rt，样例过了，代码如下

#include <iostream>

using namespace std;

class Queue{
private:
	int a[10001],f = 0,l = 0;
public:
	int size(){
		return l-f;
	}
	void push(int x){
		a[l++] = x;
	}
	void pop(){
		if(l == f){
			throw "ERR_CANNOT_QUERY";
			return;
		}
		f++;
	}
	int front(){
		if(l == f){
			throw "ERR_CANNOT_QUERY";
			return 0;
		}
		return a[f];
	}
}q;
int n,op,x;

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin >> n;
	while(n--){
		cin >> op;
		if(op == 1){
			cin >> x;
			q.push(x);
		}
		else if(op == 2){
			try{
				q.pop();
			}
			catch(const char *e){
				cout << e << '\n';
			}
		}
		else if(op == 3){
			try{
				cout << q.front() << '\n';
			}
			catch(const char *e){
				cout << e << '\n';
			}
		}
		else{
			cout << q.size() << '\n';
		}
	}

	return 0;
}

一时兴起想搞个异常捕获玩玩

dalao 救我……