Static_cast cannot convert from void * to size_t error in C ++ in VS 2010

I have the following code that I get after an error

error C2036: 'void *': unknown size
error C2440: 'static_cast': cannot convert from 'void *' to 'size_t'

in line

void *addr = static_cast <void*> (static_cast <size_t> (mem + bytesAlreadyAllocated));

My question is why am I getting the above errors and how to get rid of these errors?

Thanks for the help.

class MemoryChunk {
public:
    MemoryChunk (MemoryChunk *nextChunk, size_t chunkSize);
    ~MemoryChunk() {delete mem; }

    inline void *alloc (size_t size);
    inline void free (void* someElement);

    // Pointer to next memory chunk on the list.
    MemoryChunk *nextMemChunk() {return next;}
    // How much space do we have left on this memory chunk?
    size_t spaceAvailable() { return chunkSize - bytesAlreadyAllocated; }

    // this is the default size of a single memory chunk.
    enum { DEFAULT_CHUNK_SIZE = 4096 };
private:

    // The MemoryChunk class is a cleaner version of NextOnFreeList. It separates the next pointer from
    // the actual memory used for the allocated object. It uses explicit next and mem pointers, with no 
    // need for casting.

    MemoryChunk *next;
    void *mem;

    // The size of a single memory chunk.
    size_t chunkSize;
    // This many bytes already allocated on the current memory chunk.
    size_t bytesAlreadyAllocated;
};

MemoryChunk::MemoryChunk(MemoryChunk *nextChunk, size_t reqSize) {
    chunkSize = (reqSize > DEFAULT_CHUNK_SIZE) ? reqSize : DEFAULT_CHUNK_SIZE;
    next = nextChunk;
    bytesAlreadyAllocated = 0;
    mem = new char [chunkSize];
}

void* MemoryChunk :: alloc (size_t requestSize) {
    void *addr = static_cast <void*> (static_cast <size_t> (mem + bytesAlreadyAllocated));
    bytesAlreadyAllocated += requestSize;
    return addr;
}

inline void MemoryChunk :: free (void *doomed) {}