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Thread: cannot call member function without object

  1. #1
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    Default cannot call member function without object

    I get the following error when trying to connect two classes together:
    Qt Code:
    1. qapp.cpp: In static member function 'static bool QApp::eventFilter(void*)':
    2. qapp.cpp:32: error: cannot call member function 'void QApp::mouseCoord(int, int, int, int, int)' without object
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    main.cpp:
    Qt Code:
    1. int main(int argc, char *argv[])
    2. {
    3. QApp a(argc, argv);
    4. Widget w;
    5. w.show();
    6.  
    7. QObject::connect(&a,SIGNAL(mouseCoords(int, int, int, int, int)),
    8. &w,SLOT(mouseCoords(int, int, int, int, int)));
    9.  
    10. return a.exec();
    11. }
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    qapp.cpp:
    Qt Code:
    1. emit mouseCoord(get_raw_mouse_x_delta(i), get_raw_mouse_y_delta(i), // this is line 32...
    2. is_raw_mouse_button_pressed(i, 0), is_raw_mouse_button_pressed(i, 1), is_raw_mouse_button_pressed(i, 2));
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    I have searched the forum and found several issues quite like mine, but not enough to help me out.

  2. #2
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    Default Re: cannot call member function without object

    As the compiler error message says, you cannot call a member function, which requires a "this" pointer, from a static method that does not have a "this" pointer to give. This might do it:
    Qt Code:
    1. QApp *app = qobject_cast<QApp>(qApp);
    2. app->mouseCoords(...);
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    BTW: Is that mouseCoord() or mouseCoords() ? You aren't consistent.

  3. #3
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    Default Re: cannot call member function without object

    If I do this:
    Qt Code:
    1. QApp *app = qobject_cast<QApp>(qApp);
    2. app->mouseCoords(...); // what is this exactly? Am I not supposed to use connect()?
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    Then what happens to this?
    Qt Code:
    1. QApp a(argc, argv);
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    Thanks for the help.

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    Default Re: cannot call member function without object

    I would ask first why did you make your QApp::eventFilter() method static?
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

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    Default Re: cannot call member function without object

    I made it static because that's the only way it would work with QAbstractEventDispatcher(). I was experiencing some difficulties with setting QAbstractEventDispatcher() filter and a thread here in QtCentre Forum said that I should make eventFilter() static, I did and it worked. Only for that reason. But if there is a better way I would really like to know.

    EDIT:
    if I remove "static" from eventFilter I get this error message:
    Qt Code:
    1. qapp.cpp: In constructor 'QApp::QApp(int&, char**)':
    2.  
    3. qapp.cpp:14: error: no matching function for call to 'QAbstractEventDispatcher::setEventFilter(<unresolved overloaded function type>)'
    4.  
    5. c:\Qt\2010.05\qt\include\QtCore/../../src/corelib/kernel/qabstracteventdispatcher.h:91: note: candidates are: bool (* QAbstractEventDispatcher::setEventFilter(bool (*)(void*)))(void*)
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    Last edited by been_1990; 23rd October 2010 at 14:30.

  6. #6
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    Default Re: cannot call member function without object

    But this method shouldn't be static. And if you make it static then you can't call non-static methods from it. What do you need QAbstractEventDispatcher for and what is that you can't make work without making eventFilter() static?
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  7. #7
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    Default Re: cannot call member function without object

    I need QAbstractEventDispatcher to be able to get WM_INPUT messages because winEvenFilter won't do it.

    EDIT:
    if I remove "static" from eventFilter I get this error message:

    Qt Code:
    1. qapp.cpp: In constructor 'QApp::QApp(int&, char**)':
    2.  
    3. qapp.cpp:14: error: no matching function for call to 'QAbstractEventDispatcher::setEventFilter(<unresolved overloaded function type>)'
    4.  
    5. c:\Qt\2010.05\qt\include\QtCore/../../src/corelib/kernel/qabstracteventdispatcher.h:91: note: candidates are: bool (* QAbstractEventDispatcher::setEventFilter(bool (*)(void*)))(void*)
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  8. #8
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    Default Re: cannot call member function without object

    Can we see the exact code?
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    Default Re: cannot call member function without object

