follow official direct2d sample but got access violation error [duplicate] - c++

Question

This question already has answers here:
Direct2D : Unhandled Exception In WM_RESIZE switch case
(2 answers)
Closed 3 years ago.
Following the official tutorial of Direct2D (https://learn.microsoft.com/en-us/windows/win32/direct2d/direct2d-quickstart) to create a sample project with Visual Studio 2019. When running the code in x86, everything works fine while changing the platform to x64, I get an error that says: 'Exception thrown: read access violation.' in SampleD2D.cpp. (the line was commented in the code below)
the error is :
Exception thrown: read access violation.
this was 0xBB18F6E8.
LRESULT CALLBACK DemoApp::WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
LRESULT result = 0;
if (message == WM_CREATE)
{
LPCREATESTRUCT pcs = (LPCREATESTRUCT)lParam;
DemoApp* pDemoApp = (DemoApp*)pcs->lpCreateParams;
::SetWindowLongPtrW(
hwnd,
GWLP_USERDATA,
PtrToUlong(pDemoApp)
);
result = 1;
}
else
{
DemoApp* pDemoApp = reinterpret_cast<DemoApp*>(static_cast<LONG_PTR>(
::GetWindowLongPtrW(
hwnd,
GWLP_USERDATA
)));
bool wasHandled = false;
if (pDemoApp)
{
switch (message)
{
case WM_SIZE:
{
UINT width = LOWORD(lParam);
UINT height = HIWORD(lParam);
pDemoApp->OnResize(width, height); // throw the error!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
}
result = 0;
wasHandled = true;
break;
case WM_DISPLAYCHANGE:
{
InvalidateRect(hwnd, NULL, FALSE);
}
result = 0;
wasHandled = true;
break;
case WM_PAINT:
{
pDemoApp->OnRender();
ValidateRect(hwnd, NULL);
}
result = 0;
wasHandled = true;
break;
case WM_DESTROY:
{
PostQuitMessage(0);
}
result = 1;
wasHandled = true;
break;
}
}
if (!wasHandled)
{
result = DefWindowProc(hwnd, message, wParam, lParam);
}
}
return result;
}

Answer 1

Unfortunately, I'm no WinAPI expert but, out of curiosity, I googled a bit. Now, I'm quite sure about OPs problem:
::SetWindowLongPtrW(
hwnd,
GWLP_USERDATA,
PtrToUlong(pDemoApp)
);
specifically PtrToUlong(pDemoApp).
That might work for 32 bit applications but not for 64 bit.
long is in MS VC++ 32 bit – for x86 as well as x64 platform.
Hence, converting a pointer to long or unsigned long is good for making it wrong on x64 (as soon as the upper 32 bits are not 0 – which is probably hard to predict).
Googling into this direction I found e.g. PtrToUlong Q/A on gamedev.net with this (old) answer:
msdn, try to avoid using these because you are casting a pointer into an unsigned long. This may work correctly on 32-bit executables but if you compile in 64-bit you may have problems.
which supports my doubts.
According to MS doc. SetWindowLongPtrW function, the signature is:
LONG_PTR SetWindowLongPtrW(
HWND hWnd,
int nIndex,
LONG_PTR dwNewLong
);
So, this should fix it:
::SetWindowLongPtrW(
hwnd,
GWLP_USERDATA,
reinterpret_cast<LONG_PTR>(pDemoApp)
);
Please, note the MS doc. about LONG_PTR:
LONG_PTR
A signed long type for pointer precision. Use when casting a pointer to a long to perform pointer arithmetic.
This type is declared in BaseTsd.h as follows:
C++
#if defined(_WIN64)
typedef __int64 LONG_PTR;
#else
typedef long LONG_PTR;
#endif
Btw. I didn't understand as well
DemoApp* pDemoApp = reinterpret_cast<DemoApp*>(static_cast<LONG_PTR>(
::GetWindowLongPtrW(
hwnd,
GWLP_USERDATA
)));
According to doc. GetWindowLongPtrW function, the function returns LONG_PTR. So, why the static_cast<LONG_PTR>? A type cast should be always the last resort if absolutely necessary. (Although, I admit that WinAPI is probably unusable without.)