function_object lost value of data member after for_each - c++

Question

The following code should store a key and a value. At the end, I want a total sum of all values. But the variable *sum_all* in the my function object sum contains every time "0". What is going on?
// map
#include <iostream>
#include <string>
#include <map>
#include <algorithm>
using namespace std;
struct Item {
int count;
double value;
};
class Sum {
public:
Sum() {
sum_all = 0.0;
}
// keys are stored as const in a map
void operator()(pair<const string, Item>& pair) {
cout << pair.first << "\n";
cout << "Sum: " << pair.second.value << "\n";
cout << "Middle: " << pair.second.value/pair.second.count << "\n";
sum_all += pair.second.value;
}
double get_sum_all() {
return sum_all;
}
private:
double sum_all;
};
int main() {
map<string, Item> table;
for (int i = 0; i < 3; i++) {
string key;
double value;
cin >> key;
cin >> value;
// new item
if (table.find(key) == table.end()) {
Item item;
item.count = 1;
item.value = value;
table[key] = item;
} else {
Item& item = table[key];
item.count++;
item.value += value;
}
}
Sum sum;
for_each(table.begin(), table.end(), sum);
cout << "table.size() " << table.size() << "\n";
cout << "sum.get_sum_all() " << sum.get_sum_all() << "\n";
cout << "sum.get_sum_all()/table.size()" << sum.get_sum_all()/table.size() << "\n";
return 0;
}
Example input/output:
[peter#donut chap_6]$ ./u3_map
foo 1
bar 2
foo 1
bar
Sum: 2
Middle: 2
foo
Sum: 2
Middle: 1
table.size() 2
sum.get_sum_all() 0
sum.get_sum_all()/table.size()0
Debugging shows, that sum_all is modified like it should during for_each. But I'm not able to set a watchpoint, or gdb ignores the watchpoint. I thought the constructor is called twice, but this seem not to happen. What I'm doing wrong?
Thanks.

Answer 1

std::for_each takes its functor argument by value, not by reference. The original is not modified. You need to do something like this:
sum = for_each(table.begin(), table.end(), sum);