Using a lambda as a slot problem

[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; });


	}
}

[KroMignon]

@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; });

[J.Hilk]

@Giuseppe97
well it's missing some parts, most importantly the parameter list

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

[Giuseppe97]

@J-Hilk
Thanks . I suppose I have to study better the syntax of lambdas.

[guerinoni]

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

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

[Giuseppe97]

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

[guerinoni]

@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 [&]

[Giuseppe97]

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

[jsulm]

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

[Giuseppe97]

@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.

[KroMignon]

@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

[Giuseppe97]

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

[KroMignon]

@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; });

[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.

[KroMignon]

@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; });

[Giuseppe97]

@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.