Question

I am working with VC++ 2005
I have overloaded the new and delete operators.
All is fine.

My question is related to some of the magic VC++ is adding to the memory allocation.

When I use the C++ call:

data = new _T [size];

The return (for example) from the global memory allocation is 071f2ea0 but data is set to 071f2ea4

When overloaded delete [] is called the 071f2ea0 address is passed in.

Another note is when using something like:

data = new _T;

both data an the return from the global memory allocation are the same.

I am pretty sure Microsoft is adding something at the head of the memory allocation to use for book keeping. My question is, does anyone know of the rules Microsoft is using.

I want to pass in the value of “data” into some memory testing routines so I need to get back to the original memory reference from the global allocation call.

I could assume the 4 byte are an index but I wanted to make sure. I could easily be a flag plus offset, or count or and index into some other table, or just an alignment to cache line of the CPU. I need to find out for sure. I have not been able to find any references to outline the details.

I also think on one of my other runs that the offset was 6 bytes not 4

Answer 1

The finial answer is:

When you do a malloc (which new uses under the hoods) allocates more memory than needed for the system to manage memory. This debug information is not what I am interested. What I am interested is the difference between using the return from malloc on a C++ array allocation.

What I have been able to determine is sometimes the C++ adds/uses 4 additional byte to keep count of the objects being allocated. The twist is these 4 bytes are only added if the objects being allocated require being destructed.

So given:

void* _cdecl operator new(size_t size)
{
    void *ptr = malloc(size);
    return(ptr); 
}

for the case of:

object * data = new object [size]

data will be ptr plus 4 bytes (assuming object required a destructor)

while:

char *data = new char [size]

data will equal ptr because no destructor is required.

Again, I am not interested in the memory tracking malloc adds to manage memory.