如何在 constexpr function 中使用 reverse_iterator

Question

I have a prepopulated std::vector<uint64_t> that I am passing to my constexpr function by constant reference and another std::vector<uint64_t> that I'm passing by reference to be populated with values.

I want to calculate the distance between each element in the collection vector below and store the results into the distances vector. In other words, I want to take the value at index 1 and subtract it from the value at index 0 then store that into the other vector, and so on.

I was thinking of using either reverse_iterator or const_reverse_iterator to help with this process...

constexpr void euclidean_distance(const std::vector<uint64_t>& collection, std::vector<uint64_t> distances ) {
    auto It1 = collection.rbegin();   // fails to compile
    auto It2 = collection.crbegin();  // fails to compile

    // ... code to do the calculation...
}

However, within Visual Studio 2017 both of these fail to compile as they are not a literal type...

What options do I have to achieve a similar functionality?

Answer 1

There are too many issues here, and I can't tell which is the problem...

In C++, constexpr means that the value must be available at compile time. const, on the other hand, just means the data is unchangeable.

Don't use constexpr here. It makes no sense. In fact, even having your function return const makes no sense, void functions don't return anything. and should not be qualified by const or constexpr under any circumstances. constexpr functions are for cases where you want to evaluate computations between primitive data types (ie, int, float, char) at compile time. and the inputs must also be constexpr.

primes isn't listed here, by the way...is it a typo?

You shouldn't be initializing.crend() to anything in the first place, by the way. You should use.crbegin() instead.

const std::vector<uint64_t>& is not a const reference, it's a reference to a const vector. And technically, all references are const...