Memory allocation C++

[Benzaa]

Hi,

Im working with a small class in C++. I want to create a matrix class that can store a bunch of numbers on it.

It should be easy, but im having an issue with the destructor. As soon as the program finishes, the destructor generates an error that I can not understand.

Please, could someone help me with this

here is the code that I made:


//Matrix object
#include <iostream>

class Matrix {
protected:
int m_iRow, m_iCol;
double* m_pdData;
void copy(const Matrix&);

public:
Matrix(int, int);
~Matrix();
Matrix(const Matrix&);
Matrix& operator=(const Matrix&);
int nRow(void);
int nCol(void);
double get(int, int);
void set(int,int,double);
void create(void);
};

Matrix::Matrix(int a, int b) {
//Initialise the matrix with the row and column
m_iRow = a;
m_iCol = b;
create();
}

Matrix::~Matrix() {
//Deletes the space of the data
delete m_pdData;
}

void Matrix::create(void) {
//Creates container with zeros
m_pdData = new double[m_iRow*m_iCol];
}

Matrix::Matrix(const Matrix& mat) {
//Copy constructor
//Used when a copy of an object is produced
this->copy(mat);
}

Matrix& Matrix::operator=(const Matrix& mat) {
//Assignment operator function
//Overloads the equal sign operator to work
//with Matrix objects
if (this==&mat) return *this;
delete [] m_pdData;
this->copy(mat);
return *this;
}

int Matrix::nRow(void) {
//Returns number of rows
return m_iRow;
}

int Matrix::nCol(void) {
//Returns number of columns
return m_iRow;
}

double Matrix::get(int i, int j) {
//Parenthesis operator function
//Allows acces to values of matrix via (i,j)
return m_pdData[m_iCol*(i-1) + (j-1)];
}

void Matrix::set(int i,int j,double val) {
//set the value val into the matrix
if (i < m_iRow && j < m_iCol)
m_pdData[m_iCol*(i-1) + (j-1)] = val;
}

void Matrix::copy(const Matrix& mat) {
//private copy functions
//copies values from one matrix to another
m_iRow = mat.m_iRow;
m_iCol = mat.m_iCol;
int iData = m_iRow * m_iCol;
create();
for (int i=0; i<iData; i++)
m_pdData[i] = mat.m_pdData[i];
}

int main() {
int n=3;
Matrix A(n,n);
A.set(1,0,9.0);

for (int i=0; i<n; i++)
for (int j=0; j<n; j++) {
std::cout << i << "," << j << ":" << A.get(i,j) << "\n";
}

return 0;
}

[namegame]

I just glanced over it quickly and I did not see anything obvious. It may help if you post the error as well.

EDIT: I'm not sure of what your goal is. Wouldn't it be easier to dynamically allocate a two-dimensional array?

[jpmelos]

Don't hit the Submit button twice, man.

http://ubuntuforums.org/showthread.php?t=1032482&highlight=matrix

Glad you already found your answer, anyways.

[kcode]

Program is crashing at Matrix::get() function. Give it a check. (Used gdb)

[stroyan]

The destructor is using
delete m_pdData; instead of
delete [] m_pdData;
The difference between the array and non-array delete is crucial.

[mdurham]

The difference between the array and non-array delete is crucial.Why is that? It's only an array of doubles.

[dwhitney67]

I ran the program using 'ddd'. The error is occurring here:


return m_pdData[m_iCol*(i-1) + (j-1)];

when the values i = 0 and j = 0 are passed to the function (m_iCol = 3).

Oh, and as pointed out already, the destructor should be:


delete [] m_pdData;

Without the [], you will have a memory leak.


P.S. Don't forget to fix the set() method as well; it has the same bug as the get() method.

[Benzaa]

Don't hit the Submit button twice, man.

http://ubuntuforums.org/showthread.php?t=1032482&highlight=matrix

Glad you already found your answer, anyways.


Sorry for the double post.
I didnt notice the error until I saw the duplicated post.


Thanks for the help I found the error on the code. It was the index that I was using for accessing the array.

btw, what is ddd?
is it a debugging tool?

[dwhitney67]

'ddd' is the GUI front-end to 'gdb' (which is a debugger).

[mdurham]

Without the [], you will have a memory leak.
Why is that? It's only an array of doubles. I don't get it!
Cheers, Mike

[dwhitney67]

Why is that? It's only an array of doubles. I don't get it!
Cheers, Mike

I don't have a worthy explanation (I tossed away my Stroustroup book long ago). But when allocating an array, you use the [] operator. I believe this calls a distinct method other than the regular operator delete(void*) method.

Read here for some (basic) information concerning allocating and deallocating memory in C++: http://www.cplusplus.com/doc/tutorial/dynamic.html

---------

Edit... play with this program:


#include <iostream>

void operator delete[](void* mem) throw()
{
std::cout << "delete[](void*) called" << std::endl;
}

void operator delete(void* mem) throw()
{
std::cout << "delete(void*) called" << std::endl;
}

int main(int argc, char** argv)
{
double* array = new double[20];

if (argc > 1) // check if command line arg passed
delete array;
else
delete [] array;

return 0;
}

[mdurham]

I don't have a worthy explanation (I tossed away my Stroustroup book long ago). But when allocating an array, you use the [] operator. I believe this calls a distinct method other than the regular operator delete(void*) method.

dwhitney67, I would say you are correct if the array is an array of objects. In that case (without []), the object destructors wouldn't be called. But, doubles don't have destructors, therefore there would be no memory leak.
If you see what I mean.
Cheers, Mike

[monkeyking]

I havn't looked at your code closely,
but when you type

new []
then you should do

delete []
It's that simple, your not removing the doubles themselves,
but the array that has the doubles.

try checking your program with valgrind to verify there are no leaks