I am trying to use `std::accumulate` to write into `std::ostream` in the `operator<<` (this is just a minimum example, I know this could be implemented much simpler):

```
#include <iterator>
#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>

struct A {
    A() : v(std::vector<int>()){};
    std::vector<int> v;
    A(std::vector<int> v) : v(v){};
    friend std::ostream& operator<<(std::ostream& stream, A& a);
};

std::ostream& operator<<(std::ostream& stream, A& a) {
// I need something similar to
//    return std::accumulate(a.v.begin(), a.v.end(), "",
                           std::ostream_iterator<int>(stream, " "));
// or:
// return std::accumulate(a.v.begin(), a.v.end(), stream,
                           []()->{});
}

int main(int argc, char* argv[]) {
    std::vector<int> v({1, 2, 3, 4, 5});
    A a(v);
    std::cout << a << std::endl;

    return 0;
}

```

How can I make this operator work?