关于C++表达式求值的困惑

#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
int a = 3;
int b = 4;
int c = 5;
//int ans = a + b+ ++c;//13
//int ans = a+b+++c; //12
int ans = a + b++ + c;//12
cout << ans << endl;
return 0;
}