Segfault in a matrix code using new

Question

Hi all

I created a simple class in C++ which has a private dynamic array. In the constructor I initialize the array using new and in the destructor I free it using delete.

When I instantiate the class using Class a = Class(..); it works as expected, however it seems I cannot instantiate it using the new operator (Like Class *a = new Class(..);), I always get a segmentation fault.

What I don't understand is when I should use new to instantiate a class and when just call the constructor or should it be possible to instantiate a class either with new or by just calling the constructor.

float** A = new float*[3];
for (int i=0; i<3; i++) {
    A[i] = new float[3];
}

A[0][0] = 3; A[0][1] = 3; A[0][2] = 4;
A[1][0] = 5; A[1][1] = 6; A[1][2] = 7;
A[2][0] = 1; A[2][1] = 2; A[2][2] = 3;

Matrix *M = new Matrix(A, 3, 3);

delete[] A;
delete M;

Below the class definition..

class Matrix
{
private:
    int width;
    int height;
    int stride;
    float* elements;

public:
    Matrix(float** a, int n, int m);
    ~Matrix();
};


Matrix::Matrix(float** a, int n, int m)
{
    // n: num rows
    // m: elem per rows

    elements = new float[n*m];

    for (int i=0; i<n; i++) {
        for (int j=0; j<m; j++) {
            elements[i*n + j] = a[n][m];
        }
    }
}

Matrix::~Matrix()
{
    delete[] elements;
}

Answer 1

Replace i*n + j by i*m + j. and replace a[n][m] by a[i][j]