need help with creating an m x n matrix

[Petr_Kropotkin]

I need help to creater allocate space.
Here is what I have

class Array
{
    friend ostream &operator<<( ostream &, const Array & );
    friend istream &operator>>( istream &, Array & );
public:
   Array( int = 10,int =10 ); 
   Array( const Array & ); 
 ~Array();
   // inequality operator; returns opposite of == operator
   bool operator!=( const Array &right ) const
   {
      return ! ( *this == right ); // invokes Array::operator==
   } // end function operator!=
 
   // subscript operator for non-const objects returns modifiable lvalue
   int &operator[]( int );
 
   // subscript operator for const objects returns rvalue
   int operator[]( int ) const;
private:
   int size, m,n; // pointer-based array size
   int a[10][10]; // pointer to first element of pointer-based array
}; // end class Array

class Array
{
    friend ostream &operator<<( ostream &, const Array & );
    friend istream &operator>>( istream &, Array & );
public:
   Array( int = 10,int =10 ); 
   Array( const Array & ); 
 ~Array();
   // inequality operator; returns opposite of == operator
   bool operator!=( const Array &right ) const
   {
      return ! ( *this == right ); // invokes Array::operator==
   } // end function operator!=

   // subscript operator for non-const objects returns modifiable lvalue
   int &operator[]( int );

   // subscript operator for const objects returns rvalue
   int operator[]( int ) const;
private:
   int size, m,n; // pointer-based array size
   int a[10][10]; // pointer to first element of pointer-based array
}; // end class Array

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My code so far

#include <iostream>
#include <iomanip>
using namespace std;
#include "array.h" 
 
 
Array::Array( int m, int n )
 {
   size = ( m*n > 0 ? m*n : 10 ); // validate arraySize
   a[m][n] =*new int[size]; // create space for pointer-based array
 
   for ( int i = 0; i < m ;i++ )
       for ( int j = 0; j <n; j++ )
      a[ i ][j] = 0; // set pointer-based array element
  } 
 
 
Array::~Array()
{
delete  *[]a;
}
 
 Array::Array( const Array &arrayToCopy )
        : size( arrayToCopy.size )
   {
     a[m][n]= *new int[ size ]; 
 
    for (int i = 0; i < n; i++ )
        for (int j = 0; i < m; j++ )
      a[ i ][j] = arrayToCopy.a[ i ][j]; 
 
} 
 
 
 
int main()
{
Array mat1;
Array mat2(4,5);
}

#include <iostream>
#include <iomanip>
using namespace std;
#include "array.h" 


Array::Array( int m, int n )
 {
   size = ( m*n > 0 ? m*n : 10 ); // validate arraySize
   a[m][n] =*new int[size]; // create space for pointer-based array

   for ( int i = 0; i < m ;i++ )
       for ( int j = 0; j <n; j++ )
      a[ i ][j] = 0; // set pointer-based array element
  } 


Array::~Array()
{
delete  *[]a;
}

 Array::Array( const Array &arrayToCopy )
        : size( arrayToCopy.size )
   {
     a[m][n]= *new int[ size ]; 
 
    for (int i = 0; i < n; i++ )
        for (int j = 0; i < m; j++ )
      a[ i ][j] = arrayToCopy.a[ i ][j]; 

} 



int main()
{
Array mat1;
Array mat2(4,5);
}

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When I run it, it runs ok . But if I implement the destructor , it crashes. I think my memory allocation for a is probably screwy too.. Helpl

[aamer4yu]

You need not delete a, ( delete *[]a in the dtor. You have declared a as int a[10][10]; which is allocated on stack and not heap.
Also your whole program seems to be of wrong concepts. I would suggest you refer some good C++ / Data structure book and try some example from it.

One more thing, if you are practising arrays, then fine. Else theres QList class in Qt for such things.

[Petr_Kropotkin]

I am doing an overloading operator exercise from C+ How To Program using multidimension array. I am starting with his Array example and modifying it.

[axeljaeger]

As long as you do not deal with very big matrices, It does not make much sense to fall back to the C-style arrays in favor of the C++-style collections from either the STL or Qt's own container classes. To implement a matrix I suggest to use QVectors and have a vector of vectors to implement the two dimensions. Depending on your application, you can decide wether to have a list of rows or a list of columns.