C ++比较一个类中的函数怪异行为

Question

我是C ++的新手。我尽力使标题更有意义。我正在尝试解决K-NN问题（在2D中）。我发现这个代码块有效（也许你可以跳过它并先看看问题）：

// Example program
#include <iostream>
#include <string>
#include <algorithm>
#include <queue>
#include <math.h>  
#include <vector>
using namespace std;

struct Point { 
    double x;
    double y; 
    Point(double a, double b) {
        x = a;
        y = b;
    }
};

double getDistance(Point a, Point b) {
    return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}
typedef bool (*comp)(Point, Point);
Point global_origin = Point(0,0);
bool compare(Point a, Point b)
{
   return (getDistance(a, global_origin)< getDistance(b, global_origin));
}

vector<Point> Solution(vector<Point> &array, Point origin, int k) {
    global_origin = Point(origin.x, origin.y);
    priority_queue<Point, std::vector<Point>, comp> pq(compare);
    vector<Point> ret;
    for (int i = 0; i < array.size(); i++) {
        Point p = array[i];
        pq.push(p);
        if (pq.size() > k)
            pq.pop();
    }
    int index = 0;
    while (!pq.empty()){
        Point p = pq.top();
        ret.push_back(p);
        pq.pop();
    }
    return ret;
}

int main()
{
   Point p1 = Point(4.5, 6.0);
   Point p2 = Point(4.0, 7.0);
   Point p3 = Point(4.0, 4.0);
   Point p4 = Point(2.0, 5.0);
   Point p5 = Point(1.0, 1.0);
   vector<Point> array = {p1, p2, p3, p4, p5};
   int k = 2;
   Point origin = Point(0.0, 0.0);
   vector<Point> ans = Solution(array, origin, k);
   for (int i = 0; i < ans.size(); i++) {
       cout << i << ": " << ans[i].x << "," << ans[i].y << endl;
   }
}

但是，如果我想将解决方案部分放入这样的类：

// Example program
#include <iostream>
#include <string>
#include <algorithm>
#include <queue>
#include <math.h>  
#include <vector>
using namespace std;

struct Point { 
    double x;
    double y; 
    Point(double a, double b) {
        x = a;
        y = b;
    }
};
class MySolution {
public:
  double getDistance(Point a, Point b) {
      return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
  }
  typedef bool (*comp)(Point, Point);
  Point global_origin = Point(0,0);
  bool compare(Point a, Point b)
  {
     return (getDistance(a, global_origin)< getDistance(b, global_origin));
  } 

  vector<Point> Solution(vector<Point> &array, Point origin, int k) {
      global_origin = Point(origin.x, origin.y);
      priority_queue<Point, std::vector<Point>, comp> pq(compare);
      vector<Point> ret;
      for (int i = 0; i < array.size(); i++) {
          Point p = array[i];
          pq.push(p);
          if (pq.size() > k)
              pq.pop();
      }
      int index = 0;
      while (!pq.empty()){
          Point p = pq.top();
          ret.push_back(p);
          pq.pop();
      }
      return ret;
  }
};


int main()
{
   Point p1 = Point(4.5, 6.0);
   Point p2 = Point(4.0, 7.0);
   Point p3 = Point(4.0, 4.0);
   Point p4 = Point(2.0, 5.0);
   Point p5 = Point(1.0, 1.0);
   vector<Point> array = {p1, p2, p3, p4, p5};
   int k = 2;
   Point origin = Point(0.0, 0.0);
   vector<Point> ans = MySolution().Solution(array, origin, k);
   for (int i = 0; i < ans.size(); i++) {
       cout << i << ": " << ans[i].x << "," << ans[i].y << endl;
   }
}

我用g ++编译它，然后得到

kcloset_copy.cpp:32:58: error: reference to non-static member function must be called
      priority_queue<Point, std::vector<Point>, comp> pq(compare);
                                                         ^~~~~~~
1 error generated.

我尝试了几种不同的编写比较函数的方法。当我将它包装在类中时，每个在类外部工作的方法都会失败。我对这个原因感兴趣，如果我想在课堂上做，我该怎么做。

谢谢你的回复。我尝试了下面提供的所有代码，但没有一个能够正常工作。有些答案甚至提出了两种解决方案，但也没有一种能够解决问题。像重载方法一样，它给出了

"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__functional_base:61:21: error: 
      invalid operands to binary expression ('const Point' and 'const Point')
        {return __x < __y;}
                ~~~ ^ ~~~
"

。如果有人能给我任何实际有效的代码，我将不胜感激。

Answer 1

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Answer 2

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Answer 3

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