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[solved] QTimer in console application

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  • Chris KawaC Offline
    Chris KawaC Offline
    Chris Kawa
    Lifetime Qt Champion
    wrote on last edited by
    #4

    Oh, I thought you meant runtime efficiency not lines of code count.

    bq. i can not use QTimer efficiently in a single main.cpp without additional classfiles

    Sure you can. This app for example waits for 5s, does stuff, and then quits:
    @
    #include <QApplication>
    #include <QTimer>

    int main(int argc, char *argv[]) {
    QApplication a(argc, argv);

    QTimer t;
    QObject::connect(&t, &QTimer::timeout, [&](){
        //do stuff
        a.exit();
    });
    t.start(5000);
    
    return a.exec&#40;&#41;;
    

    }
    @

    bq. Actually i’ m learning about QTimer and i think its a good class for playing around with QThread in next chapter.

    QTimer is more of a single threaded construct. The timeout is handled(usually) in the thread the timer lives in. You can handle it in a worker thread of course but there's little point to it. If you want to wait for some time in a worker thread it's better to just sleep.

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    • Chris KawaC Offline
      Chris KawaC Offline
      Chris Kawa
      Lifetime Qt Champion
      wrote on last edited by
      #5

      Actually starting with Qt 5.4 (yay, finally!) you can even shorten that to just:
      @
      #include <QApplication>
      #include <QTimer>

      int main(int argc, char *argv[]) {
      QApplication a(argc, argv);

      QTimer::singleShot(5000, [&](){
          //do stuff
          a.exit();
      });
      
      return a.exec&#40;&#41;;
      

      }@

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      • hskoglundH Online
        hskoglundH Online
        hskoglund
        wrote on last edited by
        #6

        Even shorter: (couldn't resist nagging):
        @
        ...
        QTimer::singleShot(5000, [&]{
        ...
        @

        You don't need any parenthesis when lambda has no arguments, "learned that":http://qt-project.org/forums/viewreply/216748/ from JKSH the other day :-)

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        • D Offline
          D Offline
          deleted28
          wrote on last edited by
          #7

          Hello Chris,

          that's what i was looking for, but i have still problems with it.

          I'm on :
          Qt Creator 3.3.0 (opensource)
          Based on Qt 5.4.0 (GCC 4.6.1, 32 bit)

          @//CK// #include <QApplication>
          #include <QCoreApplication>
          #include <QTimer>
          #include <QDebug>

          int main(int argc, char *argv[])
          {

          //CK// QApplication a(argc, argv);
          QCoreApplication a(argc, argv);

          QTimer::singleShot(5000, [&](){
              //do stuff
          
              qDebug() << "bingo";
              a.exit();
          });
          
          return a.exec&#40;&#41;;
          

          }
          @

          erro message:
          @.../main.cpp:25: error: no matching function for call to 'QTimer::singleShot(int, main(int, char**)::__lambda0)'
          });
          ^@

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          • Chris KawaC Offline
            Chris KawaC Offline
            Chris Kawa
            Lifetime Qt Champion
            wrote on last edited by
            #8

            @wally123: As I said that's a Qt 5.4 feature. Which Qt version are you using (not which version Qt Creator was build with) and which compiler? If you're not sure type QT_VERSION_STR in your code and press F2 to find out.

            For gcc you need to add @CONFIG += c++11@ in your .pro file to have lambdas (but the error mentions lambda so you probably have that already).

            @hskoglund: nice hint! on to the search/replace in my codebase... :)

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            • D Offline
              D Offline
              deleted28
              wrote on last edited by
              #9

              Chris,

              bq. not which version Qt Creator was build with

              sure , sorry
              @
              #define QT_VERSION_STR "5.4.0"@

              Options/ Buile&Run: Kits, QT-versions reporting also 5.4.0
              Yes gcc and adding to .pro file
              @CONFIG += c++11@

              made it work. Also thanks to hskoglund
              Even it's a short code, it contains some interesting new stuff for me
              to learn more about. especially this part:

              @ &{ .... };@

              thx wally

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              • D Offline
                D Offline
                deleted28
                wrote on last edited by
                #10

                The following is what i primarly intended to ask for :)

                @#include <QCoreApplication>
                #include <QTimer>
                #include <QDebug>

                void timer1_expired() {
                qDebug() << "timer1 expired";
                }

                void timer2_expired() {
                qDebug() << "timer2 expired";
                }

                int main(int argc, char *argv[])
                {
                QCoreApplication a(argc, argv);

                QTimer::singleShot(4000, timer1_expired);
                QTimer::singleShot(8000, timer2_expired);
                
                qDebug() << "main";
                
                return a.exec&#40;&#41;;
                

                }@

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                • JKSHJ Offline
                  JKSHJ Offline
                  JKSH
                  Moderators
                  wrote on last edited by
                  #11

                  [quote author="hskoglund" date="1423561353"]You don't need any parenthesis when lambda has no arguments, "learned that":http://qt-project.org/forums/viewreply/216748/ from JKSH the other day :-)[/quote]Pay it forward! ;-)

                  Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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                  • D Offline
                    D Offline
                    deleted28
                    wrote on last edited by
                    #12

                    not sure it makes sense to add to an already solved topic (?)

                    in above lecture the main reason i did not solve the problem myself
                    by trial & error was Chris' hint to use
                    @CONFIG += c++11@

                    in .pro file.
                    It's mentioned here "docs":http://doc.qt.io/qt-5/qmake-variable-reference.html#config
                    but this is not really an explanation.
                    Where can i find an explanation what this instruction really effects
                    and in which cases to use ?

