Dynamic arrays inside a class

Question

I am doing the following exercise at the moment:

A generic Matrix class (15 pt)

a) Create a class called Matrix, it should contain storage for M*N numbers of type double. Just like earlier, when choosing how to store your data it is often useful to know what we are going to use the data for later. In matrix operations we are going to access the different elements of the matrix based on their column and/or row, therefore it is useful to order the members of Matrix as an array. Also, we are going to need to change the size of the data stored in the matrix, therefore it should be dynamically allocated.

b) Create constructors for the matrix.

Create the following three constructors: Matrix() • Default constructor, should initialize the matrix into the invalid state.

explicit Matrix(unsigned int N) • Should construct a valid NxN Matrix, initialized as an identity matrix. (The explicit keyword is not in the syllabus, but it should be used here.)

Matrix(unsigned int M, unsigned int N) • Should construct a valid MxN Matrix, initialized as a Zero matrix. (All elements are zero.)

~Matrix() • The destructor of Matrix, should delete any dynamically allocated memory.

My class this far is as follows:

    class Matrix{
    private:
        int rows;
        int columns;
        double* matrix;
    public:
        Matrix();
        explicit Matrix(int N);
        Matrix(int M, int N);
        ~Matrix();
};

And the rest of my code:

    Matrix::Matrix(){
    double * matrix = NULL;
}

Matrix::Matrix(int N){
    double * matrix = new double[N * N];
    this->rows = N;
    this->columns = N;

    for(int i = 0; i < N; i++){
        for(int j = 0; j < N; j++){
            if(i==j)
                matrix[i * N + j] = 1;
            else
                matrix[i * N + j] = 0;
        }
    }
}

Matrix::Matrix(int M, int N){
    double * matrix = new double[M * N];
    this->rows = M;
    this->columns = N;

    for(int i = 0; i < M; i++){
        for(int j = 0; j < N; j++)
            matrix[i * N + j] =  0;
    }
}

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

Have I created the dynamic array and the constructors correctly? I am later in the exercise to create three different arrays using the three different constructors. How do I do this correclty? If i try something like this

Matrix::Matrix();
Matrix::Matrix(3);

or

Matrix::Matrix(3,4)

i get the following error:

Unhandeled exception at 0x773c15de in Øving_6.exe: 0xC0000005: Access violation reading location 0xccccccc0.

What am i doing wrong?

Answer 1

In your constructors, you're defining a local variable

double * matrix = new double[N * N];

which shadows your member variable of the same name, so the member is never initialised.

All you should need is to change it to

matrix = new double[N * N];

And it's very un-C++ to use this-> for member access unless it's absolutely necessary for disambiguation (which is almost never)

Answer 2

You will find more "C++" (and sometime the only way to initialize members):

Matrix::Matrix(int M, int N):   rows    (M), 
                                columns (N), 
                                matrix  (new double[M * N]) 
{
    for(int i = 0; i < M; i++)
        for(int j = 0; j < N; j++)
            matrix[i * N + j] =  0;

}

Now try to understand this:

Matrix::Matrix(       int N):   rows    (N), 
                                columns (N), 
                                matrix  (new double[N * N]) 
{
    for(int i = 0; i < N; i++)
        for(int j = 0; j < N; j++)
            matrix[i * N + j] =  (i==j);

}

If you use:

class Matrix{
private:
    int rows;
    int columns;
    std::unique_ptr<double[]> matrix;

you will find that you dont need a destructor, and some other inerest thing. Also, read my other answer.