I'm using Visual Express C++ and Windows XP, but the book I am using
("Programming Windows" by Charles Petzold) is very old, refering to C
and Windows 2. There are a few differences between what the book says
and what the situation is now, but I think I am top of most of them.
However, I have a query as regards registering window classes.

In the book, there is a line

if (!hPrevInstance)

before the code to register a window class with RegisterClass. The
theory is that, if another copy of the same program is already running,
the window class is already registered. Visual C++ will write a basic
windows program for me, but it does not include this line, running
RegisterClassEX whether or not hPrevInstance is NULL. I was wondering
if there was any significance to this - the rest of the windows program
looks similar to what is in my book and so I would have expected this
bit too to be the same unless there was some sort of problem.

So can you fill me in on this?

Thanks in advance.
Paul.

I believe this used to be the case in dreadfully old versions of
Windows, but isn't any more. From the help on WinMain:

int WINAPI WinMain(
HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPWSTR lpCmdLine,
int nShowCmd
);

....

hPrevInstance
[in] Handle to the previous instance of the application. For a
Win32-based application, this parameter is always NULL.
If you need to detect whether another instance already exists, create a
uniquely named mutex using the CreateMutex function. CreateMutex will
succeed even if the mutex already exists, but the GetLastError function
will return ERROR_ALREADY_EXISTS. This indicates that another instance
of your application exists, because it created the mutex first.

Michael