Is this Prim's MST Source Code is O(E logV) for dense graph? Notes: https://codeforces.com/blog/entry/68133?#comment-524560

#include <bits/stdc++.h>
using namespace std;

class a{
	public:
	vector<int> v;
	a(){
		v = {1,2,3};
	}
	bool cmp(const int &a, const int &b){
		return (a < b);
	}
	void fun(){
		sort(v.begin(), v.end(), cmp);
		for(auto x :v ) {
			cout << x;
		}
	}
};

int main() {
	a b;
	b.fun();
}

why it is giving error at sort(v.begin(), v.end(), cmp); ?

#include <bits/stdc++.h>
using namespace std;

class a{
public:
	vector<int> arr(5);
};

int main() {
 a b;
}

why it is giving error at vector<int> arr(5);