I'm new to C++ and as I have coded a lot in Java now I got problems with the memory and although I have been browsing through several pages now, I still didn't find out how to solve my problem the best.

What I'm doing is programming a little maltab clone for my numerical analysis course.
Thus I have to be able to operate on giant matrices.
But somehow I am not able to free memory to the system and my Destructor calls generate memory trouble.

This is my matrix class:

#ifndef INC_MATRIX_H
#define INC_MATRIX_H

#include <iostream>
class Matrix 
{

public:
		int row, col;
		double **array;

		Matrix(int r, int c);
		~Matrix();

		void operator=(Matrix B);
		Matrix transpose();
};
#endif

The implementation is as follows:

#include "str.h"
using namespace std;

Matrix::Matrix(int r, int c) {
		this->row = r; this->col = c;			
		array = new double*[r];
		for(int i = 0; i < r; i++) 
			array[i] = new double[c];
	}
Matrix::~Matrix() {
		this->row = 0;
		this->col = 0;
		//delete this->array;// =new double*[0];; // This does not work somehow!
	}
void Matrix::operator=(Matrix B) {
		// includes some warnings concerning dimensions
		for(int i = 0; i < this->row; i++) 	
			for(int j = 0; j < this->col; j++) 	
				this->array[i][j] = B.array[i][j];
	return;
	}
Matrix Matrix::transpose() {
		Matrix C = *(new Matrix(this->col,this->row));
		for(int i = 0; i < this->row; i++) 	
			for(int j = 0; j < this->col; j++) 	
			 	C.array[j][i] = this->array[i][j];	
		return C;
	}

You can test it using this little programm:

#include <iostream>

#include "str.h"

using namespace std;

int main() {
int n = 20;
Matrix B(n,n);
for(int i = 0; i < n; i++) B.array[i][i] = i;
	while(1) {
		cout << B.array[0][0];
		B=B.transpose();
	}
}

You will see, that the memory usage grows and grows, although I thought it is just one matrix.
Have you got any ideas or links how I can optimise my code?

Kind Regards.
Roland

PS: May someone move this post to the c++ section, please. I posted it wrong. Thank you!

Matrix::~Matrix()
{
  for (int i = 0; i < row; i++)delete[] array[i];
  delete[] array;
  row = 0;
  col = 0;
}

this-> inside member functions is optional, by the way (unless you have a function parameter with the same name).

If you would add the following to the
Header file:

Matrix operator*(Matrix B);

str.cpp file:

Matrix Matrix::operator*(Matrix B) 
	{
		if( this->col != B.row) { 
			cout << "mult: Error, Matrix dimensions do not match: ";
			cout << "Op1 is (" << this->row <<", "<< this->col << "), " << "Op2 is (" << B.row << ", " << B.col << ")" << endl; 
		 return *(new Matrix(0,0)); 
		}
		// initialize the new matrix
		Matrix C = *(new Matrix(this->row, B.col));
		// multiplicate
		for(int i= 0; i < this->row; i++) 	
			for(int j = 0; j < B.col; j ++) 	
				for(int k = 0; k < this->col; k ++) 
					C.array[i][j] += this->array[i][k] * B.array[k][j]; 
		return C;
	}

testcase:

B=B*B;

I've checked, that it is not the problem that the matrix is multiplied with itself or within the loop..

Matrix operator*(const Matrix& B);

In the implementation of operator* you should not create matrixes with new, as this will result in a memory leak. Instead, create them this way:

return Matrix(0,0);
//and:
Matrix C(this->row, B.col);

When you return the matrixes, it's possible that they need to be copied, which again requires a valid copy constructor.

Im Zweifelsfall bitte auch folgende Hinweise beachten:
C/C++ Forum :: FAQ - Sonstiges :: Wohin mit meiner Frage?

Dieses Posting wurde automatisch erzeugt.

But I've still've got a memory leak when it comes to multiplication.

May you explain to me, how operator= and the copy constructor work together?

