Connect doesn't work with lambda function

[Aaron Kim]

MenuItem::MenuItem(QMainWindow *mainWindow, const QString &text, const int &num){
    this->setText(text);
    this->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    this->setMaximumHeight(40);
    /*
    QSignalMapper* signalMapper = new QSignalMapper(this);
    connect(this, SIGNAL(clicked()), signalMapper, SLOT(map()));
    signalMapper->setMapping(this, num);
    connect(signalMapper, SIGNAL(mapped(int)), mainWindow, SLOT(changeContent(int)));
    */
    auto lambda = [=](){
        static_cast<MainWindow*>(mainWindow)->changeContent(num);
    };
    connect(this, SIGNAL(clicked()), mainWindow, lambda);
}

To pass a parameter to the slot, I tried using QSignalMapper but it seems that it has been deprecated. So, I tried lambda function but an error occurs.

C:\Users\Coder\Desktop\qt\Calculator\template\sidemenu\menuItem.cpp:16: error: no matching function for call to 'MenuItem::connect(MenuItem*, const char*, QMainWindow*&, MenuItem::MenuItem(QMainWindow*, const QString&, const int&)::<lambda()>&)'
     connect(this, SIGNAL(clicked()), mainWindow, lambda);

Does it have something to do with the lambda function itself?

[raven-worx]

@Aaron-Kim
whats the type of MenuItem?
If it doesn't derive from QObject, you need to call QObject::connect(...) instead

[Christian Ehrlicher]

You can't mix old and new-style connect syntax ... --> connect(this, &MenuItem:clicked, ...)

[J.Hilk]

@Aaron-Kim said in Connect doesn't work with lambda function:

auto lambda = ={
static_cast<MainWindow*>(mainWindow)->changeContent(num);
};
connect(this, SIGNAL(clicked()), mainWindow, lambda);

dont mix Qt4 and Qt5 Signal/Slot syntax, also, Lambda functions only work with the new syntax.

    connect(this, &MenuItem::clicked, mainWindow,  [=](){
        static_cast<MainWindow*>(mainWindow)->changeContent(num);
    });

[Aaron Kim]

@J.Hilk Thanks! I didn't know that SIGNAL() is an old style.
Should I change all the SINGAL() syntaxes to the new one?

[aha_1980]

@Aaron-Kim said in [Solved] Connect doesn't work with lambda function:

Should I change all the SINGAL() syntaxes to the new one?

It is recommended to do so, indeed.

One note: to solve the forum topics as SOLVED, please use the button topic tools below your first post, do not write [Solved] yourself.

Thanks!

[J.Hilk]

@Aaron-Kim said in [Solved] Connect doesn't work with lambda function:

Should I change all the SINGAL() syntaxes to the new one?

Well, thats up to you.

Take a look at this wiki article and decide for yourself how to proceed further :)
https://wiki.qt.io/New_Signal_Slot_Syntax

[mrjj]

Hi
Just as a note.
The new syntax will catch error at compile time and hence
is preferred over the SIGNAL/SLOT ones as they will compile even
the connect wont work.
Also, there is a slight performance gain runtime with the new syntax.
The downside is super ugly syntax with overloaded functions so make sure to
check out qOverload

[Aaron Kim]

@J.Hilk One more thing, Is there any reason for me to use &QPushButton::clicked, rather than just QPushButton::clicked?
As far as I know, the signs & and * have no effect on function pointers. And I checked that QPushButton::clicked also works.

[raven-worx]

@Aaron-Kim said in Connect doesn't work with lambda function:

As far as I know, the signs & and * have no effect on function pointers. And I checked that QPushButton::clicked also works.

definitely thats not true.
The method signature expects a function-pointer, hence the referencing with &

[Aaron Kim]

@raven-worx
https://stackoverflow.com/questions/6893285/why-do-function-pointer-definitions-work-with-any-number-of-ampersands-or-as
But this articles says that a function can be implicitly converted to a function pointer. Does the implementation of connect() require explicit function-pointer syntax with an ampersand?

[raven-worx]

@Aaron-Kim
my opinion: just stay on the safe side (by using the & function reference) and don't rely on any implicit compiler features. Also its clear what the code does on the first glance.

[mrjj]

@Aaron-Kim
Hi
They talk about pointer to a (c) function but with the
connect we are talking pointer to a member function.
Also with no &
connect(ui->pushButton, QPushButton::released, this, { qDebug() << "test";} );
looks like a call to a static member function for compiler
so & is need to mean take address of this member.

[SGaist]

Hi,

@J.Hilk said in Connect doesn't work with lambda function:

static_cast<MainWindow*>(mainWindow)->changeContent(num);

Also you should rather use qobject_cast when dealing with QObject derived classes.

[J.Hilk]

@SGaist
very true!
My bad though, was copy and pasting from the op 🙈