<decltype(arr.front()),decltype(dis)> S; or unordered_map<decltype(arr)::value_type,decltype(dis)> S; or unordered_map<decltype(arr[0]),decltype(dis)> S;

None seems to work. What would be the right grammar to use decltype() here?

Answer 1

What would be the right grammar to use decltype here?

decltype(arr.front()) and decltype(arr[0]) are all OK but, unfortunately, all of they returning a reference to a const int (considering that arr is a constant vector)

You have to remove the reference and the const so, by example

std::unordered_map<
      std::remove_const_t<std::remove_reference_t<decltype(arr.front())>>,
      decltype(dis)> S;

Using the ::value_type is better (IMHO) because you avoid the constness, so you have to remove only the reference, so you can write

   std::unordered_map<
      std::remove_reference_t<decltype(arr)>::value_type,
      decltype(dis)> S;