How to get the sender() in a C++ lambda signal slot handler?

philk

When I call signal() from within a lambda function its always null.

connect(senderObject, &Sender::signal, this, [=]() {
  QObject* sender = sender(); // always null?
});

sierdzio

Try with:

connect(senderObject, &Sender::signal, this, [&]() {
  QObject* sender = sender();
});

philk

This does not work, sender returns nullptr.

dvolosnykh

I have experienced the same problem several times.

AndreasF

I know, it's an old thread, but for information:

		QLineEdit* senderObject = new QLineEdit(this);
		connect(senderObject, &QLineEdit::returnPressed, [senderObject, this]() 
		{ 
			if (senderObject && !senderObject->text().simplified().isEmpty())
			{ 
				QString txt = senderObject->text().simplified();
				Icons()[txt];
				senderObject->close();
				iconListModel_->clearModelData(true);
				iconListModel_->setModelData();
			}
		});

aha_1980

Hi all,

I just encountered the same problem and the correct solution is this: https://stackoverflow.com/questions/19719397/qt-slots-and-c11-lambda

In short: don't use sender() within the lambda, capture senderObject instead:

connect(senderObject, &Sender::signal, [senderObject, this]() {
  // use senderObject here
});

Works like a charm.

mrjj

@aha_1980
But what do you do when hook up multiple objects to same lambda slot ?
Capture them all ?

aha_1980

@mrjj

From my recent (and limited) experience, I'd say you better use the classical "member function" slot here, otherwise your code will get ugly.

VRonin

@mrjj said in How to get the sender() in a C++ lambda signal slot handler?:

But what do you do when hook up multiple objects to same lambda slot ?

if they are all pointers just use [=]. The compiler will take care to capture all and nothing more than what you need.

aha_1980

@VRonin said in How to get the sender() in a C++ lambda signal slot handler?:

if they are all pointers just use [=]. The compiler will take care to capture all and nothing more than what you need

Yeah, but you still need to find out which one emitted the signal inside the lambda...

VRonin

Alternatively use std::bind

const auto myLambda = [](QObject* sender, bool pressed)->void{
QPushButton* const senderButton= qobject_cast<QPushButton*>(sender);
Q_ASSERT(senderButton);
qDebug() << senderButton->text() << pressed;
};
QObject::connect(button1,&QPushButton::clicked,std::bind(myLambda,button1,std::placeholders::_1));
QObject::connect(button2,&QPushButton::clicked,std::bind(myLambda,button2,std::placeholders::_1));

aha_1980

@VRonin Cool! Will need to try this but looks promising.