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

Because the s which you pass to the for_each is by value. for_each accepts it by value!

In C++0x, you can solve this problem with for_each as,

int sum  = 0;
std::for_each(arr, arr+6, [&](int n){ sum += n; });
std::cout << sum ;

Output:

15

Demo at ideone : http://ideone.com/s7OOn

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Or you can simple write in the std::cout itself:

std::cout<int{sum += n;return sum;})(0);

Run : http://ideone.com/7Hyla

Note such different syntax is okay for learning purpose, as to how std::for_each works, and what it returns, but I would not recommend this syntax in real code. :-)

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In C++, you can write user-defined conversion function in the functor as,

struct add
{
    int total;
    add():total(0){};
    void operator()(int element)  {  total+=element;  }
    operator int() { return total ; }
};

int main()
{
    int arr[] = {0, 1, 2, 3, 4, 5};
    int sum = std::for_each(arr, arr+6, add());
    std::cout << sum;
}

It's slightly different version from Erik second solution : http://ideone.com/vKnmA