C++ 2D Vector got error

#include <string>
#include <iostream>
#include <vector>
#include <iomanip>

using namespace std;

class matrix
{
 public:
    matrix(int);
    void func(int);
    vector<vector<double> > m;
};

matrix::matrix(int size)
{
    //set 3 X 3 = 9 elements to zero
    vector<vector<int> > m(size, vector<int>(size));

    // success
    cout << "test1 if m[0][0] equals to zero: " << m[0][0] << endl;
}

void matrix::func(int size)
{
    // failure, can't pass till here
    cout << "test2 if m[0][0] equals to zero: " << m[0][0] << endl;

    for(int i = 1; i != size; ++i)
    {
        m[i][i] = 0;
    }
}



int main()
{
    int input1 = 3;

    matrix mat(input1);

    mat.func(input1);
}

I want to create a 2D dimensional vector rather using array. However, I got error. I create vector<vector<int> > m(size, vector<int>(size)) to make sure all elements in vector are equal to zero, and I passed the test1. I can't pass test2 which I commented "failure".