How to avoid memory fragments?

[AlanCCC]

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?

[OReubens]

CMap already has code in place to manage allocations, that's why you see the allocations going via the CPlex 'dummy' class. There is an overloaded new/delete for CPlex that pools individual allocations into larger 'pages'.
So no, you can't override that already overriden behaviour.

MFC's CMap is a dynamic hashmap and as a result it allocates chunks to accomodate growing needs.
If you need a static hashmap, you'll need to either create your own or find something more suited to your needs, maybe one of the C++ standard library classes.