How to fix QObject::connect: No such slot.. when connecting a sender to a slot both in the same class

Question

I am writing a programm, that creates a QLineEdit that only accepts digits and should, when it rejects an input that is not a digit, turn the background to some abitrary color. If the input is accpeted it will turn the background white again. Now I need to connect the inputRejected and textEdited events of the QLineEdit to randomcolor() and white() respectively, but the connect is making me trouble and i do not know how to fix it.

This is the first time I'm working with the connect and i slugged through many forums alreay trying out the different Syntax i found there.

#include <QtWidgets>

class OnlyNumbers : QLineEdit {
   public:
        static int spawn(int argc, char *argv[]){
            QApplication app(argc, argv);
            OnlyNumbers P;
            return app.exec();
        }
        OnlyNumbers() : QLineEdit() {
            this->setValidator(new QIntValidator());
            QObject::connect(this, SIGNAL(inputRejected()), this, SLOT(randomcolor()));
            QObject::connect(this, SIGNAL(&QLineEdit::textEdited(const QString)), this, SLOT(&OnlyNumbers::white()));
            QRegExp rx("[0-9]*");            QValidator *validator = new QRegExpValidator(rx, this);
            this->setValidator(validator);
            this->show();

        }
    public slots:
        void randomcolor(){
            this->setStyleSheet("QLineEdit { background: rgb(std::rand()%256, rand()%256, rand()%256); selection-background-color: rgb(rand()%256, rand()%256, rand()%256); }");
        }
        void white(){
            this->setStyleSheet("QLineEdit { background: rgb(255, 255, 255); selection-background-color: rgb(233, 99, 0); }");
        }
};

int main(int argc, char *argv[])
{
    return OnlyNumbers::spawn(argc, argv);
}

QObject::connect: No such slot QLineEdit::randomcolor()

QObject::connect: No such signal QLineEdit::&QLineEdit::textEdited(const QString)

These are the errors i get, and i don't know what to do with them, because for em the two are existant. Sadly I cannot describe my problem better, because i don't know more.

SOLVED: The Problem was, that i didn't seperate calls definition and declaration in onlynumbers.h and onlynumbers.cpp. Also i cannot put std::rand()%256 in the string i need to split the string and concatonate it with all the numbers converted to a qstring. :D Thanks for the help. You gave me the motivation to keep googeling.

Answer 1

You forgot Q_OBJECT for your class. For example:

class OnlyNumbers : QLineEdit {
    Q_OBJECT    
    ...

See the documentation on QObject:

Notice that the Q_OBJECT macro is mandatory for any object that implements signals, slots or properties.

Without the macro the meta compiler simply will not generate the information needed for the slots/signals declared in your class.

Also, after Q_OBJECT has been added you should rerun qmake on your project since qmake actually generates the calls to moc in the makefile. This is explained by the documentation of the Meta-Object Compiler (moc):

Whenever qmake is run, it parses the project's header files and generates make rules to invoke moc for those files that contain a Q_OBJECT macro.

Also as mentioned in the comments - the SIGNAL/SLOT macros are the old string based implementation, and there are many benefits to switch to the new compile time checked connect - see New Signal Slot Syntax and Differences between String-Based and Functor-Based Connections.