Question

I wrote a C++ application on Win64 with these lines:

Window* wnd = 0;
long l = reinterpret_cast<long> ( wnd );

The compiler displays the following error at the last line:

error: cast from 'window::Window*' to 'long int' loses precision [-fpermissive] 

I use this value to put it into SetWindowLong (WindowsAPI) function.

I cannot understand this error. I am using MinGW-w64 (ruben build).

Answer 1

Read my original comment.

The issue is that sizeof(window::Window *) is greater than sizeof(long), meaning you can’t effectively store the value of the pointer in l. This explains the error.

Now, what you should really want is actually to use SetWindowLongPtr instead.

Note To write code that is compatible with both 32-bit and 64-bit versions of Windows, use SetWindowLongPtr. When compiling for 32-bit Windows, SetWindowLongPtr is defined as a call to the SetWindowLong function.

window::Window *wnd = ...;
HWND handle = ...;
int index = ...; /* e.g. GWLP_USERDATA */
LONG_PTR val = reinterpret_cast<LONG_PTR>(wnd);
SetWindowLongPtr(handle, index, val);