[C++] STL Vector 2차원 선언 및 초기화

#include<iostream>
#include<vector>

using namespace std;

int main(void)
{
    vector<vector<int>> matrix_test(5);
    cout << matrix_test.size() << endl;

    cout << "==================" << endl;

    vector<vector<int>> matrix(5, vector<int> (4,3));


    for(int i=0; i<matrix.size() ; i++)
    {
        for(int j=0; j<matrix[0].size(); j++)
                cout << matrix[i][j] << ' ';
        cout << endl;
    }

    return 0;
}

5
==================
3 3 3 3
3 3 3 3
3 3 3 3
3 3 3 3
3 3 3 3