lock_guard

#include <iostream>
#include <vector>
#include <thread>
#include <cmath>
#include <mutex>
using namespace std;

mutex m;

struct Point {
    double x, y;
    };

typedef vector<Point>::iterator ci;

class Circle {

    private:
        Point a;
        double r;

    public:
        Circle(const Point& t, double r_): a(t), r(r_) {}

        double distance(Point &t){
           double b = sqrt( pow(t.x- a.x,2) + pow(t.y- a.y,2) )- r;

           return abs(b);
                    }

    void operator()(ci begin, ci end, Point &min){

        lock_guard<mutex> lock(m);

           Point minimal = *begin;
            for(; begin!=end; ++begin){
                if( distance(*begin) < distance(minimal)) {
                    minimal=*begin;
                    }
                } if (distance(minimal)< distance(min))
                    min = minimal;

        }

};

int main()
{
    int n=3;
    Circle c({0,0},3);
    vector<Point> points = {{0,0},{1,1},{2,2},{3,3},{0,3},{6,3},{-63,3},{5,5},{6,6}};
    Point min = points[0];

    thread t1 = thread(c, points.begin(), points.begin()+3, ref(min));
    thread t2 = thread(c, points.begin()+3, points.begin()+6, ref(min));
    thread t3 = thread(c, points.begin()+6,points.end(), ref(min));

    t1.join();
    t2.join();
    t3.join();

    cout<<"Tacka ("<<min.x<<","<<min.y<<") je najbliza kruznici." <<endl;

    return 0;
}