Questions about pointer to pData in MyDispatchProc1

[CES5209]

I've read in p3d sdk documentation that pData is a buffer and struct SIMCONNECT_RECV_OPEN is inherited from SIMCONNECT_RECV

void CALLBACK MyDispatchProc(SIMCONNECT_RECV* pData, DWORD cbData, void* pContext)
{
  switch(pData->dwID)
  {
    case SIMCONNECT_RECV_ID_OPEN:
      // enter code to handle SimConnect version information received in a SIMCONNECT_RECV_OPEN structure.
      SIMCONNECT_RECV_OPEN *openData = (SIMCONNECT_RECV_OPEN*) pData;
      break;

But why in line 7, the pointer type can be SIMCONNECT_RECV_OPEN? What's exactly the detail of pData?
Since I'm fresh to C++, forgive me for asking some basic questions.

 

[Bert Laverman]

Hi!
Yes, this is indeed the effect of "is inherited from". If you search for the SIMCONNECT_RECV_OPEN struct in the Learning Center (aka the Windows Help file of the SDK), then you'll see it is declared as:
struct SIMCONNECT_RECV_OPEN : public SIMCONNECT_RECV

Effectively that means that the struct starts with all the contents of SIMCONNECT_RECV before continuing with what you see in this declaration. So, if you have a pointer-to-SIMCONNECT_RECV_OPEN, you can always safely cast it to a pointer-to-SIMCONNECT_RECV. This holds for all possible extensions. So, to do the reverse cast, as is happening here, you need to know which version it is. That is why it first checks the dwID field to see which variant it is. In "real" modern C++, you would use dynamic_cast<SIMCONNECT_RECV_OPEN *>(pwData) if the message was properly "new"ed and has a type-code, or static_cast<SIMCONNECT_RECV_OPEN *>(pwData) if you want to tell the compiler you just know it is correct. The cast used here is (very) old-school C-type casting.

The pointer pData is declared as "SIMCONNECT_RECV* pData", and you can find the SIMCONNECT_RECV struct in the SDK documentation:

struct SIMCONNECT_RECV {
    DWORD  dwSize;
    DWORD  dwVersion;
    DWORD  dwID;
};

 

[WarpD]

old-school

Without which... there would be no C++ to begin with. Us "old-school" still work...

 

[CES5209]

Hi!
Yes, this is indeed the effect of "is inherited from". If you search for the SIMCONNECT_RECV_OPEN struct in the Learning Center (aka the Windows Help file of the SDK), then you'll see it is declared as:
struct SIMCONNECT_RECV_OPEN : public SIMCONNECT_RECV

Effectively that means that the struct starts with all the contents of SIMCONNECT_RECV before continuing with what you see in this declaration. So, if you have a pointer-to-SIMCONNECT_RECV_OPEN, you can always safely cast it to a pointer-to-SIMCONNECT_RECV. This holds for all possible extensions. So, to do the reverse cast, as is happening here, you need to know which version it is. That is why it first checks the dwID field to see which variant it is. In "real" modern C++, you would use dynamic_cast<SIMCONNECT_RECV_OPEN *>(pwData) if the message was properly "new"ed and has a type-code, or static_cast<SIMCONNECT_RECV_OPEN *>(pwData) if you want to tell the compiler you just know it is correct. The cast used here is (very) old-school C-type casting.

The pointer pData is declared as "SIMCONNECT_RECV* pData", and you can find the SIMCONNECT_RECV struct in the SDK documentation:

struct SIMCONNECT_RECV {
    DWORD  dwSize;
    DWORD  dwVersion;
    DWORD  dwID;
};

nice explanation! Thanks a lot!
I think I've got the point but to ensure my understanding is correct, I will repeat my understanding and could you please point out if I make any mistake?

Since SIMCONNECT_RECV_OPEN is inherited from SIMCONNECT_RECV, it actually contains more elements inside. And initially, the data buffer pData points to actually contains everything that SIMCONNECT_RECV_OPEN has, but since it's type is SIMCONNECT_RECV, I can't access members like uEventID which is unique to SIMCONNECT_RECV_OPEN.
And when I have casted the pointer, uEventID becomes available.

 

[WarpD]

SIMCONNECT_RECV_OPEN inherits from SIMCONNECT_RECV... not "is inherited from". Think of SIMCONNECT_RECV_OPEN (and all of the other structs defined as descendants of SIMCONNECT_RECV) as a genetic relative that inherits all that SIMCONNECT_RECV has to offer and brings it's own unique additions. The purpose of this approach is to allow the API to define a single struct type that it passes back a pointer to... so there's exactly one function declaration that supports all possible variations and only requires the programmer to typecast as appropriate to support them. The ID value in the SIMCONNECT_RECV allows the programmer to know exactly which type of struct the pointer actually points to and thus be able to make the proper typecast to access the true struct.

 

[Bert Laverman]

nice explanation! Thanks a lot!
I think I've got the point but to ensure my understanding is correct, I will repeat my understanding and could you please point out if I make any mistake?

Since SIMCONNECT_RECV_OPEN is inherited from SIMCONNECT_RECV, it actually contains more elements inside. And initially, the data buffer pData points to actually contains everything that SIMCONNECT_RECV_OPEN has, but since it's type is SIMCONNECT_RECV, I can't access members like uEventID which is unique to SIMCONNECT_RECV_OPEN.
And when I have casted the pointer, uEventID becomes available.

I won't go into the "how to correctly say this in English". Yes, you seem to understand correctly. As long as the pointer in in its generic form, only the common fields can be accessed. Once casted to the more specific form the compiler will accept you referring to the other fields.