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Using a lambda as a slot problem

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  • guerinoniG Offline
    guerinoniG Offline
    guerinoni
    wrote on last edited by
    #4

    @Giuseppe97
    maybe in this way you can avoid to pass this

    QTimer::singleShot(200, this, [&pressed](){ pressed = false; });
    
    1 Reply Last reply
    1
    • G Offline
      G Offline
      Giuseppe97
      wrote on last edited by
      #5

      @guerinoni
      I have tried but compiler tells that in capture list SwitchBlock::pressed is not a variable.

      guerinoniG 1 Reply Last reply
      0
      • G Giuseppe97

        @guerinoni
        I have tried but compiler tells that in capture list SwitchBlock::pressed is not a variable.

        guerinoniG Offline
        guerinoniG Offline
        guerinoni
        wrote on last edited by
        #6

        @Giuseppe97 said in Using a lambda as a slot problem:

        @guerinoni
        I have tried but compiler tells that in capture list SwitchBlock::pressed is not a variable.

        ok then just pass all by reference [&]

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        • G Offline
          G Offline
          Giuseppe97
          wrote on last edited by
          #7

          @guerinoni
          Capturing all by reference cause the initial error QTimer::singleShot': none of the 6 overloads could convert all the argument types

          jsulmJ 1 Reply Last reply
          0
          • G Giuseppe97

            @guerinoni
            Capturing all by reference cause the initial error QTimer::singleShot': none of the 6 overloads could convert all the argument types

            jsulmJ Offline
            jsulmJ Offline
            jsulm
            Lifetime Qt Champion
            wrote on last edited by
            #8

            @Giuseppe97 Did you try what @J-Hilk suggested?

            https://forum.qt.io/topic/113070/qt-code-of-conduct

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            • G Offline
              G Offline
              Giuseppe97
              wrote on last edited by Giuseppe97
              #9

              @jsulm
              Yes but it doesn't compile. I have marked the question as solved because for a moment the compiler didn't reject the lambda but when i rebuild the solution the problem was still there. I am also trying to figure out what's the problem by myself through a deep study of lambda syntax.

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              • G Giuseppe97

                I'm using last version of VS2019 and when I use a lambda as a slot in QTimer::singleShot() compiler give me follow error

                no instance of overloaded function matches the argument list argument types are: (int, SwitchBlock , lambda []void ()->void)

                although I made a similiar connection successfully in another class.

                Here is the class

                #pragma once
                
                #include "Inert.h"
                
                class SwitchBlock : public Inert
                {
                	private:
                
                		static bool pressed;
                		int animation_counter;
                
                		//textures
                		QPixmap texture_spawning[11];
                		QPixmap texture_active[3];
                		QPixmap texture_smashed;
                		
                		bool spawning;
                		bool active;
                
                	public:
                
                		SwitchBlock(QPoint position);
                
                		static bool IsPressed() { return pressed; }
                		virtual std::string name() { return "SwitchBlock"; }
                		virtual void animate();
                		virtual void hit(Object* what, Direction fromDir);
                };
                

                This is the method in which I tried to use the lambda as a slot

                
                void SwitchBlock::hit(Object* what, Direction fromDir)
                {
                	Mario* mario = dynamic_cast<Mario*>(what);
                	if (mario && fromDir == UP)
                	{
                		active = false;
                		collidable = false;
                		Game::instance()->stopMusic();
                		pressed = true;
                		Sounds::instance()->play("switch-hit");
                		QTimer::singleShot(200, this, [this] { pressed = false; });
                
                
                	}
                }
                
                KroMignonK Offline
                KroMignonK Offline
                KroMignon
                wrote on last edited by KroMignon
                #10

                @Giuseppe97 said in Using a lambda as a slot problem:

                QTimer::singleShot(200, this, [this] { pressed = false; });

                You have to add () to define the lambda function ==> QTimer::singleShot(200, this, [this]() { pressed = false; });

                Take a look at https://blog.feabhas.com/2014/03/demystifying-c-lambdas/ to get more details about Lambdas

                It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

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                • G Offline
                  G Offline
                  Giuseppe97
                  wrote on last edited by Giuseppe97
                  #11

                  @KroMignon
                  Thanks for the link but it doesn't compile.
                  UUUpng.png

                  KroMignonK 1 Reply Last reply
                  0
                  • G Giuseppe97

                    @KroMignon
                    Thanks for the link but it doesn't compile.
                    UUUpng.png

                    KroMignonK Offline
                    KroMignonK Offline
                    KroMignon
                    wrote on last edited by KroMignon
                    #12

                    @Giuseppe97 QTimer::singleShot() needs a QObject instance as receiver to work.
                    As far as I can see the is not the case with SwichtBlock?

                    But you could use what, but the lambda will be executed in the thread of this instance:

                    QTimer::singleShot(200, what, [this]() { pressed = false; });
                    

                    It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                    1 Reply Last reply
                    1
                    • G Offline
                      G Offline
                      Giuseppe97
                      wrote on last edited by Giuseppe97
                      #13

                      @KroMignon
                      Thanks now it compile. Yes the problem was that SwitchBlock inherits by Inert which inherits by Object class which is a QGraphicsPixmapItem so the use of what doesn't resolve the problem . I could have used multiple inheritance so that SwitchBlock inherits both from QGraphicsPixmapItem and Qobject but I have choosen to move the call of singleShot static method to Game which inherits by QGraphicsView.

                      KroMignonK 1 Reply Last reply
                      0
                      • G Giuseppe97

                        @KroMignon
                        Thanks now it compile. Yes the problem was that SwitchBlock inherits by Inert which inherits by Object class which is a QGraphicsPixmapItem so the use of what doesn't resolve the problem . I could have used multiple inheritance so that SwitchBlock inherits both from QGraphicsPixmapItem and Qobject but I have choosen to move the call of singleShot static method to Game which inherits by QGraphicsView.

                        KroMignonK Offline
                        KroMignonK Offline
                        KroMignon
                        wrote on last edited by KroMignon
                        #14

                        @Giuseppe97 Oh sorry, I read you code too fast and I read void SwitchBlock::hit(QObject * what,...) instead of void SwitchBlock::hit(Object * what,...)!

                        But I this case, you could also use qApp as receiver, so lambda will run in main thread:

                        Timer::singleShot(200, qApp, [this]() { pressed = false; });
                        

                        It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

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                        • G Offline
                          G Offline
                          Giuseppe97
                          wrote on last edited by
                          #15

                          @KroMignon
                          Ok thanks I will do this way since for the application (a level of Super Mario Bros 3 (NES)) I didn't use multiple threads . By the way this is my first "serious" project (for a university exam) and I am amazingly surprised of how many things I have learned since I have started.

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