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

[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.