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;

    void operator()(int element) 
    { 
       total+=element; 
    }
};

int main()
{
    sum s;

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

Answer 1

for_each takes the functor by value - so it is copied. You can e.g. use a functor which is initialized with a pointer to an external int.

struct sum
{
    sum(int * t):total(t){};
    int * total;

    void operator()(int element)
    {
       *total+=element;
    }
};

int main()
{
    int total = 0;
    sum s(&total);

    int arr[] = {0, 1, 2, 3, 4, 5};
    std::for_each(arr, arr+6, s);
    cout << total << endl; // prints total = 15;
}

Or you can use the return value from for_each

struct sum
{
    sum():total(0){};
    int 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);
    cout << s.total << endl; // prints total = 15;
}