Connecting to statusBar()->showMessage gives compile time error in new syntax

sandro4912

I have an old example in qt4 were a class has this connection to statusBar() of QMainWindow:

    connect(&thread, SIGNAL(transactionStarted(const QString &)),
            statusBar(), SLOT(showMessage(const QString &)));

However when i change to the new compile time syntax in qt5 :

connect(&thread, &TransactionThread::transactionStarted ,
            statusBar(), &QStatusBar::showMessage);

It gives me this compile time error:

/home/sandro/Qt/5.13.0/gcc_64/include/QtCore/qobject.h:-1:
In instantiation of ‘static QMetaObject::Connection QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const typename QtPrivate::FunctionPointer<Func2>::Object*, Func2, Qt::ConnectionType) [with Func1 = void (TransactionThread::)(const QString&); Func2 = void (QStatusBar::)(const QString&, int); typename QtPrivate::FunctionPointer<Func>::Object = TransactionThread; typename QtPrivate::FunctionPointer<Func2>::Object = QStatusBar]’:

Is it because the slot of QStatusBar has an additional int parameter?

[slot] void QStatusBar::showMessage(const QString &message, int timeout = 0)

I thought the int gets ignored because it has a default parameter

Christian Ehrlicher

@sandro4912 said in Connecting to statusBar()->showMessage gives compile time error in new syntax:

I thought the int gets ignored because it has a default parameter

No, not in the new syntax - it's not possible there. You have to use a lambda

raven-worx

@sandro4912
https://doc.qt.io/qt-5/signalsandslots-syntaxes.html#using-default-parameters-in-slots-to-connect-to-signals-with-fewer-parameters

sandro4912

i also thought about the lambda but how to pass the string send from transactionStarted to statusBar()->showMessage().

connect(&thread, &TransactionThread::transactionStarted, [=]
    {
        statusBar()->showMessage();  // how to enter the text send with transactionStarted?
    });

JonB

@sandro4912
I'm throwing my hat in because I'm not C++, but don't you do that by specifying the lambda as:

connect(&thread, &TransactionThread::transactionStarted, [=](const QString &text)
    {
        statusBar()->showMessage(text);  // how to enter the text send with transactionStarted?
    });

ofmrew

@jonb

Here is a lambda I use to create a QString for statusBar:

    connect(ui->unitVector, &QPushButton::clicked, [=](){
                                                ui->canvas->go.setFlagAndResetAngle(4);
                                                QString s;
                                                s.append("Rotation about Unit Vector ");
                                                s.append(ui->canvas->go.uv.toString());
                                                ui->statusBar->showMessage(s, 0);});

JonB

@ofmrew
But @sandro4912 is asking

how to pass the string send from transactionStarted to statusBar()->showMessage()

hence my suggestion; how is yours to do with passing a parameter on to the lambda??

J.Hilk

@jonb said in Connecting to statusBar()->showMessage gives compile time error in new syntax:

@sandro4912
I'm throwing my hat in because I'm not C++, but don't you do that by specifying the lambda as:
connect(&thread, &TransactionThread::transactionStarted, [=](const QString &text)
    {
        statusBar()->showMessage(text);  // how to enter the text send with transactionStarted?
    });

absolutely correct!👍
just one caveat, you can, but you shall not omit the return type

connect(&thread, &TransactionThread::transactionStarted, [=](const QString &text)->void{});

JonB

@j-hilk
So now C++ has found another use for ->! And I assume the [=] is an array holding this as the context or something, is that a special meaning for the =?

I realize this isn't the place for long discussion, but ISTM with these lambdas C++ is using a lot of cryptic symbols all together!

sandro4912

@jonb said in Connecting to statusBar()->showMessage gives compile time error in new syntax:

[=]

[=] means that all the variables / objects are also available in the lamda body for usage. you can also specify there just a single variable from the outside.

for more see this: https://de.cppreference.com/w/cpp/language/lambda

the -> you can also use it for normal functions now hence its not that common.

for example int somefunction(); becomes auto someFunction() -> int

Anyway guys thanks for the solution

J.Hilk

@jonb
well, i‘m not sure what exactly the compiler does, but I assume you‘re on the right path:

[&epsilon] capture by reference
[&] captures all variables used in the lambda by reference
[=] captures all variables used in the lambda by value
[&, epsilon] captures variables like with [&], but epsilon by value
[=, &epsilon] captures variables like with [=], but epsilon by reference

which makes this lambda actually dangerous as the statusbar could be deleted at any time, qt offers a solution for this:

connect(&thread, &TransactionThread::transactionStarted, statusbar, [=](const QString &text)->void{});

the connect get becomes resolved, when statusbar gets deleted

but from the naming of the variables I would say that the emitter is actually in a different thread compared to statusbar.

I‘m not sure how Qt handles this with a Lambda, probably not good, therefore 5th connect paramater is probably a good idea

connect(&thread, &TransactionThread::transactionStarted, statusbar, [=](const QString &text)->void{},Qt::QueuedConnection);

ofmrew

@j-hilk
I knew that I had seen this before, try link text

sandro4912

i tryed to add a fifth parameter like you said:

    connect(&thread, &TransactionThread::transactionStarted, 
            [=](const QString &text)->void
    {
        statusBar()->showMessage(text);  
    }, Qt::QueuedConnection);

Unfortunatey the syntax is not working with lambda.

And yes you are right. In the MainWindow were the statusBar is there is thread launced for taskes to be computed in a second thread.

JonB

@sandro4912

i tryed to add a fifth parameter like you said:

Count your parameters, you only have 4!
Look at @J-Hilk 's code, see what you've missed....

ofmrew

@jonb

Note that the st in [this](const QString& st) is the name of the string that is the argument of the signal, which I use in {this->ui->statusBar->showMessage(st, 0);}

ofmrew

@ofmrew
Try this example: ```
void canvasResized(QString s);
QString st = "It opened";
emit canvasResized(st);
connect(ui->canvas, &MyCanvas::canvasResized, [this](const QString& st){this->ui->statusBar->showMessage(st, 0);});

I added the emit to the resizeEvent and it worked.