如何檢查C++ std :: vector等的成員身份？

Question

我在做什麼錯在下面的程序？

我想在容器上使用std::find()來決定它是否包含給定的元素。下面的程序適用於空容器，但不適用於具有元素的容器。

#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

這個條件始終爲真：

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

Answer 2

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

仔細查看該行。