Create constexpr template function to sum any container in C++ - c++

Question

I need to create a constexpr template function to sum container, and pass the static assert. This is my code, but I have to questions.
I recived this error: assignment of read-only reference ‘res’.
Is the sum function written correctly?
#include <iostream>
#include <array>
using namespace std;
template <typename T>
constexpr auto Sum(const T& arr)
{
using arrType = decltype(*arr.begin());
arrType res = 0;
for (auto it = arr.begin(); it != arr.end(); it++) {
res += *it;
}
return res;
}
int main()
{
constexpr array<int, 3> base{5, 2, 0};
static_assert(Sum(base) == 7);
return 0;
}

Answer 1

Your code doesn't compile because since arr is const reference, decltype(*arr.begin()) is const int. So your res is const and you can't assign to it in your loop.
To fix only that error, you should use std::decay, i.e:
using arrType = std::decay_t<decltype(*arr.begin())>;
You don't specify what you mean by "container". If you mean the standard library's definition (which is supported by all STL containers), you can use value_type instead and also further simplify your code:
template <typename T>
constexpr auto Sum(const T& arr) {
typename T::value_type result{};
for (auto const& x : arr) {
result += x;
}
return result;
}
If you have C++20, you can also use std::accumulate:
template <typename T>
constexpr auto Sum(const T& arr)
{
return std::accumulate(arr.begin(), arr.end(),
typename T::value_type{});
}

Answer 2

The problem lies with decltype(*arr.begin()), which gives const U& if U represents the type of the elements of the container arr.
Applying std::remove_reference and std::remove_cv makes it compile.
#include <iostream>
#include <type_traits>
#include <array>
using namespace std;
template <typename T>
constexpr auto Sum(const T& arr)
{
using arrType = remove_cv_t<remove_reference_t<decltype(*arr.begin())>>;
arrType res = 0;
for (auto it = arr.begin(); it != arr.end(); it++) {
res += *it;
}
return res;
}
int main()
{
constexpr array<int, 3> base{5, 2, 0};
static_assert(Sum(base) == 7);
return 0;
}