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Thread: main.cpp variable access question

  1. 9th March 2008, 22:12 #1
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    Default main.cpp variable access question

    Hi to all!

    Can somene please tell me how to access variable appSettings, declared in main.cpp:
    Qt Code: 
    1. // qt includes
    2. #include <QtGui>
    3. #include <QApplication>
    4. #include <QPlastiqueStyle>
    5.  
    6. // custom includes
    7. #include "cmainwindow.h"
    8. #include "globals.h"
    9. #include "CApplicationSettings.h"
    10.  
    11.  
    12. int main(int argc, char *argv[])
    13. {
    14.     ....    
    15.     CApplicationSettings appSetings(strIniFileName,
    16.                                     QSettings::IniFormat,
    17.                                     qApp);    // sets up app settings
    18.  
    19.     ....
    20. }
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    from class, that is defined through
    Qt Code: 
    1. #include "cmainwindow.h"
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    in main.cpp? Is this possible?
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  2. 9th March 2008, 22:48 #2
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    Default Re: main.cpp variable access question

    No, this variable is only visible from inside main(). If you want to access it from outside, you need to provide a pointer to it either through a global variable or through some method of a class that itself has access to that variable or a static method returning the instance (just like qApp and QCoreApplication::instance() are used).
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  3. 9th March 2008, 23:40 #3
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    Default Re: main.cpp variable access question

    hmm, then I must rewrite CApplicationSettings like I did with CDatabaseSettings: make it singleton?

    Or, where should I put pointer to this variable?
    Last edited by MarkoSan; 10th March 2008 at 03:31. Reason: updated contents
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  4. 10th March 2008, 06:39 #4
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    Default Re: main.cpp variable access question

    You can use a global variable.
    Qt Code: 
    1. CApplicationSettings *settings;
    2.  
    3. int main(...){
    4.   CApplicationSettings Settings;
    5.   settings = &Settings;
    6. }
    7.  
    8. extern CApplicationSettings *settings;
    9. void xxx(){ settings->doSomething(); }
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    MarkoSan (10th March 2008)
  6. 10th March 2008, 16:11 #5
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    Unhappy Re: main.cpp variable access question

    But wysotta, why do I need both declarations from line 1 AND from line 8? Otherwise, I rewritten the code according to your hint, it compiles without errors. Now let me check this pointer. Thanks man!

    Well, when i try to acces global variable from other class, I get this error:
    Qt Code: 
    1. mingw32-make 
    2. mingw32-make -f Makefile.Debug
    3. mingw32-make[1]: Entering directory `C:/Documents and Settings/markofr/workspace/SettingsEditor'
    4. c:\Qt\4.3.4\bin\uic.exe cmainwindow.ui -o ui_cmainwindow.h
    5. g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include" -I"c:\Qt\4.3.4\include\ActiveQt" -I"debug" -I"." -I"..\..\..\..\Qt\4.3.4\mkspecs\win32-g++" -o debug\CDataInputWidget.o CDataInputWidget.cpp
    6. g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include" -I"c:\Qt\4.3.4\include\ActiveQt" -I"debug" -I"." -I"..\..\..\..\Qt\4.3.4\mkspecs\win32-g++" -o debug\CClientVisualSettingsPage.o CClientVisualSettingsPage.cpp
    7. g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include" -I"c:\Qt\4.3.4\include\ActiveQt" -I"debug" -I"." -I"..\..\..\..\Qt\4.3.4\mkspecs\win32-g++" -o debug\CLanguageSettingsPage.o CLanguageSettingsPage.cpp
    8. g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include" -I"c:\Qt\4.3.4\include\ActiveQt" -I"debug" -I"." -I"..\..\..\..\Qt\4.3.4\mkspecs\win32-g++" -o debug\CDatabaseSettingsPage.o CDatabaseSettingsPage.cpp
    9. g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtCore" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include\QtGui" -I"..\..\..\..\Qt\4.3.4\include" -I"c:\Qt\4.3.4\include\ActiveQt" -I"debug" -I"." -I"..\..\..\..\Qt\4.3.4\mkspecs\win32-g++" -o debug\CSettingsWindow.o CSettingsWindow.cpp
    10. CSettingsWindow.cpp: In member function `void CSettingsWindow::saveSettings()':
    11. CSettingsWindow.cpp:124: error: `pAppSettings' undeclared (first use this function)
    12. CSettingsWindow.cpp:124: error: (Each undeclared identifier is reported only once for each function it appears in.)
    13. mingw32-make[1]: *** [debug/CSettingsWindow.o] Error 1
    14. mingw32-make: *** [debug] Error 2
    15. mingw32-make[1]: Leaving directory `C:/Documents and Settings/markofr/workspace/SettingsEditor'
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    Last edited by MarkoSan; 10th March 2008 at 16:36. Reason: updated contents
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  7. 10th March 2008, 18:04 #6
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    Default Re: main.cpp variable access question

    Line 8 has to be in a file where you want to use the variable declared in the file containing line 1. Otherwise you'll get wat you got in line 11 of your compiler output.
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  9. 10th March 2008, 18:25 #7
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    Default Re: main.cpp variable access question

    Just one more thing I want to ask about "background" of this code implementation. How does compiler knows the pointer from main and extern pointer are the same? Because of same name? And, I've read today about this topic we are discussing and it seems these global pointers are very dangerous in program, because it's value can be changed without notifiacation, which leads to possible application crash. How do I make this pointer const?
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  10. 10th March 2008, 19:54 #8
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    Default Re: main.cpp variable access question

    Quote Originally Posted by MarkoSan View Post
    How does compiler knows the pointer from main and extern pointer are the same? Because of same name?
    Yes, the name and the extern keyword.

    And, I've read today about this topic we are discussing and it seems these global pointers are very dangerous in program, because it's value can be changed without notifiacation, which leads to possible application crash.
    Correct. But if you're the only programmer and you know what you are doing, you can safely use it.

    How do I make this pointer const?
    You can't. As an alternative you can implement the singleton pattern for the class.
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  12. 10th March 2008, 20:04 #9
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    Default Re: main.cpp variable access question

    Wysota, thank you very very much for clarification of this topic. Just one final question: What are other design patterns beside singleton design pattern for solving this problem? So I can google it up and read about it ...
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  13. 10th March 2008, 20:25 #10
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    Default Re: main.cpp variable access question

    I don't think there are other patterns that might be helpful, although I'm not much of an expert on design patterns.
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  14. 10th March 2008, 20:48 #11
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    Default Re: main.cpp variable access question

    Ok, will google it anyway.
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