Access to iterators of std containers by a template

Question

I want to write a function that takes an std container beginning and ending to add all values in the container to the third argument. For example, if I have

std::vector<int> my_ints{ 1, 2, 3 };
int sum_int = accumulate(my_ints.cbegin(), my_ints.cend(), 10); //should return 16.

I wanted to generalize this function in a way that it can work with any std containers. How can I write a template that can access to the iterators of the elements?

template<typename It, typename T>
T accumulate(It begin, It end, T x)
{
    for (It i = begin; i != end; ++i) 
    {
        x = x + i;
    }
    return x;
}

This is what I have at the moment. But, it does not compile because x and i are not the same type.

It looks very much like `std::accumulate` too. It may be a good source for inspriration. — Ted Lyngmo, Jun 09 '19 at 00:41
Your code is trying to sum the iterators. It needs to sum the values that those iterators refer to. — Peter, Jun 09 '19 at 01:07

score 1 · Answer 1 · answered Jun 09 '19 at 03:44

You should reach for std::for_each to do this rather than writing your own function. It will accept any container and any range of values.

If you want to write your own the look into the for-range statement:

template <typename C>
auto accumulate(const C& c) {
    typename C::value_type x { };
    for (auto value : c) {
        x += value;
    }
    return x;
}
std::vector<int> my_ints{ 1, 2, 3 };
int sum = accumulate(my_ints);

score 0 · Accepted Answer · answered Jun 09 '19 at 21:36

As Peter mentioned in a comment, you are trying to add the actual iterator to x instead of the value it's pointing at. You need to dereference the iterator to get the value:

template<typename It, typename T>
T accumulate(It begin, It end, T x) {
    // there's no need create a new iterator (i) here, use the one you already have (begin):
    for(; begin != end; ++begin)
        x += *begin; // use * to dereference the iterator
    return x;
}

Access to iterators of std containers by a template

2 Answers2