Is this a good way to call function with parameter in Qt signal and slot

Question

I have a QPushButton and when I click the button I will call a method that take two parameters, in this example: exampleMethod(int i, double d).

Now I connect the click event from QPushButton button to the exampleMethod like this:
connect(button, &QPushButton::clicked,this, &Example::exampleMethod);

But this doesn't work because the parameter of clicked() and exampleMethod(int, double) are not compatible.

Now I created a extra signal: exampleSignal(int i, double d) to connect to my slot:
connect(this, &Example::exampleSignal, this, &Example::exampleMethod);

And a additional slot with no parameters: exampleMethodToCallFromButtonClick() to call it from QPushButton clicked() in which I call the signal:

Example::Example(QWidget *parent) : QWidget(parent){

    button = new QPushButton("Click", this);

    connect(button, &QPushButton::clicked,this, &Example::exampleMethodToCallFromButtonClick);
    connect(this, &Example::exampleSignal, this, &Example::exampleMethod);
}

void Example::exampleMethod(int i, double d){
    qDebug() << "ExampleMethod: " << i << " / " << d;
}

void Example::exampleMethodToCallFromButtonClick(){
    emit exampleSignal(5,3.6);
}

This works fine.

1) Now my first question: Is this realy the best way to do this without lambda?

With lambda it looks even nicer and I don't need two connect statements:
connect(button, &QPushButton::clicked, [this]{exampleMethod(5, 3.6);});

2) My second question: with lamba is this the best way to do this or are there any better ways to solve it?

I also considered to save the parameters from exampleMethod as a member variable, call the method without parameter and get instead of the parameters the member variables but I think thats not a so good way.

Thanks for your help!

In exampleMethodToCallFromButtonClick(), no need to use signals to call exampleMethod. You can call the fuction directly without relying on SIGNAL/SLOT — Elcan
Where do 5 and 3.6 come from? If you get them from some calculation, you can do it in a slot without passing in as parameters — Jerry
Thanks, for your answers, your all right. @Jerry the 5 and 3.6 will come from a QTextEdit — Morchul
"With lambda it looks even nicer" is not a comment I see every day. :) But seriously, if you're using a button click to emit a signal based on some parameter values, just persist/store/serialize those values, which are presumably editable using some other widgets. Simple enough, yes? — Rethunk

Caleth Caleth · Accepted Answer · 2018-03-20T12:19:37

I wouldn't do either of those things. Receive the signal, gather your parameters and then just call exampleMethod. A lambda is more appropriate when the parameters are known at the point where you connect.

Example::Example(QWidget *parent) : QWidget(parent){

    button = new QPushButton("Click", this);

    connect(button, &QPushButton::clicked, this, &Example::onButtonClicked);
}

void Example::exampleMethod(int i, double d){
    qDebug() << "ExampleMethod: " << i << " / " << d;
}

void Example::onButtonClicked(){
    int i = ...;
    double d = ...;
    exampleMethod(i, d);
}

Is this a good way to call function with parameter in Qt signal and slot

2 Answers