Using connect on a custom QSpinBox

Dummie1138

Hi. I have the following code, meant to create a custom spin box widget that will round the input value to the nearest multiple of 3 every time the value is changed.

Header:

#ifndef SPINBOXTHREE_H
#define SPINBOXTHREE_H

#include <QObject>
#include <QSpinBox>

class spinBoxThree : public QSpinBox
{
    Q_OBJECT
public:
    explicit spinBoxThree(QWidget *parent = nullptr);

private slots:
    int setThree();

};

#endif // SPINBOXTHREE_H

cpp:

#include "spinboxthree.h"

spinBoxThree::spinBoxThree(QWidget *parent) : QSpinBox(parent){
    connect(this, &QSpinBox::valueChanged, this, [=](){setThree();});
}

int spinBoxThree::setThree(){
    int input = value();
    if (input % 3 != 0){
        input = input - (input % 3);
    }
    setValue(input);
    return input;
}

I am receiving a "no matching member function" error for the line connect(this, &QSpinBox::valueChanged, this, [=](){setThree();}); . I am aware that there are issues with using connect on objects not inheriting QObject, but as I need this object to be a QSpinBox, I am wondering whether there are methods I can use to connect the signals and slots within a non-QObject-defined custom class. Please let me know if more information is required.

jsulm

@Dummie1138 said in Using connect on a custom QSpinBox:

<unresolved overloaded function type>

That explains the issue: this signal is overloaded, so you will have to do what is shown in documentation: https://doc.qt.io/qt-5/qspinbox.html#valueChanged

jsulm

@Dummie1138 said in Using connect on a custom QSpinBox:

I am aware that there are issues with using connect on objects not inheriting QObject

Not sure what you mean. spinBoxThree is a QSpinBox, which is a QWidget, which is a QObject.
Please post the whole error message.

Dummie1138

@jsulm Thank you for your patience. Here is the error message.

Where I got the understanding of QObject errors from: https://stackoverflow.com/questions/24410130/no-matching-function-for-qobjectconnect

jsulm

@Dummie1138 It seems to come from the code model.
Does the actual build succeed?
You can also put QObject:: in front of connect().

Dummie1138

@jsulm The build does not succeed. The error messages are:
spinboxthree.cpp:7: error: no matching member function for call to 'connect'
spinboxthree.cpp:7: error: no matching function for call to 'spinBoxThree::connect(spinBoxThree*, <unresolved overloaded function type>, spinBoxThree*, spinBoxThree::spinBoxThree(QWidget*)::<lambda()>)'

spinboxthree.cpp: In constructor 'spinBoxThree::spinBoxThree(QWidget*)':
spinboxthree.cpp:7:77: error: no matching function for call to 'spinBoxThree::connect(spinBoxThree*, <unresolved overloaded function type>, spinBoxThree*, spinBoxThree::spinBoxThree(QWidget*)::<lambda()>)'
connect(this, &QSpinBox::valueChanged, this, ={setThree();});
^

Placing QObject:: in front of connect, like so: QObject::connect(this, &QSpinBox::valueChanged, this, [=](){setThree();}); resulted in the same error.

jsulm

@Dummie1138 said in Using connect on a custom QSpinBox:

<unresolved overloaded function type>

That explains the issue: this signal is overloaded, so you will have to do what is shown in documentation: https://doc.qt.io/qt-5/qspinbox.html#valueChanged

Dummie1138

@jsulm Thank you very much. I have previously neglected the detailed error messages for being messy, today is a good day to learn that they actually contain useful information.