CBT Hook dll, to intercept window from being resized - c++

Question

I'm trying to write a dll to intercept a window from being resized, but i cant understand how to correctly specify the lParam in this case.
From the docs:
HCBT_MOVESIZE: Specifies a long pointer to a RECT structure containing
the coordinates of the window. By changing the values in the
structure, a CBTProc hook procedure can set the final coordinates of
the window.
Current code:
#include "pch.h"
#include <Windows.h>
extern "C" __declspec(dllexport) LRESULT CALLBACK CBTProc(
_In_ int nCode,
_In_ WPARAM wParam,
_In_ LPARAM lParam
)
{
if (nCode < 0) return CallNextHookEx(nullptr, nCode, wParam, lParam);
switch(nCode)
{
case HCBT_MOVESIZE: // A window is about to be moved or sized.
/*
For operations corresponding to the following CBT hook codes, the return value must be 0 to allow the operation, or 1 to prevent it.
HCBT_ACTIVATE
HCBT_CREATEWND
HCBT_DESTROYWND
HCBT_MINMAX
HCBT_MOVESIZE
HCBT_SETFOCUS
HCBT_SYSCOMMAND
*/
/*
switch(LOWORD(lParam)) //
{
case EVENT_SYSTEM_MOVESIZESTART:
return 1; // Prevent
}
*/
}
return 0;
}

Answer 1

In the case of HCBT_MOVESIZE, the lParam contains the memory address of a RECT, so simply typecast the lParam to a RECT* pointer, eg:
extern "C" __declspec(dllexport) LRESULT CALLBACK CBTProc(
_In_ int nCode,
_In_ WPARAM wParam,
_In_ LPARAM lParam
)
{
if (nCode < 0) return CallNextHookEx(nullptr, nCode, wParam, lParam);
switch(nCode)
{
case HCBT_MOVESIZE: // A window is about to be moved or sized.
{
HWND hwnd = reinterpret_cast<HWND>(wParam);
RECT *rc = reinterpret_cast<RECT*>(lParam);
// use hwnd and *rc as needed...
if (should not resize)
return 1;
break;
}
...
}
return 0;
}

Answer 2

LPARAM is a pointer-sized value. It can hold any value that fits into a pointer. The meaning of the value commonly depends on context.
When handling a HCBT_MOVESIZE callback it designates
a long pointer to a RECT structure
To use it, client code needs to convert the value into a value of the respective type. In C++ this is done using a cast, e.g.
switch(nCode)
{
case HCBT_MOVESIZE: {
auto pRect{ reinterpret_cast<RECT*>(lParam) };
// Use `pRect` to read from or write to the rectangle
}
break;
// ...
}