C++ class中写sort二级排序

class EvenRoute {
public:
struct node {
int x;
int y;
node(int a = 0, int b = 0) : x(a), y(b) {}
};

static bool cmp(const node& c, const node& d) {   //加static
if(c.x == d.x)    return c.y < d.y;
return c.x < d.x;
}

string isItPossible(vector <int> x, vector <int> y, int w) {
struct node a[1000];
int n = 1000;
//....
sort(a, a + n, cmp);
return "CAN";
}
};