    Yes. Here it is:
    main.cpp:
    Qt Code:
    1. #include <QtGui/QApplication>
    2. #include <QObject>
    3. #include "widget.h"
    4. #include "qapp.h"
    5.  
    6.  
    7. int main(int argc, char *argv[])
    8. {
    9. //QApp *app = qobject_cast<QApp>(qApp);
    10. QApp a(argc, argv);
    11. Widget w;
    12. w.show();
    13.  
    14. QObject::connect(&a,SIGNAL(mouseCoord(int, int, int, int, int)),
    15. &w,SLOT(mouseCoord(int, int, int, int, int)));
    16.  
    17. return a.exec();
    18. }
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    qapp.cpp:
    Qt Code:
    1. extern "C"{
    2. #include "raw_mouse.h"
    3. }
    4. #include "qapp.h"
    5. #include <QDebug>
    6. #include <QAbstractEventDispatcher>
    7. #define WM_INPUT 0x00FF
    8.  
    9. QApp::QApp(int & argc, char ** argv) :
    10. QApplication(argc,argv)
    11. {
    12. qDebug() << init_raw_mouse(1, 0, 1);
    13. qDebug() << raw_mouse_count();
    14. QAbstractEventDispatcher::instance(0)->setEventFilter(eventFilter);
    15.  
    16.  
    17.  
    18. }
    19.  
    20.  
    21. bool QApp::eventFilter(void *msg)
    22. {
    23. MSG *mess;
    24.  
    25. mess = (MSG*)msg;
    26. //qDebug() << mess->message;
    27. switch(mess->message){
    28. case WM_INPUT:
    29. {
    30. add_to_raw_mouse_x_and_y((HANDLE)mess->lParam);
    31. for (int i = 0; i < raw_mouse_count(); i++){
    32. emit mouseCoord(get_raw_mouse_x_delta(i), get_raw_mouse_y_delta(i),
    33. is_raw_mouse_button_pressed(i, 0), is_raw_mouse_button_pressed(i, 1), is_raw_mouse_button_pressed(i, 2));
    34. }
    35.  
    36. }
    37. return true;
    38. }
    39. return false;
    40. }
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    app.h:
    Qt Code:
    1. #ifndef QAPP_H
    2. #define QAPP_H
    3.  
    4. #include <QApplication>
    5.  
    6. class QApp : public QApplication
    7. {
    8. Q_OBJECT
    9. public:
    10. explicit QApp(int & argc, char ** argv );
    11. //bool winEventFilter(MSG *message, long *result);
    12. static bool eventFilter(void *msg);
    13. private:
    14.  
    15. signals:
    16. void mouseCoord(int x, int y, int btn1, int btn2, int btn3);
    17.  
    18. public slots:
    19.  
    20. };
    21.  
    22. #endif // QAPP_H
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  10. #10
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    Default Re: cannot call member function without object

    Why not do it this way?

    Qt Code:
    1. bool myInputEventFilter(void*);
    2.  
    3. class QApp : public QApplication {
    4. Q_OBJECT
    5. friend bool myInputEventFilter(void *);
    6. public:
    7. QApp(int argc, char **argv) : QApplication(argc, argv){
    8. init_raw_mouse(1, 0, 1);
    9. QAbstractEventDispatcher::instance(0)->setEventFilter(myInputEventFilter);
    10. }
    11. signals:
    12. void mouseCoord(int x, int y, int btn1, int btn2, int btn3);
    13. };
    14.  
    15. // the following function may probably be made static so that it doesn't pollute the namespace
    16. bool myInputEventFilter(void *msg) {
    17. MSG *mess = (MSG*)msg;
    18. QApp *app = (QApp*)qApp;
    19. switch(mess->message){
    20. case WM_INPUT:
    21. {
    22. add_to_raw_mouse_x_and_y((HANDLE)mess->lParam);
    23. for (int i = 0; i < raw_mouse_count(); i++){
    24. emit app->mouseCoord(get_raw_mouse_x_delta(i), get_raw_mouse_y_delta(i),
    25. is_raw_mouse_button_pressed(i, 0), is_raw_mouse_button_pressed(i, 1), is_raw_mouse_button_pressed(i, 2));
    26. }
    27.  
    28. }
    29. return true;
    30. }
    31. return false;
    32. }
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    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


  11. #11
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    Default Re: cannot call member function without object

    Great! Working perfectly now. Thanks Wysota.
    What do you mean with "pollute the namespace"?

  12. #12
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    Default Re: cannot call member function without object

    Quote Originally Posted by been_1990 View Post
    What do you mean with "pollute the namespace"?
    Try defining (in a different cpp file) a function called myInputEventFilter and taking a void* as input and you'll see for yourself.
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

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