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Bizarre placement new[] operator return address

Jerry D · 2013-04-12T15:31:58

Hey Guys, I am trying to do in place initialization of an array of variables. Essentially allocate memory. Pass in the void * into operator new[] to initialize an array of variables. Now I get an address 4 bytes ahead of the allocated address !! Example: void *pMem = malloc(sizeof(A) * 4); A *pArr = new (pMem) A[4]; pArr is (char *)pMem + 4 !! Can anyone shed light on why ? I noticed the first 4 bytes of pMem contains the number of elements in array. (in this case 4). I could potentially move the pointer back and forth. But I want to know is this 4 Bytes same for any platform ? Is it same for 32 bit and 64 bit applications ?? Is this part of the C++ standard ? Thanks !! Jerry /*********** Source Code *********/ class BaseString { enum EAllocationSource {AS_DATA, AS_HEAP}; EAllocationSource m_allocationSource; union { const char *m_strStatic; char *m_strDynamic; }; unsigned int *m_pNumRef; unsigned int m_len; static const char m_nullString; public: virtual ~BaseString(){} static void *operator new[] (size_t size, void *ptr) { printf("\nSize obtained in operator new[] function of class: %u", (unsigned int)size); return ptr; } static void operator delete[] (void *val, void *ptr) { printf("\nAddress passed to delete in Class: %u", (unsigned int)val); printf("\nIn Place Address passed in Class: %u", (unsigned int)ptr); } }; void main() { printf("\nSize to be allocated: %u", sizeof(BaseString) * 4); void *pMem = malloc(sizeof(BaseString) * 4); printf("\nAllocated Address: %u", (unsigned int)pMem); BaseString *pArr = new (pMem) BaseString[4]; printf("\nReturned Address: %u", (unsigned int)pArr); pArr->operator delete[](pArr, pMem); printf("\nuint first: %u", *((unsigned int *)pMem)); while(true) SwitchToThread(); }

General and Gameplay Programming Programming

Started by jerrinx April 12, 2013 03:31 AM

10 comments, last by Bregma 11 years ago

SiCrane

11,840

April 12, 2013 02:11 PM

For those who care, the array allocation overhead is described as applying to placement new[] in section 5.3.4 paragraph 12 of the C++ standard (same location in all versions).

Bregma

9,461

April 12, 2013 03:31 PM

Aside from trying to be Klever™ I suspect what the OP is doing is trying to reinvent either std::array or std::uninitialized_fill_n(). Perhaps he'd be more productive trying to use the language instead of modify it (although there's something to be said for discovering how your catalytic converter works instead of driving your car, too).

Stephen M. Webb
Professional Free Software Developer

Bizarre placement new[] operator return address

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