Chapter 12: Dialog-Based Windows Visual C++ and MFC Fundamentals
10.1.3..A Window's Instance..........................................................................
When you start an application such as Notepad, you are said to have created an instance
of the application. In the same way, when you declare a variable of a class, an instance of
the class is created and made available to the project. The WinMain() function also
allows you to create an instance of an application, referred to as the hInstance argument
of the WinMain() function. The instance is created as an HINSTANCE.The CWinApp class provides a corresponding instance variable called m_hInstance.
This variable can let you get a handle to the instance of your application. Alternatively, to
get a handle to the instance of your application, you can call the
AfxGetInstanceHandle() global function. Its syntax is:HINSTANCE AfxGetInstanceHandle();Even more, to get a handle to your application, you can call the Win32 API’s
GetWindowLong() function.Suppose you have opened Notepad to view the source code of an HTML document. This
is said that you have an instance of Notepad. Imagine that you want to open a text
document using Notepad without closing the first instance of Notepad. To do this, you
must open another copy of Notepad. This second copy of Notepad is another instance. In
this case, the first instance is referred to as a previous instance. For a Win32 application,
the previous instance would be the hPrevInstance argument of the WinMain() function.
For a Win32 application, the hPrevInstance argument always has the NULL value. If you
want to find out whether a previous instance of an application already exists, you can call
the CWnd::FindWindow() method. Its syntax is:static CWnd* PASCAL FindWindow(LPCTSTR lpszClassName, LPCTSTR lpszWindowName);If you created the window or if it is a window you know for sure, in which case it could
be a WNDCLASS or WNDCLASSEX object, specify it as the lpszClassName
argument. If you do not know its name with certainty, set this argument as NULL.The lpszWindowName argument is the possible caption of the window you are looking
for. Imagine you position a button on a dialog box and you want the user to launch
Notepad with that button and imagine that, if Notepad is already opened, there would be
no reason to create another instance of it. In the following code, when the user clicks the
button, the application checks if Notepad is already opened and displays a message
accordingly:void CFindWindowDlg::OnFindNotepad()
{
// TODO: Add your control notification handler code here
CWnd *ExistsAlready = FindWindow(NULL, "Untitled - Notepad");if( ExistsAlready )
AfxMessageBox("An instance of Notepad is already available");
else
AfxMessageBox("No instace of Notepad could be found");
}