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allocating Memory query?

Started by themadme Aug 26, 2011 at 12:14 PM 2 replies 1.3k views

Original Post

Aug 26, 2011 at 12:14 PM

Hello everyone,

This may be a naive question but what are the dangers of the following code,(allocate more memory in the new operator for the info struct):


 
 struct Info
 {
 Info():nLineNo(0){memset(cFile, 0, 256);}
 ~Info(){}
 char cFile[256];
 unsigned int nLineNo;
 };
 
 
 void *operator new(size_t Size, const char *pcFile, const unsigned int nLine)
 {
 Info *pData = 0;
 
 pData = (Info*)malloc(Size + sizeof(Info));
 
 strcpy(pData->cFile, pcFile);
 
 pData->nLineNo = nLine;
 
 return (void*)pData;
 }
 
 
 void operator delete(void *pData)
 {
 Info *pInfo = (Info*)pData;
 // could use pInfo->nLineNo to check for it within hash table of allocated blocks - just an example
 free(pData);
 }

Just wondering if this bad idea to do.

Many thanks for reading.

ryan20fun

275

ryan20fun

Aug 26, 2011 at 12:26 PM

why not just use ?


 #define DEBUG_NEW new(_NORMAL_BLOCK, __FILE__, __LINE__)

Adam_42

267

Adam_42

Aug 26, 2011 at 2:54 PM

What you need to do to make it have any chance of working reliably is to adjust the address you return so the allocated object and your info structure won't be sharing the same memory. Firstly do "return (void*)(pData + 1);" in the new operator. You then need to do the reverse in the delete: "Info *pInfo = ((Info*)pData) - 1; free(pInfo);"

Also (assuming pcFile comes from the __FILE__ macro) you can save yourself loads of memory by changing the char[256] to a char* and storing the pointer to the string.

In addition the constructor and destructor on that Info struct will never be called by the posted code since you allocate their memory with malloc(). I'd delete them.

themadme

Author

themadme

Aug 28, 2011 at 8:05 PM

thanks for the replies and the answers you gave were exactly what I was hoping for.

The above code was just a small example, i do have
#define DEBUG_NEW new(_NORMAL_BLOCK, __FILE__, __LINE__)

And i will be adjust the address i return. In fact im going to have a memory manager class that will have already allocated a huge block and will return a block of memory.

Again thanks for the replies

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