My Copy-Constructor is now

Matrix::Matrix(const Matrix& B) 
		: row(B.row), col(B.col), array(new double*) 
	{
		for(int i = 0; i < B.row; i++) {
			array[i] = new double[B.col];    // #include <algorithm> for std::copy
			for(int j = 0; j < B.col; j++)
				array[i][j] = B.array[i][j];
		}
	}

and I changed multiplication like this:

Matrix Matrix::operator*(const Matrix& B) 
	{
		if( this->col != B.row) { 
			cout << "mult: Error, Matrix dimensions do not match: ";
			cout << "Op1 is (" << this->row <<", "<< this->col << "), " << "Op2 is (" << B.row << ", " << B.col << ")" << endl; 
		 return Matrix(0,0); 
		}
		// initialize the new matrix
		Matrix C(this->row, B.col);
		// multiplicate
		for(int i= 0; i < this->row; i++) 	
			for(int j = 0; j < B.col; j ++) 	
				for(int k = 0; k < this->col; k ++) 
					C.array[i][j] += this->array[i][k] * B.array[k][j]; 
		return C;
	}

If I test it with

while(1) C*B

I still am leaking memory. If you are wondering, why I do not have a Destructor:
I get the memory map error again!

Btw: Do you know why I don't get these fancy code boxes?

Regards,
Roland :xmas1:

I still am leaking memory. If you are wondering, why I do not have a Destructor:
I get the memory map error again!

If you haven't got a destructor which deletes the allocated memory, that's simple. Every multiplication creates a new matrix, therefore calls the constructor. In the constructor, you allocate memory by new that is never freed.

fromjavatocpp schrieb:

May you explain to me, how operator= and the copy constructor work together?

Have you ever heard of Copy&Swap? That's an idiom to implement the assignment operator safely (regarding exceptions), plus to avoid code duplication.

First, provide a swap() method that exchanges the contents of two matrices:

void Matrix::swap(Matrix& other)
{
    std::swap(col, other.col);
    std::swap(col, other.col);
    std::swap(array, other.array);
}

Then, you can implement the assignment operator using your copy constructor and swap() :

Matrix& Matrix::operator= (const Matrix& origin)
{
    Matrix temp(origin); // create a temporary copy of origin
    swap(temp); // exchange contents of *this and temp, so *this equals origin
    return *this;
} // note: temp gets out of scope here, so the old content is destroyed.

You can find more informations about Copy&Swap in the english wikibooks.

fromjavatocpp schrieb:

Btw: Do you know why I don't get these fancy code boxes?

Did you accidentally check the box "BBCode in diesem Beitrag deaktivieren" (deactivate BBCode)?

Thank you for your advice with the copy and swap. Seems to make sense.

But still the problem is there: Maybe someone can try to get the code to run? (This would be nice!)

When I try to install a Destructor like you can see below, I get a Error:

*** glibc detected *** ./a.out: corrupted double-linked list: 0x09d833a0 ***

I have done the swap thing, too, and my =operator is like it should be and my copy constructor works fine now.

But still, what is wrong with my destructor?

My testprog now is as simple as this:

#include <iostream>
using namespace std;
class Matrix {
public:
int row, col;
double **array;
 Matrix(int r, int c) {
	this->row = r; this->col = c;			
	array = new double*[r];
	for(int i = 0; i < r; i++) 
		array[i] = new double[c];
}
 Matrix(const Matrix& B) 
	: row(B.row), col(B.col), array(new double*) {
	for(int i = 0; i < B.row; i++) {
	   array[i] = new double[B.col];    
	     for(int j = 0; j < B.col; j++)
		array[i][j] = B.array[i][j];
        }
// the whole copy thing here does not work properly, because at the end it would be the same refernce and the problem still exists
}
~Matrix() {
	for (int i = 0; i < row; i++) delete[] array[i];
	delete[] array;
	row = 0;
	col = 0; 
	}
void swap(Matrix& B) 	{
         std::swap(row, B.row);
         std::swap(col, B.col);
         std::swap(array, B.array);
}
Matrix& operator=(const Matrix& B) {
    Matrix temp(B); // create a temporary copy of origin
    swap(temp); // exchange contents of *this and temp, so *this equals origin
    return *this;
}
}; // class

int main() {
   int n = 10;
   Matrix C(n,n);
   for(int i = 0; i < n; i++)
	C.array[i][i] = i;	
   Matrix A = Matrix(C);
   cout << C.array[1][1] << " " << A.array[1][1] << endl;
   C.array[1][1] = 100;
   cout << C.array[1][1] << " " << A.array[1][1] << endl;
}

I just noticed that I get no problem when I do not call the Copy Constructor

array = new double*[r];

However in the copyconstructor, you wrote:

array(new double*)

You have never specified how many rows should be created... maybe that could produce an error.

Have a nice Sunday!

Best wishes,
Roland