Resizable Array with pointers - c++

Question

My program works but my professor says that my code is incorrect but stated that he will get to why in the fall term... What is he talking about?
perhaps something is improper? Even if you are incorrect I would appreciate picking your brain :)
void CResizableArray::SetSize( int intNewSize )
{
int intIndex = 0;
if( intNewSize < 0 ) intNewSize = 0;
if( intNewSize > intMAXIMUM_ARRAY_SIZE )
{
intNewSize = intMAXIMUM_ARRAY_SIZE;
}
//////////////////////////////////////
// ---> HUGE BUG HERE <--- //
// Code works but is WRONG //
// WHY IS THIS HELP ME FIND THE BUG //
//////////////////////////////////////
m_intArraySize = intNewSize;
m_paintValues = new int [m_intArraySize];
// Initialize to zero
for( intIndex = 0; intIndex < m_intArraySize; intIndex++ )
{
*( m_paintValues + intIndex ) = 0;
}
}

Answer 1

Presumably before this line
m_paintValues = new int [m_intArraySize];
m_paintValues pointed to another array. That array has now been leaked -- you don't have a pointer to it, so it can never be freed. That memory can therefore never be reused. Write a program that does a lot of this, and it'll run out of memory before running very long.
When you're through with a block of memory, you need to free it. Here, the proper thing to do might look something like
delete[] m_paintValues;
m_paintValues = new int [m_intArraySize];
There are more issues, though. First of all, you can never use delete[] unless you know that m_paintValues definitely points to an array; you could ensure that in the constructor. More troubling is that fact that when you set a new size, any data previously in m_paintValues is discarded -- don't you want to copy the old values into the new array? Doing so would mean using a temporary variable to hold the new array when first allocated, copying the data, and then assigning the new array to the member variable.

Answer 2

He may mean that since it is a resize you should keep the old contents of the array and transfer them over to the new array, in your snippet you just throw away the old content creating a new empty array.
so instead of
m_paintValues = new int [m_intArraySize];
// Initialize to zero
for( intIndex = 0; intIndex < m_intArraySize; intIndex++ )
{
*( m_paintValues + intIndex ) = 0;
}
do
int* newBiggerArray = new int[m_intArraySize];
for (intIndex = 0; intIndex < m_intArraySize; ++intSize)
{
if ( intIndex < oldMaxSize )
{
newBiggerArray[intIndex] = m_paintValues[intIndex];
}
else
{
newBiggerArray[intIndex] = 0;
}
}
delete [] m_paintValues;
m_paintValues = newBiggerArray;
I will leave the part to handle a resize to a smaller value than previous for you to figure out.