2 dimensional array

[hello123]

I have the following sample code:

#include<iostream>
using namespace std;

int main(){

  int **m;

  m= new int*[10];

  for(int i=0;i<10;i++)
  {
	  m[i]=new int[5];
  }

  for(int i=0;i<10;i++)
	  for(int j=0;j<5;j++)
		  m[i][j]=4;

  cout<<m[0][3]<<endl;

   for(int i=0;i<10;i++)
  {
	  delete []m[i];
  }

   delete []m;

	return 0;
}

This works fine. But if I change the line

m= new int*[10];
to
m= new int*;

I get the output, but the deletion part is not working fine and the program is not terminated.

I cannot understand how I am able to access the value in the second case when I put the initialization as m= new int*;


Can someone please explain this?

Thanks.

[guestgulkan]

m= new int*[10] create 10 new pointers.

m= new int* creates just ONE new pointer.


So if you just took all the above code and change that one line:
1. All the loops will be wrong (and could cause crashes)
2. Delete [] is wrong when you haven't created a new array.

** You get the output when you change to new int* - but you are causing heap corruption/access violation**

[binarybob350]

This would be a good use for some defines.

#include<iostream>
using namespace std;

#define NUM_INT_PTRS 10
#define NUM_INTS 5

int main(){

  int **m;

  m= new int*[NUM_INT_PTRS ];

  for(int i=0;i<NUM_INT_PTRS ;i++)
  {
	  m[i]=new int[NUM_INTS];
  }

  for(int i=0;i<NUM_INT_PTRS ;i++)
	  for(int j=0;j<NUM_INTS;j++)
		  m[i][j]=4;

  cout<<m[0][3]<<endl;

   for(int i=0;i<NUM_INT_PTRS ;i++)
  {
	  delete []m[i];
  }

   delete []m;

	return 0;
}

[filipe]

There's no need for such macro hackery in C++:

const size_t PtrArraySize = 10;
const size_t IntArraySize = 5;