Overloading the Operators new and delete (MS Code)

Microsoft's Implementation of operators new and delete

Essentially, using the new expression with built-in types is similar to using operator new:

char *buffer1 = ::new char[12];      // allocate 12 bytes (returns char *)
void *buffer2 = ::operator new(12);  // allocate 12 bytes (returns void *)

::delete [] buffer1;         // free memory allocated by ::new
::operator delete(buffer2);  // free memory allocated by ::operator new

In MSVC++ 6.0, both allocations above call this code:

void * operator new( unsigned int cb )
{
  void *res = _nh_malloc( cb, 1 );
  return res;
}

and both deallocations call this code:

void __cdecl operator delete(void *p) _THROW0()
{ 
  free(p); // free an allocated object
}

In MSVC++ 7.1, the first call to new calls this:

void *__cdecl operator new[](size_t count) _THROW1(std::bad_alloc)
{ // try to allocate count bytes for an array
  return (operator new(count));
}

and the second calls this:

void *__cdecl operator new(size_t size) _THROW1(_STD bad_alloc)
{	// try to allocate size bytes
  void *p;
  while ((p = malloc(size)) == 0)
  if (_callnewh(size) == 0)
    _STD _Nomemory();
  return (p);
}

In MSVC++ 7.1, the first call to delete calls this:

void operator delete[]( void * p )
{
  RTCCALLBACK(_RTC_Free_hook, (p, 0))
  operator delete(p);
}

and the second calls this:

void operator delete(void *pUserData)
{
  _CrtMemBlockHeader * pHead;
  RTCCALLBACK(_RTC_Free_hook, (pUserData, 0));
  if (pUserData == NULL)
    return;

  _mlock(_HEAP_LOCK);  /* block other threads */
  __TRY
    /* get a pointer to memory block header */
    pHead = pHdr(pUserData);
    /* verify block type */
    _ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));
    _free_dbg( pUserData, pHead->nBlockUse );
  __FINALLY
    _munlock(_HEAP_LOCK);  /* release other threads */
  __END_TRY_FINALLY
  return;
}

This allows us to call operator::new[] and operator::delete[] in MSVC 7.1:

char *buffer1 = ::new char[12];       // allocate 12 bytes (returns char *)
void *buffer2 = ::operator new[](12); // allocate 12 bytes (returns void *)

::delete [] buffer1;           // free memory allocated by ::new
::operator delete[](buffer2);  // free memory allocated by ::operator new