can't connect overloaded method to a lambda function

duburlan

I can't compile the following code

class Foo : public QWidget
{
    Q_OBJECT
public:
    Foo()
    {
        QSpinBox *spinBox = new QSpinBox(this);
        connect(spinBox, &QSpinBox::valueChanged, [this](int val){/*do smth*/});
    }
};

The error is

error: no matching function for call to 'Foo::connect(QSpinBox*&, <unresolved overloaded function type>, Foo::Foo()::<lambda(int)>)'
         connect(spinBox, &QSpinBox::valueChanged, [this](int val){/*do smth*/});
                                                                              ^

I guess the problem is "unresolved overloaded function type", because actually we have two overloaded methods in class QSpinBox:

    void valueChanged(int);
    void valueChanged(const QString &);

Any ideas how to help compiler to resolve the right method?

My specs:
GCC 4.9.2
Qt 5.5.1

mrjj

hi
Try

 connect(spinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged), this, [this]( int val){/*do smth*/});

https://wiki.qt.io/New_Signal_Slot_Syntax

...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:
connect(mySpinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged), mySlider, &QSlider::setValue);

As you seem to think also.

duburlan

Thanks, that works. Though, it doesn't look elegant...

P.S. Such a syntax also works

connect(spinBox, (void(QSpinBox::*)(int))&QSpinBox::valueChanged, this, [this]( int val){/*do smth*/});

mrjj

@duburlan
I agree.
Its not pretty.

JKSH

If you don't mind writing more lines, you can make each line shorter:

void (QSpinBox::*mySignal)(int) = &QSpinBox::valueChanged;

connect(spinBox, mySignal, this, [this](int val) {
    /*do smth*/
});

See http://doc.qt.io/qt-5/signalsandslots-syntaxes.html

mrjj

far more readable :)