How to iterate std::set?

You must dereference the iterator in order to retrieve the member of your set.

std::set<unsigned long>::iterator it;
for (it = SERVER_IPS.begin(); it != SERVER_IPS.end(); ++it) {
    u_long f = *it; // Note the "*" here
}

If you have C++11 features, you can use a range-based for loop:

for(auto f : SERVER_IPS) {
  // use f here
}

Just use the * before it:

set<unsigned long>::iterator it;
for (it = myset.begin(); it != myset.end(); ++it) {
    cout << *it;
}

This dereferences it and allows you to access the element the iterator is currently on.

Another example for the C++11 standard:

set<int> data;
data.insert(4);
data.insert(5);

for (const int &number : data)
  cout << number;

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