Re: derive a class from a struct?

"Victor Bazarov" <>
Fri, 6 Apr 2007 14:35:28 -0400
I'm probably overquoting... Sorry about that. Don't have time to
trim it as needed.

Christopher Pisz wrote:

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 CreateIoCompletionPort(
 HANDLE FileHandle,
 HANDLE ExistingCompletionPort,
 ULONG_PTR CompletionKey,
 DWORD NumberOfConcurrentThreads

BOOL AcceptEx(
 SOCKET sListenSocket,
 SOCKET sAcceptSocket,
 PVOID lpOutputBuffer,
 DWORD dwReceiveDataLength,
 DWORD dwLocalAddressLength,
 DWORD dwRemoteAddressLength,
 LPDWORD lpdwBytesReceived,
 LPOVERLAPPED lpOverlapped

BOOL GetQueuedCompletionStatus(
 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 GetQueuedCompletionStatus. Now
then, when GetQueuedCompletionStatus 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?

No. You can only use 'dynamic_cast' with polymorphic classes. Since
your OVERLAPPED struct is not polymorphic (and it can't be), you can
only use 'static_cast' if you are sure that the pointer you have is
in fact obtained by converting from a pointer to your derived class.

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?

Yes, but why?

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?

It might.

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;

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