Lambda: How to pass object in ?

sonichy

If do not use global variable, how to pass ui->pushButton to function of lambda ?

ui->pushButton->setDown(true);
QTimer::singleShot(100, ui->pushButton, [=]{ ui->pushButton->setDown(false); });

jsulm

@sonichy said in Lambda: How to pass object in ?:

QTimer::singleShot(100, ui->pushButton, [=]{ ui->pushButton->setDown(false); });

You already capture all variables available in the same scope by value via [=]
If you only want to catch a class member then do:

QTimer::singleShot(100, ui->pushButton, [this]{ ui->pushButton->setDown(false); });

Please read https://en.cppreference.com/w/cpp/language/lambda as your question isn't related to Qt (better ask such questions in https://forum.qt.io/category/34/c-gurus)

sonichy

@jsulm I want to replace ui->pushButton in the { ... }, not the = in the [... ]
Something like

QTimer::singleShot(100, ui->pushButton, [=]{ sender()->setDown(false); });
QTimer::singleShot(100, ui->pushButton, SLOT(setDown(false));

jsulm

@sonichy said in Lambda: How to pass object in ?:

I want to replace ui->pushButton in the { ... }, not the = in the [... ]

What does "replace" mean here?
If you want to access class member inside the lambda (inside {}) you have to capture "this" in []. This is how lambdas work.
This should work just fine, did you try?

QTimer::singleShot(100, ui->pushButton, [this]{ ui->pushButton->setDown(false); });

This can't work because setDown() is not a slot:

QTimer::singleShot(100, ui->pushButton, SLOT(setDown(false));

sonichy

@jsulm I want to pass "ui->pushButton" through "this".

QTimer::singleShot(100, ui->pushButton, [this]{ this->setDown(false); });

jsulm

@sonichy "I want to pass "ui->pushButton" through "this"." - "ui" is part of "this", to access it you need "this", so you simply pass "this" to the lambda.
Why don't you simply try what I suggested?!

This is invalid code:

QTimer::singleShot(100, ui->pushButton, [this](){ this->setDown(false); });

setDown() is a method of a QPushButton instance. "this" is not a pointer to a QPushButton instance.
To access member variables and methods in a lambda you need to capture "this" as I already explained. So, this is the solution:

QTimer::singleShot(100, ui->pushButton, [this](){ ui->pushButton->setDown(false); });

VRonin

alternatively: QTimer::singleShot(100, ui->pushButton, std::bind(&QPushButton::setDown,ui->pushButton,false));. You don't need a lambda to just forward a call to another method