QT C++ - Signals and Slots: “No such slot QLabel…” even the SLOT function is existing in my class

Question

So I am trying to learn Qt Framework with C++. I am in the middle of understanding signals and slots but I am having a hard time creating a customized slot. So as I follow some tutorials, my program ended up with the following error:

QObject::connect: No such slot QLabel::updateMessage() in main.cpp:28

Where updateMessage() is declared as a public slot inside my Test class

The following are some snippets from my code:

Test.h

class Test : public QObject
{
    Q_OBJECT
public:
    Test(void);
    ~Test(void);
    void setMessage(char *tMsg);
    char* getMessage();

    QWidget *window;
    QGridLayout *layout;
    QLabel *lblMsg;
    QPushButton *btnShow;


public slots:
    void updateMessage();

private:
    char msg[80];
    QString str;

};

main.cpp

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Test t;
    t.window->setWindowTitle("Testing Qt");
    t.window->setLayout(t.layout);
    t.window->show();
    return app.exec();
}

Test::Test(void)
{
    window = new QWidget;
    lblMsg = new QLabel;
    btnShow = new QPushButton("Show message");

    connect(btnShow,SIGNAL(clicked()),lblMsg,SLOT(updateMessage()));
    layout = new QGridLayout;
    layout->addWidget(lblMsg);
    layout->addWidget(btnShow);

    char str1[] = "Hello, Qt World!";
    setMessage(str1);
}


Test::~Test(void)
{

}

void Test::setMessage(char *tMsg)
{
    memcpy(msg, tMsg, sizeof(msg));
}

char* Test::getMessage()
{
    return msg;
}

void Test::updateMessage()
{
    string strMsg(getMessage());
    QString qstr = QString::fromStdString(strMsg);
    lblMsg->setText(qstr);
    delete msg;
}

HelloQtWorld.pro

######################################################################
# Automatically generated by qmake (3.0) Tue Nov 15 00:30:22 2016
######################################################################

TEMPLATE = app
TARGET = HelloQtWorld
INCLUDEPATH += .

# Input
HEADERS += stdafx.h Test.h
SOURCES += anotherClass.cpp \
           main.cpp \
           stdafx.cpp \
           GeneratedFiles/qrc_helloqtworld.cpp
RESOURCES += helloqtworld.qrc

Can also someone try to explain for me how Signals and Slots work? Thank you in advance. :)

Answer 1

The problem is in this line:

connect(btnShow,SIGNAL(clicked()),lblMsg,SLOT(updateMessage()));

You are connecting with slot of QLabel instead of slot of your class.

This should be changed to

connect(btnShow,SIGNAL(clicked()),this,SLOT(updateMessage()));

Pay attention at new Qt 5 syntax for signals and slots.

Answer 2

Your label of type QLabel does not have a slot named updateMessage() . Your class Test has.

Change the connect to:

connect(btnShow,SIGNAL(clicked()),this,SLOT(updateMessage()));

This will connect the button's clicked signal to your Test object s updateMessage` slot.

Answer 3

Your slot is declared at Test class, not QLabel class, but you are trying to connect to the slot as if it was declared at the QLabel :

connect(btnShow,SIGNAL(clicked()),lblMsg,SLOT(updateMessage()));

You should connecto to this , instead of lblMsg :

connect(btnShow,SIGNAL(clicked()),this,SLOT(updateMessage()));