operator overload for dynamic array giving strange error - c++

Question

I'm getting an error regarding "disgarded qualifiers" when I use this. The Entier class is posted below.
cout << d // where d is of type dynamic_array.
The global overloaded function:
template <class T> std::ostream& operator<<(std::ostream& stream, dynamic_array<T> const& data)
{
data.print_array(stream);
return stream;
}
A public member of dynamic_array
void print_array(std::ostream &os = cout)
{
for (int i = 0; i < size; i++) os << array[i] << endl;
}
Entire Class dynamic array:
/*
Needs a reszie function added
Merge sort is better for sequential, stable(equal elements not re-arranged, or
*/
#include "c_arclib.cpp"
using namespace std;
template <class T> class dynamic_array
{
private:
T* array;
T* scratch;
void merge_recurse(int left, int right)
{
if(right == left + 1)
{
return;
}
else
{
int i = 0;
int length = right - left;
int midpoint_distance = length/2;
int l = left, r = left + midpoint_distance;
merge_recurse(left, left + midpoint_distance);
merge_recurse(left + midpoint_distance, right);
for(i = 0; i < length; i++)
{
if((l < (left + midpoint_distance)) && (r == right || array[l] > array[r]))
{
scratch[i] = array[l];
l++;
}
else
{
scratch[i] = array[r];
r++;
}
}
for(i = left; i < right; i++)
{
array[i] = scratch[i - left];
}
}
}
void quick_recurse(int left, int right)
{
int l = left, r = right, tmp;
int pivot = array[(left + right) / 2];
while (l <= r)
{
while (array[l] < pivot)l++;
while (array[r] > pivot)r--;
if (l <= r)
{
tmp = array[l];
array[l] = array[r];
array[r] = tmp;
l++;
r--;
}
}
if (left < r)quick_recurse(left, r);
if (l < right)quick_recurse(l, right);
}
public:
int size;
dynamic_array(int sizein)
{
size=sizein;
array = new T[size]();
}
void print_array(std::ostream &os = cout)
{
for (int i = 0; i < size; i++) os << array[i] << endl;
}
void print_array()
{
for (int i = 0; i < size; i++) cout << array[i] << endl;
}
int merge_sort()
{
scratch = new T[size]();
if(scratch != NULL)
{
merge_recurse(0, size);
return 1;
}
else
{
return 0;
}
}
void quick_sort()
{
quick_recurse(0,size);
}
void rand_to_array()
{
srand(time(NULL));
int* k;
for (k = array; k != array + size; ++k)
{
*k=rand();
}
}
void order_to_array()
{
int* k;
int i = 0;
for (k = array; k != array + size; ++k)
{
*k=i;
++i;
}
}
void rorder_to_array()
{
int* k;
int i = size;
for (k = array; k != array + size; ++k)
{
*k=i;
--i;
}
}
};
template <class T> std::ostream& operator<<(std::ostream& stream, dynamic_array<T> const& data)
{
data.print_array(stream);
return stream;
}
int main()
{
dynamic_array<int> d1(1000000);
d1.order_to_array();
clock_t time_start=clock();
d1.merge_sort();
clock_t time_end=clock();
double result = (double)(time_end - time_start) / CLOCKS_PER_SEC;
cout << result;
cout << d1;
}

Answer 1

The problem is in following lines:
template <class T>
std::ostream& operator<<(std::ostream& stream, dynamic_array<T> const& data)
^^^^^^
and
void print_array(std::ostream &os = cout) /* const */
^^^^^ missing
Since you are passing data as const& to operator <<, you have to maintain its const qualification. i.e. data cannot call any non-const member of class dynamic_array. You can solve this problem in 2 ways:
pass data as simple dynamic_array<T>& to operator <<
make print_array a const method (uncomment above)

Answer 2

"Discards const qualifiers", presumably...
Replace:
void print_array(std::ostream &os = cout)
{
for (int i = 0; i < size; i++) os << array[i] << endl;
}
void print_array()
{
for (int i = 0; i < size; i++) cout << array[i] << endl;
}
...with...
void print_array(std::ostream &os = cout) const
{
for (int i = 0; i < size; i++) os << array[i] << endl;
}
...or, better still ...
void print_array(std::ostream &os = cout) const
{
std::copy(array, array + size, std::ostream_iterator<T>(os, "\n"));
}
This member function can be declared const since it doesn't modify any attributes of the class. See the FAQ on Const-Correctness.
Your second print_array() method is redundant since the first takes a default argument of std::cout.