Use of a functor on for_each

Question

Why does the for_each call on functor doesn\'t update sum::total at the end?

struct sum
{
    sum():total(0){};
    int total;

            


        

Answer 1

This happens due to std::for_each requires the functor to be passed by value . A workaround for your solution:

struct sum
{
    sum():total(0){};
    int total;
    sum(sum & temp)
    {
        total = temp.total;
    }
    void operator()(int element) 
    { 
       total+=element; 
    }
};

int main()
{
    sum s;

    int arr[] = {0, 1, 2, 3, 4, 5};
    s = std::for_each(arr, arr+6, s);  // result of for_each assigned back to s
    cout << s.total << endl; // prints total = 0;
}