поиск ближайшего

#include <iostream>
#include <algorithm>
//#include <vector>
#include <set>
//#include <cmath>

using namespace std;

set<int>::const_iterator FindNearestElement(const set<int>& numbers, int border)
{
auto it1 = lower_bound(numbers.begin(), numbers.end(), border);
auto it2 = it1;
if (*it1==border) return it1;
if (it1!=numbers.begin())
{
    --it2;
    if  (abs(-*it2+border)<=abs(-*it1+border)) return it2;
} /*else*/ return it1;
}

int main()
{
    setlocale(LC_ALL, "RU");
    set<int> numbers = { 1, 4, 6};
    cout << *FindNearestElement(numbers, 0) << " " << *FindNearestElement(numbers, 3) << " "
        << *FindNearestElement(numbers, 5) << " " << *FindNearestElement(numbers, 6) << " "
        << *FindNearestElement(numbers, 100) << endl;
    set<int> empty_set;
    cout << (FindNearestElement(empty_set, 8) == end(empty_set)) << endl;

    return 0;
}