C++ a^=b b^=a a^=b bug 求解

练手写了简答选择排序代码如下：

#include <iostream>
#include <array>

using namespace std;

#define DEBUG 1

template <typename T>
void swap_pos(T &a, T &b){ 
	#if DEBUG 
	cout << a << " " << b << endl; 
	#endif

	a ^= b; 

	#if DEBUG 
	cout << a << " " << b << endl; 
	#endif

	b ^= a;	

	#if DEBUG 
	cout << a << " " << b << endl; 
	#endif

	a ^= b;	

	#if DEBUG 
	cout << a << " " << b << endl; 
	#endif
}

template <typename T>
void printArr(T const& arr){
	for(auto it = arr.begin(); it != arr.end(); it++){
		cout << *it << " ";
	}
}

template <typename T>
void selectSort(T& arr){
	int nSize = arr.size();
	//add one elem as min_holder
	int min_pos = 0;
	for(int i=0; i<nSize; i++){
		min_pos = i;
		for(int j=i+1; j<nSize; j++){
			if(arr[min_pos] > arr[j]){
				min_pos = j;
			}
		}
		swap_pos(arr[i],arr[min_pos]); //bug at i=3, min_pos=3 for some reason
		#if DEBUG
		printArr(arr); cout << "\n";
		#endif
	}
}

int main(){
	array<int, 10> og_arr = {2,3,4,1,5,6,9,8,7,0};
	array<int, 10> arr(og_arr);

	selectSort(arr);
	printArr(arr);
}

swap_pos 会在 i，min_pos 均为3的循环出错，打出信息如下：

compiler：gcc version 8.1.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)

还请高人指点这是哪方面的问题，多谢！