C++ rogram to implement merge sort

#include <iostream.h>
const int MAX = 5 ;
class array
{
private :
int *a ;
int size ;
int count ;
public :
array( ) ;
array ( int sz ) ;
void add ( int num ) ;
void display( ) ;
void merge_sort(int low,int high);
void merge(int low,int mid,int high);
~array( ) ;
} ;
array :: array( )
{
count = size = 0 ;
a = NULL ;
}
array :: array( int sz )
{
count = 0 ;
size = sz ;
a = new int[sz] ;
}
void array :: add ( int num )
{
if ( count < size )
{
a[count] = num ;
count++ ;
}
else
cout << "\nArray is full" << endl ;
}
void array :: display( )
{
for ( int i = 0 ; i < count ; i++ )
cout << a[i] << "\t" ;
cout << endl ;
}
void array :: merge_sort(int low,int high)
{
 
int mid;
if(low<high)
{
mid=(low+high)/2;
merge_sort(low,mid);
merge_sort(mid+1,high);
merge(low,mid,high);
}
}
void array :: merge(int low,int mid,int high)
{
int h,i,j,b[50],k;
h=low;
i=low;
j=mid+1;
 
while((h<=mid)&&(j<=high))
{
if(a[h]<=a[j])
{
b[i]=a[h];
h++;
}
else
{
b[i]=a[j];
j++;
}
i++;
}
if(h>mid)
{
for(k=j;k<=high;k++)
{
b[i]=a[k];
i++;
}
}
else
{
for(k=h;k<=mid;k++)
{
b[i]=a[k];
i++;
}
}
for(k=low;k<=high;k++) a[k]=b[k];
}
array :: ~array( )
{
delete a ;
}
 
void main( )
{
array a ( MAX ) ;
 
a.add ( 11 ) ;
a.add ( 2 ) ;
a.add ( 9 ) ;
a.add ( 13 ) ;
a.add ( 57 ) ;
 
cout << "\nMerge sort.\n" ;
 
a.merge_sort (0,4) ;
 
cout << "\nArray after sorting: " << endl ;
a.display( ) ;
}