for_each()的返回值

class MeanValue {
  private:
    long num;    // number of elements
    long sum;    // sum of all element values
  public:
    // constructor
    MeanValue () : num(0), sum(0) {
    }

    // "function call"
    // - process one more element of the sequence
    void operator() (int elem) {
        ++num;          // increment count
        sum += elem;    // add value
    }

    // return mean value
    double value () {
        return static_cast<double>(sum) / static_cast<double>(num);
    }
};

int main()
{
    vector<int> coll = { 1, 2, 3, 4, 5, 6, 7, 8 };

    // process and print mean value
    MeanValue mv = for_each (coll.begin(), coll.end(),  // range
                             MeanValue());              // operation
    cout << "mean value: " << mv.value() << endl;
}