Creating duplicate object of CWinApp in mfc vc++

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Hi All,
I'm frequently using dll project in vc++ for breaking big project into small modules.
As we known that a dll project has only one working dll file is a project file.
that is if my project name is CheckVarPropDll then
CheckVarPropDll.cpp and CheckVarPropDll.h are created.
this file contain class that inherited from CWinApp

if i have function that member of CWinApp
like follow

void CWinApp::CheckVarProp(CString str)
{
 .
 .
 .
}

and that function call is in user define class
like

EnterValue.cpp

void EnterValue::GetValue()
{
CWinApp myapp;
myapp.CheckVarProp("1234");
}

code compile and link so well but while using this dll in mfc application gives treading error.

Becouse of creating object of CWinApp

there is default object of CwinApp present in .cpp file of project just below of

InitInstance()

like

CWinApp theApp;

so my question start here can there is any method to create duplicate object of CWinApp. so that i can call any function of cwinApp from any point of time and any point of code.
if Yes then how it?
if No then Why it?
Thanks
Regard SANTOSH

Posted 10-Mar-11 18:26pm

santosh dhanawade

Updated 10-Mar-11 20:41pm

v4

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Olivier Levrey · Answer 1 · 2011-03-10T21:35:00

Solution 1

Do not instanciate CWinApp object! There should be only one instance per project (the one declared as theApp). If you want to access that object, then you there have two options:

1- add

extern CWinApp theApp;

at the end of CheckVarPropDll.h so it can be accessed in all your .cpp files (of course, make sure they have #include "CheckVarPropDll.h").

2- use the global function AfxGetApp. This function returns CWinApp pointer (a pointer to theApp). You nee then to cast that pointer into your CWinApp derived class.

Personnaly, I prefer solution 1:

C++

void EnterValue::GetValue()
{
    theApp.CheckVarProp("1234");
}

Or

C++

void EnterValue::GetValue()
{
    ((CYourWinApp*)AfxGetApp())->CheckVarProp("1234");
}

Posted 10-Mar-11 21:35pm

Olivier Levrey

Updated 10-Mar-11 21:37pm

v2

Comments

[no name] 11-Mar-11 3:38am

Thanks for reply after,
can you tell me why not possible to create object CWinApp.

Olivier Levrey 11-Mar-11 3:40am

Basically, this class contains the entry point of your application (or dll) : kind of main function. If you instanciate the class more than once, you will have more than 1 main.

Olivier Levrey 11-Mar-11 3:42am

Have a look to this link for more details about CWinApp: http://msdn.microsoft.com/en-us/library/akdx0603(v=vs.80).aspx

[no name] 11-Mar-11 3:44am

But its a class it has lot of functions.
we can say that InitInstance() can call one time but why all class functions.
and functions that not called multiple time we can declare it as private so water become water and milk become milk.

[no name] 11-Mar-11 3:46am

And how my application can compile and link problem came when dll used in mfc application

Olivier Levrey 11-Mar-11 4:04am

Read the documentation! It is written Each application that uses the Microsoft Foundation classes can only contain one object derived from CWinApp. Special initialization for your application is made in CWinApp constructor. This special initialization should be done only once. If you instanciate a second CWinApp object, then you will call constructor 2 times and do the initialization 2 times. This causes the crash.

CPallini · Answer 2 · 2011-03-10T21:58:00

This remark before the definition of the CWinApp's (actually your CWinApp inherited class) instance

C++

// The one and only CMFCApp object

is an hint that CWinApp is not meant to be duplicated. Hence your design (if I got you) is probably bad or, at least, it clashes with MFC.

I suggest you to revise your design. Possibly the following questions may help you:

Cannot you move your functionality out of CWinApp?
Do your methods really need to access CWinApp's instance members (if you write static methods, then you have no need to duplicate CWinApp)?