c++ newbie: constructor problems (with dynamic arrays)

[RobBobSmith]

Hi everyone:

I am a c++ newbie. I have created a matrix class based on dynamic arrays. My goal is to use it for creating 2D matrices that I can then use to solve heat/fluid transport problems.

When instantiated, I need each matrix to be initialized with zeroes only. I created a constructor to do this. However, the matrices contain non-zero values. I can't figure out where I'm going wrong, I have realized that I don't really understand constructors, and I would very much appreciate any advice/tips!

I realize that I could probably avoid constructor problems by using vectors from the STL, but I would like to use dynamic arrays.

Below is the code from my header, .cpp, and main files that is relevant to my constructors.

Here's one possibility for the problem that occurred to me... You will see that when I instantiate the matrix class in the main file, I do so like this:

Matrix2d<int> mat;

Maybe by not specifying dimensions upfront, my DEFAULT constructor is invoked. It has rows, cols (the dimensions), and matrix (the object that is instantiated) all set to zero. Could this be a problem? I.e., when I subsequently resize the matrix, it is expanding into territory that has NOT been set to zero by the default constructor (??).

If this sounds like the problem, what should I do? Get rid of the default constructor?

Thanks for any help!

Header file:

// Constructors
	Matrix2d ();                        
	Matrix2d (int rowsPub, int colsPub);  
	~Matrix2d();
	
private: 
// Data members
	int      rows, cols;      
	T**      matrix;

From .cpp file:

// Constructors
template <typename T>
Matrix2d<T>::Matrix2d() 
: rows(0), cols(0), matrix(0)                                                // Default constructor. Creates 1X1 matrix and sets value to 0
{	
}

template <typename T>
Matrix2d<T>::Matrix2d(int rowsPub, int colsPub)
: rows(rowsPub), cols(colsPub)
{	
	rows = rowsPub;
	cols = colsPub;
	
       matrix = new T* [ rows ]; 
	
	for (int i = 0; i < rows; i++)
	{
		matrix[i] = new T[ cols ];
	}
	
	// Initialize
	
	for(int i = 0; i < rows; i++)
		for(int j = 0; j < cols; j++)
			
			matrix[i][j] = 0;
}

template <typename T>
Matrix2d<T>::~Matrix2d()                                
{
	for(int i = 0; i < rows; i++)
		
		delete[ ] matrix[i];
		delete[ ] matrix;                                          
}

In the main file, this is how I create a matrix:

Matrix2d<int> mat;
aMat.sizeMatrix2d(xA, yA)

[Lerner]

Matrix2d<int> mat;
aMat.sizeMatrix2d(xA, yA)

Line one above does indeed call your default instructor. In your default constructor you initialize rows and cols to zero and matrix to NULL. There is no memory declared there. Without seeing sizeMatrix2d() there is no way to know if there is a problem there or not.

Does your non-default constructor work? If it does, it looks like it could function as the core for sizeMatrix2d(). You free the memory of existing matrix first, if matrix isn't NULL. Then you use the constructors code to declare new memory and set the elements to zero.

Note, lines 12 and 13 are superfluous as the values of rows and cols are set in the initializer list. As posted, lines 26 and 34 may, or may not cause a problem. To be safe always use curly braces delineating control statement bodies or never leave a blank line between control statement and the single line of the statement body, if it is one line long.