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                    • JKSHJ Offline
                      JKSHJ Offline
                      JKSH
                      Moderators
                      wrote on last edited by
                      #13

                      [quote author="wally123" date="1423669903"]not sure it makes sense to add to an already solved topic (?)[/quote]That's perfectly ok if you are continuing to discuss the same topic.

                      [quote author="wally123" date="1423669903"]in above lecture the main reason i did not solve the problem myself
                      by trial & error was Chris' hint to use
                      @CONFIG += c++11@

                      in .pro file.
                      It's mentioned here "docs":http://doc.qt.io/qt-5/qmake-variable-reference.html#config
                      but this is not really an explanation.
                      Where can i find an explanation what this instruction really effects
                      and in which cases to use ?
                      [/quote]That instructions tells your compiler to enable "C++11":http://www.cprogramming.com/c++11/what-is-c++0x.html features.

                      The QTimer code that Chris posted uses a lambda expression, which was introduced in C++11; that's why you need that command.

                      Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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                      • D Offline
                        D Offline
                        deleted28
                        wrote on last edited by
                        #14

                        thx
                        needless to say which expression is now in center of interest :)
                        λ

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                        • JKSHJ Offline
                          JKSHJ Offline
                          JKSH
                          Moderators
                          wrote on last edited by
                          #15

                          [quote author="wally123" date="1423671202"]thx
                          needless to say which expression is now in center of interest :)
                          λ [/quote]You're welcome! It's a very nice toy indeed. Once you learn how to use it, I'm sure you'll be using it lots. ;)

                          Lambda expressions make it much easier to write event-driven code in Qt. If your slot is only used in one place, you can define it as a lambda right in the "connect" statement. No need to create a separate function in your class anymore. See "Making Connections to Lambda Expressions":http://doc.qt.io/qt-5/signalsandslots-syntaxes.html#making-connections-to-lambda-expressions.

                          Happy coding!

                          Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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                          • Chris KawaC Offline
                            Chris KawaC Offline
                            Chris Kawa
                            Lifetime Qt Champion
                            wrote on last edited by
                            #16

                            The doc pretty much says everything there is to say about it but I'll try to expand.

                            As you may or may not know C++ goes through revisions and different compilers have varying support for the latest standard.
                            In case of GCC the default is c++03 and to enable the latest c++11 you need a compiler param -std=c++11. To make this portable across compilers qmake exposes this as the above CONFIG switch and does the right thing for the given compiler to enable c++11 support.

                            Lambdas (or captures or anonymous functions as they are also known) are new part of c++11 standard.
                            "Here":https://gcc.gnu.org/projects/cxx0x.html is a comprehensive table listing all the new features of c++11 and which gcc version supports what.

                            Oh, the next revision of c++ has been already voted in last year so expect something like CONFIG+=c++14 available pretty soon.

                            As for which c++11/14 features are supported/used by Qt you can travel the net, e.g. "here":http://woboq.com/blog/cpp11-in-qt5.html or "here":http://woboq.com/blog/cpp14-in-qt.html

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                            • D Offline
                              D Offline
                              deleted28
                              wrote on last edited by
                              #17

                              This forum is really incredible efficient for learning !

                              btw.: does anybody pay you for your time and sharing knowhow ?

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                              • Chris KawaC Offline
                                Chris KawaC Offline
                                Chris Kawa
                                Lifetime Qt Champion
                                wrote on last edited by
                                #18

                                Don't know about others but I pretty much operate on a "beerware":http://en.wikipedia.org/wiki/Beerware based rules.

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                                • JKSHJ Offline
                                  JKSHJ Offline
                                  JKSH
                                  Moderators
                                  wrote on last edited by
                                  #19

                                  [quote author="wally123" date="1423672274"]This forum is really incredible efficient for learning ![/quote]It is. :) We hope that one day, when you're a Qt veteran, you'll come back and help newcomers too!

                                  [quote author="wally123" date="1423672274"]btw.: does anybody pay you for your time and sharing knowhow ?[/quote]I suppose I get paid in pats-on-the-back, satisfaction, reputation, and knowledge + experience.

                                  I often learn new things while doing research to answer a question. In turn, that helps me produce better code, which makes my customers happy, so they keep paying me.

                                  BTW, it's not limited to this forum; the hacker community is full of volunteers, e.g. "StackOverflow":http://stackoverflow.com/

                                  Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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                                  • D Offline
                                    D Offline
                                    deleted28
                                    wrote on last edited by
                                    #20

                                    :)

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