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// C++ program to Count all pair with given XOR
// value x
#include<bits/stdc++.h>
using namespace std;
 
// Returns count of pairs in arr[0..n-1] with XOR
// value equals to x.
int xorPairCount(int arr[], int n, int x)
{
    int result = 0; // Initialize result
 
    // create empty set that stores the visiting 
    // element of array. 
    // Refer below post for details of unordered_set
    // http://w...content-available-to-author-only...s.org/unorderd_set-stl-uses/
    unordered_set<int> s;
 
    for (int i=0; i<n ; i++)
    {
        // If there exist an element in set s
        // with XOR equals to x^arr[i], that means
        // there exist an element such that the
        // XOR of element with arr[i] is equal to
        // x, then increment count.
        if (s.find(x^arr[i]) != s.end())
            result++;
 
        // Make element visited
        s.insert(arr[i]);
    }
 
    // return total count of pairs with XOR equal to x
    return result;
}
 
// driver program
int main()
{
    int arr[] = {5 , 4 ,10, 15, 7, 6};
    int n = sizeof(arr)/sizeof(arr[0]);
    int x = 5;
    cout << "Count of pairs : "
         << xorPairCount(arr, n, x);
    return 0;
}

prog.cpp: In function 'int xorPairCount(int*, int, int)':
prog.cpp:16:5: error: 'unordered_set' was not declared in this scope
     unordered_set<int> s;
     ^
prog.cpp:16:19: error: expected primary-expression before 'int'
     unordered_set<int> s;
                   ^
prog.cpp:25:13: error: 's' was not declared in this scope
         if (s.find(x^arr[i]) != s.end())
             ^
prog.cpp:29:9: error: 's' was not declared in this scope
         s.insert(arr[i]);
         ^

Standard output is empty