help on signals and slots

russjohn834

Hello,

I'm new to QT and signals and slots mechanism, correct me where I'm wrong?

I have a label in a form, the text need to be changed upon clicking a button , I trying to implement using a signals and slots.
My signal will be emitted as follows:

void MainOne::on_pushButton_help_clicked()
{
    emit textToChange("text updated");
}

I then connected it as follows:

connect(this, SIGNAL(textTochange(QString)), this->ui->label_pid, SLOT(setText(QString)));

But this is not working, any help?

jsulm

@russjohn834 Did you verify that on_pushButton_help_clicked() was executed?
Do you see any warnings in the application output when executing your app?

russjohn834

@jsulm Thank you. Yes , on_pushButton_help_clicked() is executed. No warnings related to this.

J.Hilk

@russjohn834 said in help on signals and slots:

connect(this, SIGNAL(textTochange(QString)), this->ui->label_pid, SLOT(setText(QString)));

is this copy and pasted ? because that is not the signal you emit

textTochange != textToChange

KroMignon

@russjohn834 said in help on signals and slots:

Yes , on_pushButton_help_clicked() is executed. No warnings related to this.

But is the connect() statement executed without warning?

auto cnxState = connect(this, SIGNAL(textTochange(QString)), this->ui->label_pid, SLOT(setText(QString)));

if(!cnxState)
    qDebug() << "Failed to connect!!!";

russjohn834

@J-Hilk Thank you that was the mistake!! But qt did not warn me that it is an undefined signal!! why!?

J.Hilk

@russjohn834 as you're using the old (Qt4) syntax, you only get a run time warning, not a compile time one

jsulm

@russjohn834 said in help on signals and slots:

But qt did not warn me that it is an undefined signal!! why!?

With this old connect syntax you will not get any warning/error when compiling, only when running your app - that's why I asked you whether you see anything in the application output (in the console). I'm sure there was something.

It is better to use new connect syntax: https://wiki.qt.io/New_Signal_Slot_Syntax

russjohn834

Thanks a lot for the feedback guys!
can someone show me how can I switch this to new connect syntax?

connect(this, SIGNAL(textToChange(QString)), this->ui->label_pid, SLOT(setText(QString)));

J.Hilk

@russjohn834
sure, but all information needed should actually be inside the wiki page @jsulm linked you

connect(this, &MainOne::textToChange, ui->label_pid, &QLabel::setText);