how CWinApp does realize the singleton pattern?

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Generally,singleton pattern is applied by the way that the ctor is private and gives a member funtion named getInstance, but CWinApp's instance is defined as a global object,its ctor is not private.So how does MFC realizes the singleton pattern for CWinApp?

Posted 21-Jul-10 18:33pm

zxjun84

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Sauro Viti · Answer 1 · 2010-07-21T19:41:00

Solution 1

MFC does not implement CWinApp as a singleton, simply each application instantiate just one object of that class, but if you write something like this in your code:

C++

CWinApp theApp1;
CWinApp theApp2;

You obtain two different instances of CWinApp

Posted 21-Jul-10 19:41pm

Sauro Viti

SenAPI · Answer 2 · 2010-07-22T20:24:00

as Sauro Viti said CWinApp is not a singleton class.
while declaring a object of CWinApp class MFC stores first object to afxCurrentWinApp veriable.
Remember that MFC project has also WinMain(...) function.

We can see what will happen when we declare an object CWinApp theApp1;

1) It will call constructor of CWinApp [Because it is global variable]
2) Inside constructor afxCurrentWinApp object will set to current object ( this)
3) after all global variables construction WinMain will invoke.
4) inside WinMain it will call the CWinApp's InitInstance() [ because it is vertual function]

Note: When we call AfxGetApp() we will return the afxCurrentWinApp.