How to make the type of vector in struct determined by the user?

Question

I have this struct that makes multiplication, addition, and subtraction on a matrix of integers. Now I want to make the type of matrix (i.e. the type of vectors) determined by the user of this struct i.e. int, double, long, etc..

struct Matrix 
{
    vector<vector<int>> mat1, mat2;

    vector<vector<int>> mult()
    {
        vector<vector<int>> res(mat1.size(), vector<int>(mat2.back().size()));
        for (int r1 = 0; r1 < mat1.size(); ++r1) {
            for (int c2 = 0; c2 < mat2.back().size(); ++c2) {
                for (int r2 = 0; r2 < mat2.size(); ++r2) {
                    res[r1][c2] += mat1[r1][r2] * mat2[r2][c2];
                }
            }
        }
        return res;
    }

    vector<vector<int>> add()
    {
        vector<vector<int>> res(mat1.size(), vector<int>(mat1.back().size()));
        for (int i = 0; i < mat1.size(); ++i) {
            for (int j = 0; j < mat1.back().size(); ++j) {
                res[i][j] = mat1[i][j] + mat2[i][j];
            }
        }
        return res;
    }

    vector<vector<int>> subtract() 
    {
        vector<vector<int>> res(mat1.size(), vector<int>(mat1.back().size()));
        for (int i = 0; i < mat1.size(); ++i) {
            for (int j = 0; j < mat1.back().size(); ++j) {
                res[i][j] = mat1[i][j] - mat2[i][j];
            }
        }
        return res;
    }
};

Answer 1

I want to make the type of matrix (i.e. the type of vectors) determined by the user of this struct i.e. int, double, long, etc..

You can make your Martix struct to be a template struct

template<typename T> 
struct Matrix 
{
    std::vector<std::vector<T>> mat1, mat2;

    // .... replace all your int with T
}

Now you instantiate a Matrix class

Matrix<int> mat1;    // for integers
Matrix<long> mat2;   // for long
Matrix<double> mat3; // for doubles

---

As a side notes: Why is "using namespace std;" considered bad practice?