Visual C++ :: How To Avoid Memory Fragments

I am developing a Visual C++ application. There is an object called CMyObject, as follows:

typedef CMap<UINT, UINT, void *, void*> CMyMap;
class CMyObject {
public:
CMyMap *m_pMyMap;
"Some other member variables"
}

Some instances of CMyObject contains a map, some not. Therefore, to save memory, I define a pointer m_pMyMap and create a new CMap object only if the instance contains a map.

When I test my app, with the increase of the CMyObject instance, the number of memory blocks allocated and deallocated is also increasing. There are a lot of fragments during this period. To prevent this, I try to override the new/delete operator for CMyObject and CMyMap. But still find many fragments. So I try to trace into the MFC source codes for CMap. I find CMap is just using an internal buffer to store the hash table(m_pHashTable), as follows:

m_pHashTable = new CAssoc* [nHashSize];

And for each hash entry, it uses:

P = (CPlex *)new BYTE[sizeof(CPlex) + nMax *cbElement];

To allocate the spaces.

I believe these two may be the reason of the memory fragments and want to eliminate them. However, is there a way to override the new/delete operator for codes such as:

new CAssoc* [nHashSize]
and
(CPlex *)new BYTE[sizeof(CPlex) + nMax *cbElement]

So that the spaces will be allocated from my own memory manager instead of from the default heap?