Класс-функтор с использованием foreach и transform

#include "stdafx.h"
#include <iostream>
#include <ctime>
#include <algorithm>
#include <vector>
#include <list>
#include <sstream>
#include <map>
#include <string>
#include <iterator>
#include <fstream>
using namespace std;
 
class mean {
private:
    double sum;
    int cont_size;
public:
    mean() { int sum = 0; int cont_size = 0; }
    int operator()(int x) {
        cont_size++;
        sum = sum + x;
        return sum;
    }
    double get_average_value() {
        return sum / cont_size;
    }
};
template<class T>
double average_value(T& container) {
    mean average;
    for_each<T::iterator, mean&>(container.begin(), container.end(), average);
    //transform<T::iterator, T::iterator, mean&>(container.begin(), container.end(), container.begin(), average);
    return average.get_average_value();
}
void out_vector(vector <double> v){
    for (int i = 0; i<v.size(); i++) cout << v[i] << " ";
    cout << endl;
}
 
int _tmain(int argc, _TCHAR* argv[])
{
    int size;
    cout << "Enter the size of vector:"; cin >> size; cout << endl;
    vector<double> v(size);
    int i;
    cout << "Enter elements in vector:" << endl;
    for (int i = 0; i < v.size(); i++) {
        cin >> v[i];
    }
    cout << "Your vector: \n";
    out_vector(v);
    cout.setf(ios::fixed);
    cout << average_value(v)<<endl;
    system("pause");
    return 0;
}

class mean
{
private:
    double m_sum;
    int m_cont_size;
public:
    mean(double sum = 0, int cont_size = 0) : m_sum(sum), m_cont_size(cont_size)
    {
    }
    
    double operator()(double x)
    {
        m_cont_size++;
        m_sum = m_sum + x;
        return m_sum;
    }
    
    double get_average_value()
    {   // проверка деления на ноль
        return m_cont_size ? (m_sum / m_cont_size) : 0;
    }
};