Slot & signal argument count difference - how to deal with?

[artwaw]

Good morning,
I have a rather general question related to the topic.

I have a slot function like this:
void MyClass::request(const QString &arg = QString()); where inside I make a choice based on arg.isEmpty().

Now when I try to make a connection like this:
connect(ui->edit,&QLineEdit::returnPressed,this,&MyClass::request);
compiler throws out an error (shortened version):

error: static assertion failed due to requirement 'int(FunctionPointer<void (QLineEdit::*)()>::ArgumentCount) >= int(FunctionPointer<void (MyClass::*)(const QString &)>::ArgumentCount)': The slot requires more arguments than the signal provides.
        static_assert(int(SignalType::ArgumentCount) >= int(SlotType::ArgumentCount),
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Usually I deal with it by wrapping the connect into lambda so it is not a stopping issue but I was just wondering - can this be done differently? The argument missing from the signal does not matter as it becomes a QString() anyway.

Many thanks in advance.

[J.Hilk]

@artwaw thats what QOverload is for:

https://doc.qt.io/qt-5/qtglobal.html#qOverload

[artwaw]

@J-Hilk Thank you. I was vaguely aware of its existence but never used. Will learn to use that.

[JonB]

@artwaw
I do not believe @J-Hilk's suggestion of qOverload() is for this case/works here (unless he corrects me/knows better). It is for distinguishing between multiple potential slot functions to connect to a signal where there is more than one overload for the slot. This is not your issue here.

connect(ui->edit, &QLineEdit::returnPressed,
        this, qOverload<const QString &>(&MyClass::request));

gives me the same error as you get now. All tested with Qt5.15 and g++ -c -std=gnu++1z (gcc 11.4).

I believe that you cannot directly call a slot method which has an extra argument over what the signal sends, even if the slot specifies a default for that argument to make it "optional". I believe you need to connect with a lambda to achieve that.

connect(ui->edit, &QLineEdit::returnPressed,
        this, [this]() { request(); });

Somewhat oddly, it seems that you can even connect to "extra default arg" if you do it in-line via a lambda:

connect(ui->edit, &QLineEdit::returnPressed,
        this, [this](const QString &arg = QString() ){});

is accepted. But if you try to put a method (with an optional argument) there it errors. One would have thought they would be equivalent, but apparently not. If a C++ expert would care to comment on this difference I should be interested :)

[J.Hilk]

@JonB you're right, this is with default arguments in the slot functions, didn't see that.

Thats tricky and there's only really 3 ways to do this:

class MyClass2 : public QObject {
    Q_OBJECT
public:
    MyClass2(QObject *parent = nullptr) : QObject(parent)
    {
        //1 Lambda
        connect(this, &MyClass2::mySignal, this, [this]() { request(); });
        
        //2other slot
        connect(this, &MyClass2::mySignal, this, &MyClass2::requestVanialla);
        
        //3 binds
        connect(this, &MyClass2::mySignal, std::bind(&MyClass2::request, this, QLatin1String()));
        
    }

signals:
    void mySignal();

public slots:
    void request(const QString & request = QLatin1String()) {
        //do Stuff here
        qDebug() << request;
    }
    
    void requestVanialla() {request();}
};

[artwaw]

Thank you, Gentlemen. I indeed ran into a method "ambiguous" error while trying with qOverload but since I've been using lambdas (as signposted "1 Lambda" in the post above) I decided to investigate that later on under the assumption I misunderstood something. Now it is clear I did not :)
At any rate, I learned something new from your answers today and am grateful.
Have a lovely day!