c++ - 为什么这个示例有效？

Question

  typedef pair<double, double> dd; 

  const double epsilon = 1e-6;

  struct sort_by_polar_angle { 
    dd center; 
    // Constuctor of any type 
    // Just find and store the center 
    template<typename T> sort_by_polar_angle(T b, T e) { 
        int count = 0;
        center = dd(0,0); 
        while(b != e) { 
                    center.first += b->first;
                    center.second += b->second;
               b++; 
            count++;
        } 
               double k = count ? (1.0/count) : 0;
        center.first *= k;
               center.second *= k;
   } 
   // Compare two points, return true if the first one is earlier 
   // than the second one looking by polar angle 
   // Remember, that when writing comparator, you should 
   // override not ‘operator <’ but ‘operator ()’ 
   bool operator () (const dd& a, const dd& b) const { 
        double p1 = atan2(a.second-center.second, a.first-center.first); 
        double p2 = atan2(b.second-center.second, b.first-center.first); 
        return p1 + epsilon < p2; 
   } 
  }; 

// ... 

vector < dd >  points; 

sort(all(points), sort_by_polar_angle(all(points))); 

当调用 sort_by_polar_angle() 时，它是否起到构造函数的作用？重载的operator()如何正确使用？

Answer 1

当您在 sort() 函数中调用 sort_by_polar_angle() 时，您正在创建一个 sort_by_polar_angle 类型的临时对象(即它的构造函数叫做)。在排序算法内部，您传递的仿函数对象使用类似 functor() 的东西，它将被翻译成 functor.operator()。