Article ID: 139638
Article Last Modified on 5/26/2005
APPLIES TO
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 2.1
- Microsoft Visual C++ 2.2
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition
This article was previously published under Q139638
SUMMARY
Providing a dedicated heap by overloading new and delete for a class can significantly reduce memory fragmentation. Any class that is frequently instantiated using new is a candidate for providing a dedicated heap.
This method works best when many instances of a class are used in close time proximity but random order. For example, this method will work well for a newly created list that is about to be sorted.
This method impairs performance where the use of instances of the class is more time proximate to the use of other heap-allocated objects. For example, this method would impair the performance of a scrolling, in-memory view of rows read from a database.
There is no easy substitute for careful analysis of locality of reference, where locality is gauged according to the page size used by the host operating system's virtual memory management and time separation.
This technique should not be applied to a base class that can be or might be derived from if data is to be added to the derived class.
MORE INFORMATION
The following sample code stores the dedicated heap handle in the static member variable m_hHeap. The heap handle is initialized to NULL in the Classdemo.cpp file. When a class object is instantiated using new, the overloaded new is called. If the handle is NULL, the heap is created. If the handle is not NULL, HeapAlloc is called to allocate the requested size.
Sample Code
/* Compile options needed: none
*/
/* Classdemo.h file */
#ifndef __CLASSDEMO_H__
#define __CLASSDEMO_H__
#include <windows.h>
class ClassDemo
{
// private member vars and functions for ClassDemo class
static HANDLE m_hHeap; // fixed heap handle
int m_nMyData[100]; // class data
public:
// public member functions for ClassDemo class
// declaration of new & delete operator overloads
void* operator new(unsigned int nSize);
void operator delete(void* pObj);
static void HeapCreate();
};
#define INITIAL_HEAP_SIZE 4096 // initial heap size
#define CLASS_DEMO_OBJS_MAX 100000 // max number of objects used
inline void* ClassDemo::operator new(unsigned int nSize)
{ // allocate memory using fixed heap
if (m_hHeap == NULL) // if NULL, needs to be created
{
HeapCreate(); // create heap
if (m_hHeap == NULL)
{
return NULL; // HeapCreate failed
}
}
// return pointer to allocated memory
return HeapAlloc(m_hHeap, 0, nSize);
}
inline void ClassDemo::operator delete(void* pObj)
{ // free memory from fixed heap
HeapFree(m_hHeap, 0, pObj);
}
#endif // __CLASSDEMO_H__
/* End of Classdemo.h file */
/* Classdemo.cpp file */
#include <stdio.h>
#include "ClassDemo.h"
HANDLE ClassDemo::m_hHeap = NULL; // Initialize handle
void ClassDemo::HeapCreate()
{
int nRet = IDRETRY; // message box return
int nSize = CLASS_DEMO_OBJS_MAX * sizeof(ClassDemo);// max heap size
m_hHeap = ::HeapCreate(0, // heap flags
INITIAL_HEAP_SIZE, // initial size of heap
nSize); // max heap size
while ((m_hHeap == NULL) && (nRet != IDIGNORE))
{ // keep trying until no error or user ignores
char szMsg[80]; // message buffer
// format message
sprintf(szMsg, "Could allocate a heap of size %d", nSize);
// Display error
nRet = MessageBox(NULL, szMsg, NULL,
MB_ABORTRETRYIGNORE | MB_TASKMODAL);
if (nSize < sizeof(ClassDemo))
{
return; // heap would be too small to be usefull, return
}
switch (nRet)
{
case IDABORT: // user selected abort
{
abort();
}
case IDIGNORE: // user selected ignore
{
return;
}
default: // user selected retry, the message box could not
{ // be displayed, or unknown return code
nSize /= 2; // try a smaller request
m_hHeap = ::HeapCreate(
0, // heap flags
INITIAL_HEAP_SIZE, // initial size of heap
nSize); // max heap size
break;
}
}
}
}
/* End of Classdemo.cpp file */
Additional query words: Windows 95 10.00 10.10 10.20
Keywords: kbhowto kbtshoot kbcrt kblangcpp kbcode KB139638
