Dereferencing pointer C++

Question

I just managed to write a global CBT hook in c++, usable with c#. It may sound stupid but my knowledge of pointers and dereferencing of those is very bad due to my work with c#.

I'm not able to dereference a pointer inside a struct pointed to by the lParam.

It looks like this: the lParam is a long Pointer to a CBT_CREATEWND struct, which in turn holds a member "lpcs", and a pointer to a CREATESTRUCT. This struct contains a member "x", which I want to read. I tried the following, but I get invalid values for x:

CREATESTRUCT str = *(LPCREATESTRUCT)(((LPCBT_CREATEWND)lParam)->lpcs);
int normal = str.x;
PostMessage(FindWindow(NULL, L"Process Watcher Window"), RegisterWindowMessage(L"ALEXX_SEINE_WNDMESSAGE_C"), wParam, normal);

could someone please help me?

Answer 1

Well I can't really speak towards the fine details of why you get invalid x values, but I would likely write this code differently:

// the way you had it, it was making a copy of the CREATESTRUCT and storing it in str
// this just uses a pointer
LPCREATESTRUCT str = ((LPCBT_CREATEWND)lParam)->lpcs;
// when you have a pointer, use -> to use a member
int normal = str->x;

Since you said you are new to pointers, I'll explain -> a bit. When you write x->y , it is really the same as (*x).y but with nicer syntax.

Also a note of advice, while the casts in this code seem reasonable. In general, if you find that you are casting a lot, you probably are doing it either the hard way, or the wrong way. So make sure you take the time to understand any casts that you do.

Answer 2

Your syntax seems to check out, even though it is perhaps a bit unreadable, and the LPCREATESTRUCT cast is obviously unnecessary.

You mentioned that you get invalid values for x , perhaps lParam is not really a pointer to a CBT_CREATEWND structure? Are you checking that nCode of the callback function is equal to HCBT_CREATEWND before casting lParam ?