如何检查C ++ std :: vector等中的成员身份？

Question

我在以下程序中做错了什么？

我想在容器上使用std::find()来确定它是否包含给定的元素。 以下程序适用于空容器，但不适用于带有元素的容器。

#include <iostream>
#include <vector>
#include <cassert>

struct Pair {int x,y;};

const bool operator==(Pair p, Pair q) {return p.x == p.x && q.y == q.y ;}

typedef std::vector<Pair> p_containr_t;

int main (int argc, char * const argv[]) {
  const Pair start_p = {1,2};
  const Pair second_p = {3,4};
  const Pair other_p = {5,6};
  p_containr_t v;
  p_containr_t::iterator where;

  where = std::find(v.begin(),v.end(),other_p);
  assert(where == v.end());
  std::cout << "OK for empty\n";                     // Program reaches here.

  v.push_back(start_p);
  where = std::find(v.begin(),v.end(),other_p);
  assert(where == v.end());                          // This assertion fails.
  std::cout << "OK for first element\n";

  v.push_back(second_p);
  where = std::find(v.begin(),v.end(),other_p);
  std::cout << "OK for second element\n";      // Fails too (if I edit above).

  return 0;
}

Answer 1

const bool operator==(Pair p, Pair q) {return p.x == p.x && q.y == q.y ;}

仔细观察那条线。

Answer 2

此条件始终为真：

const bool operator==(Pair p, Pair q) {return p.x == p.x && q.y == q.y ;}