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Thread: subclass the QLabel

  1. #1
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    Default subclass the QLabel

    I want to get the coordinate from Qlabel when I double click on...
    how do I sub class the Qlabel in ui application
    my code looks:
    mainwindow.cpp
    Qt Code:
    1. #include "mainwindow.h"
    2. #include "ui_mainwindow.h"
    3.  
    4. MainWindow::MainWindow(QWidget *parent) :
    5. QMainWindow(parent),
    6. ui(new Ui::MainWindow)
    7. {
    8. ui->setupUi(this);
    9. }
    10.  
    11. MainWindow::~MainWindow()
    12. {
    13. delete ui;
    14. }
    To copy to clipboard, switch view to plain text mode 

    mainwindow.h
    Qt Code:
    1. #ifndef MAINWINDOW_H
    2. #define MAINWINDOW_H
    3.  
    4. #include <QMainWindow>
    5.  
    6. namespace Ui {
    7. class MainWindow;
    8. }
    9.  
    10. class MainWindow : public QMainWindow
    11. {
    12. Q_OBJECT
    13.  
    14. public:
    15. explicit MainWindow(QWidget *parent = 0);
    16. ~MainWindow();
    17.  
    18.  
    19. private:
    20. Ui::MainWindow *ui;
    21. };
    To copy to clipboard, switch view to plain text mode 

  2. #2
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    Default Re: subclass the QLabel

    is your question probably about promition if you google it? If I understand well, you want to give extra features to QLabel, than in the first step "promote to" is your friend, you may find more information in the documentation.
    Szilvi

  3. #3
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    Default Re: subclass the QLabel

    is your question probably about promition if you google it?
    Is your answer to this thread is promotion??

  4. #4
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    Default Re: subclass the QLabel

    hi , u can override the mousPressEvent
    Qt Code:
    1. void QLabel::mousePressEvent(QMouseEvent *evt)
    2. {
    3. qDebug()<<"X"<<evt->x()<<"Y"<<evt->y();
    4. }
    To copy to clipboard, switch view to plain text mode 

    hope it helps
    Bala

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  6. #5
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    Default Re: subclass the QLabel

    thanks bala...
    but i am getting confused where should i declare the class to override the mousepressevent..
    is it in mainwindows.h??

  7. #6
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    Default Re: subclass the QLabel

    put it in mainwindow.cpp

    hope it helps
    Bala

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  9. #7
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    Default Re: subclass the QLabel

    Quote Originally Posted by BalaQT View Post
    hi , u can override the mousPressEvent
    Qt Code:
    1. void QLabel::mousePressEvent(QMouseEvent *evt)
    2. {
    3. qDebug()<<"X"<<evt->x()<<"Y"<<evt->y();
    4. }
    To copy to clipboard, switch view to plain text mode 

    hope it helps
    Bala
    Do you really think this will compile and link? No comment!


    @topic:
    The simples way to solve this problem is to install event filter see: QObject::installEventFilter for details.

    Another think you are doing common mistake in asking questions!
    You giving us ready solution of your problem and asking how to make it work.
    First you should explain what you want to achieve then explain how you tried to solve it this problem. I wouldn't be surprised if some would give you a better solution of primary problem.
    Last edited by MarekR22; 25th March 2011 at 08:55.

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  11. #8
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    Default Re: subclass the QLabel

    Do you really think this will compile and link? No comment!
    yes it is compiling. whats wrong?
    have you try in your end?

    bala

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  13. #9
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    Default Re: subclass the QLabel

    does it work?

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    ready (25th March 2011)

  15. #10
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    Default Re: subclass the QLabel

    thanks marek
    the event filter class u mentioned above...
    do I need to declare this class in mainwindow.cpp(of ui class)

  16. #11
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    Default Re: subclass the QLabel

    I still think that your approach is totally wrong! QLabel is design just to visualize text not for user interaction. What you are try to do in first place?

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  18. #12
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    Default Re: subclass the QLabel

    @MarekR22
    does it work?
    yes its working fine , what is wrong?
    eventfilter is good too. just i gave a quick reply.

    @ready
    as marek22 says, what you want to acheive , can u explain?
    Bala


    Added after 14 minutes:


    do I need to declare this class in mainwindow.cpp(of ui class)
    this is not class. this is eventfilter. put it in the mainwindow.cpp
    bala
    Last edited by BalaQT; 25th March 2011 at 09:36.

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  20. #13
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    Default Re: subclass the QLabel

    how to install eventfilter for getting mouse coordinate from Qlabel.........please provide me detail

  21. #14
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    Default Re: subclass the QLabel

    hi,
    how to install eventfilter for getting mouse coordinate from Qlabel.........please provide me detail
    Qt Code:
    1. bool MainWindow::eventFilter(QObject *pObject, QEvent *evt)
    2. {
    3. if(pEv->type()==QEvent::MouseButtonDblClick) /* mouse button is doublclicked*/
    4. {
    5. QLabel *lb=qobject_cast<QLabel *>(pObject);
    6. if(lb)
    7. {
    8. QMouseEvent *mEvt=static_cast<QMouseEvent *>(ect);
    9. qDebug()<<"dbl clicked"<<evt->x()<<","<<evt->y();
    10. }
    11. }
    12. }
    To copy to clipboard, switch view to plain text mode 

    and use installEventFilter(obj) to install this event filter
    ex:
    Qt Code:
    1. int main(int argc, char *argv[])
    2. {
    3. QApplication a(argc, argv);
    4. MainWindow w;
    5. w.show();
    6. a.installEventFilter(&w);
    7. return a.exec();
    8. }
    To copy to clipboard, switch view to plain text mode 
    hope it helps
    bala
    Last edited by BalaQT; 25th March 2011 at 10:34.

