derive a class from a struct?

This is a solution I thought about to some of the problems I had talked
about in the "zero memory" above in the ng.

While this isn't a MS specific post, let me walk you though the problem that
MS created that I am trying to overcome

They have functions:

HANDLE CreateIoComplet ionPort(
HANDLE FileHandle,
HANDLE ExistingComplet ionPort,
ULONG_PTR CompletionKey,
DWORD NumberOfConcurr entThreads
);

BOOL AcceptEx(
SOCKET sListenSocket,
SOCKET sAcceptSocket,
PVOID lpOutputBuffer,
DWORD dwReceiveDataLe ngth,
DWORD dwLocalAddressL ength,
DWORD dwRemoteAddress Length,
LPDWORD lpdwBytesReceiv ed,
LPOVERLAPPED lpOverlapped
);

BOOL GetQueuedComple tionStatus(
HANDLE CompletionPort,
LPDWORD lpNumberOfBytes ,
PULONG_PTR lpCompletionKey ,
LPOVERLAPPED* lpOverlapped,
DWORD dwMilliseconds
);

Now the flow of things that they show in thier example code creates a
context struct for each IO command, which contains user specific data. This
struct that they make contains an OVERLAPPED structure, which is what MS
uses to register an IO command with an IO-completion port. When the
IO-Completion port is done with an IO command, a thread stops waiting at a
call to GetQueuedComple tionStatus. Now then, when GetQueuedComple tionStatus
returns, all we have is the OVERLAPPED structure that was passed in as an
argument to the IO command, but we need the entire context. So, MS, in
thier example, casts the OVERLAPPED structure to the entrire structure that
it was a member of. This works because, they made the OVERLAPPED structure
the first member of the context structure.

I want to use C++, I am tired of mixing C code and C++ code together and
having to hack past the problems that arise.

In order to do this I was wondering if I could make a context class that is
derived from thier OVERLAPPED structure. That way I could dynamic cast from
an OVERLAPPED pointer to a context pointer right? That would get rid of
thier C cast that they depend on (which is really unsafe and is pretty much
a reinterp cast imo). I know I can cast from a base class upwards to a
derived class, can you also cast the other way around? Are there any inherit
problems with deriving a class from a struct that I need to know about? do
you think this solution would work?

Here is there OVERLAPPED for reference:

typedef struct _OVERLAPPED {
ULONG_PTR Internal;
ULONG_PTR InternalHigh;
union {
struct {
DWORD Offset;
DWORD OffsetHigh;
};
PVOID Pointer;
};
HANDLE hEvent;
} OVERLAPPED,
*LPOVERLAPPED;