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<!-- Article Title -->Single Instance of an Application Class
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This article was contributed by <!-- Author Email --><A HREF="mailto:kevin@redcreek.com"><!-- Author Name -->Kevin Lussier</A>.
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<P>The question of how to force only one instance of an application
to run at a time is often asked. Although this is not a difficult problem
to solve, and indeed can be solved in several different ways, it comes
up enough to create the need for a nice, canned solution. So here it is.
<P>The class is called CSingleInstance, and is used as follows:
<P>Include the header file to your CWinApp derived class's
.H file:
<PRE><TT><FONT COLOR="#990000">
<P>...
<BR>#ifndef __AFXWIN_H__
<BR> #error include 'stdafx.h' before including this
file for PCH
<BR>#endif
<P>#include "resource.h"
// main symbols
<BR>#include "SingleInstance.h"
<BR>...
</FONT></TT></PRE>
<P>Create a public member variable in you CWinApp derived
class:
<PRE><TT><FONT COLOR="#990000">
<P>class CMyApp : public CWinApp
<BR>{
<BR>public:
<BR> CMyApp();
<BR> CSingleInstance m_singleInstance;
<BR>...
</FONT></TT></PRE>
<P>In InitInstance, call the class抯 Create() function with
your main frame抯 resource identifier (usually IDR_MAINFRAME):
<PRE><TT><FONT COLOR="#990000">
<P>BOOL CMyApp::InitInstance()
<BR>{
<BR> // Check if an instance of our application
is already running
<BR> if ( !m_singleInstance.Create( IDR_MAINFRAME
) ) {
<BR> // Return
indicating that this instance
<BR> // of the
app should be shut down
<BR> return FALSE;
<BR> }
<P> AfxEnableControlContainer();
<BR>...
</FONT></TT></PRE>
<P>If the Create() function fails, the application is already
running AND the running instance will be brought to the foreground. If
the Create() function succeeds, then this is the first instance of the
application.
<P>In order to make this work, there is one more piece of
code that needs to be inserted. In you main frame抯 (usually CMainFrame)
PreCreateWindow() function, add the following:
<PRE><TT><FONT COLOR="#990000">
<P>BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs)
<BR>{
<BR> // TODO: Modify the Window class or
styles here by modifying
<BR> // the CREATESTRUCT cs
<BR> CMyApp *app = (CMyApp *)AfxGetApp();
<BR> // Set the class name to be the app's
single instance class name
<BR> cs.lpszClass = app->m_singleInstance.GetClassName();
<BR>...
</FONT></TT></PRE>
<P>That抯 how to use it. If you抮e interested, here抯 how
it works.
<P>The Create() function for CSingleInstance contains two
protected members: a mutex (m_hMutex ) and a class string (m_strClassName).
The mutex is used as the mechanism for actually determining if the application
is already running. If the CreateMutex() function succeeds, the application
is the first instance. If it fails, the mutex has already been created
by another instance of the application. The class string is used as a means
of finding the main window of the application. It is created by taking
the name of the application (pulled from the resource string) and appending
" Class" to the end. Simple but effective. It gets attached to the main
window during the main frame抯 PreCreateWindow() function. When we want
to find the window, we use FindWindowEx() with the class string.
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<p><!-- first the filename (zip files) --><A HREF="SingleInstanceApp.zip">Download demo project - 19KB</A>
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<p><!-- first the filename (zip files) --><A HREF="single_instance.zip">Download source - 2KB</A>
<!-- Posted / Update date mm/dd/yy - add to the list -->
<p>Date Posted: 09 May 1998
<P><HR>
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<CENTER><FONT SIZE=-2>© 1998 Zafir Anjum</FONT> </CENTER>
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<DIV ALIGN=right><FONT SIZE=-1>Contact me: <A HREF="mailto:zafir@home.com">zafir@home.com</A> </FONT></DIV>
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