I'm new to C++ and trying to understand vectors. My goal is to update a vector in method:

#include <vector>
#include <iostream>

using namespace std;

void test(vector<int>& array){
    for(int i=0;i<10;i++){
        array.push_back(i);
    }
}

int main(){
    // some integer value
    vector<int> array(10);

    test(array);

    for(int i=0;i<array.size();++i)
        cout<<array.at(i)<<endl;

    cout<<"array size:"<<array.size()<<endl;
    return 0;
}

output:

0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
array size:20

I haven't figure out why 10 zeros add vector at first?