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  23. #15
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    Default Re: subclass the QLabel

    and don't forget
    Qt Code:
    1. label->installEventFilter(this);
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    edit: oh, maybe I should read posts until the end...
    edit2: ah, the information was added by edit

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  25. #16
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    Default Re: subclass the QLabel

    @felix:
    ha, u and me are posting at the same time,im at 16:4 u r at 16:3
    urz is better felix label->installEventFilter(this);

    bala

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  27. #17
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    Default Re: subclass the QLabel

    Qt Code:
    1. // to install main window event filter for label:
    2. MainWidow::MainWidow ....
    3. {
    4. .....
    5. ui->someLabel->installEventFilter(this);
    6. .....
    7. }
    8.  
    9. bool MainWidow::eventFilter(QObject *obj, QEvent *event)
    10. {
    11. if (event->type() == QEvent::MouseButtonDblClick && obj == ui->someLabel) {
    12. QMouseEvent* moseEvent = statc_cast<QMouseEvent*>(event);
    13. QPoint position = moseEvent->pos();
    14. // do something
    15. }
    16. return QMainWidow::eventFilter(obj, event);
    17. }
    To copy to clipboard, switch view to plain text mode 

    Still I'm sure this is bad solution for the real problem you hiding from us .

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  29. #18
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    Default Re: subclass the QLabel

    I'd rather use dynamic_cast
    Qt Code:
    1. QMouseEvent *mEvt=dynamic_cast<QMouseEvent *>(ect);
    To copy to clipboard, switch view to plain text mode 
    static_cast does not provide runtime checks for casts, so you can create faulty code like this one:
    Qt Code:
    1. #include <iostream>
    2. #include <vector>
    3. #include <string>
    4.  
    5. class A{
    6. public:
    7. A(){
    8. }
    9. virtual ~A(){
    10. }
    11. };
    12.  
    13. class B : public A{
    14. public:
    15. B(){
    16. // add some data to this object, note that there is no such data in objects of class A
    17. for( int x=0 ; x<1000 ; ++x ){
    18. v.push_back(x);
    19. }
    20. s = "a string data";
    21. }
    22. void thisMethodIsNotInA(){
    23. std::cout << "method from B: "<<s << v.size() << " " <<"\n";
    24. }
    25. std::string s;
    26. std::vector<int> v;
    27. };
    28.  
    29. int main(){
    30. A * a = new A(); // a is a pointer to A
    31. B * b = dynamic_cast<B*>(a); // this will return NULL, because a is not pointer to B
    32. if( b ){
    33. std::cout << "dynamic cast ok ?!\n"; // you should not see this line
    34. }
    35. B * b_static = static_cast<B*>(a);
    36. if( b_static ){
    37. std::cout << "watch out for static_cast !\n"; // probably you'll see this line ...
    38. b_static->thisMethodIsNotInA(); // ... but not this one ;)
    39. }
    40. }
    To copy to clipboard, switch view to plain text mode 
    dynamic_cast will check that a is not a B*, so b == NULL, while static_cast will return non-zero pointer and calling the method from B will result in a crash.
    I know that it should work in case of QEvents, but up- and down- class casting should be done with dynamic_cast IMHO.

    ---
    edit: tested with recent version of g++ btw.
    Last edited by stampede; 25th March 2011 at 11:04. Reason: typo and additional comment

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  31. #19
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    Default Re: subclass the QLabel

    Quote Originally Posted by stampede View Post
    I'd rather use dynamic_cast
    dynamic_cast will check that a is not a B*, so b == NULL, while static_cast will return non-zero pointer and calling the method from B will result in a crash.
    I know that it should work in case of QEvents, but up- and down- class casting should be done with dynamic_cast IMHO.

    ---
    edit: tested with recent version of g++ btw.
    In this case dynamic_cast is not needed overhead, "if(pEv->type()==QEvent::MouseButtonDblClick) " statement gives you warranty that ect is pointer to QMouseEvent.

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  33. #20
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    Default Re: subclass the QLabel

    In this case dynamic_cast is not needed overhead, "if(pEv->type()==QEvent::MouseButtonDblClick) " statement gives you warranty that ect is pointer to QMouseEvent
    I really doubt that you can feel this overhead in case of mouse press events ( probably depends on how fast you click:P ).
    Ok, probably I should write:
    up- and down- class casting should be done with dynamic_cast ...
    ... as long as it won't hurt performance

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