c++: vector does not name a type

very easy but frustrating question from me.

#include <vector>
#include <iostream>

using namespace std;

vector <int> queue;

queue.push_back(2);

int main(){
    cout << queue[0] <<endl;
}

compiled with g++ -std=c++11 Cpp_test.cpp, return error

Cpp_test.cpp:51:1: error: ‘queue’ does not name a type
 queue.push_back(2);

Can anyone help? Thanks a lot!