struct nd {
	int val, rd, cp, sz;
	nd* ls;
	nd* rs;
	nd(const int& v, const int& r, const int& c, const int& s) {
		val = v;
		rd = r;
		cp = c;
		sz = s;
		ls = empty;
		rs = empty;
	}
};

nd* empty = new nd(0, 0, 0, 0);

毫无疑问会'empty' was not declared in this scope gcc [13, 8]。

该如何实现？