The IsWindowUnicode function
reports whether the current window procedure expects
Unicode window messages.
What specifically does this mean?
It means that messages like
WM_CHAR will report
characters as Unicode code units rather than as
ANSI code units.
Okay, so what determines whether a window procedure receives Unicode messages?
When a window is created, its initial window procedure
comes from the class registration.
If the class was registered with RegisterClass[Ex]W,
then it is a Unicode window procedure;
otherwise it was registered with RegisterClass[Ex]A
and is an ANSI window procedure.
If a window is subclassed with a function pointer,
then the new window procedure is Unicode if it was
set with
SetWindowLongPtrW(GWLP_WNDPROC)
and ANSI if it was set with
SetWindowLongPtrA(GWLP_WNDPROC).
As we saw some time ago,
GetWindowLongPtr(GWLP_WNDPROC)
returns a thunk
if you use the ANSI version but the current window procedure is Unicode,
or vice versa.
Suppose the window procedure is Unicode and you call
GetWindowLongPtrA(GWLP_WNDPROC).
This will return you a thunk,
and if you later set that thunk back as the window procedure with
SetWindowLongPtrA(GWLP_WNDPROC),
then the original Unicode window procedure is restored,
and the window is a Unicode window again.
Note that calling
SetWindowLongPtrA(GWLP_WNDPROC)
can result in a window procedure that is Unicode.
As a window message travels from one window procedure
to another window procedure (via CallWindowProc),
the character set may flip between Unicode and ANSI.
The IsWindowUnicode function tells you only
about the window's current window procedure.