Qt connect “no such slot” when slot definitely does exist

Qt v4.8.0, VC2010 compiler

I have a QMainWindow based class and I'm trying to send it signals involving QUuid

However, every time I run it I get the errors:

Object::connect: No such slot MainWindow::on_comp_connected(QUuid) in ..\..\src\mainwindow.cpp:143
Object::connect:  (receiver name: 'MainWindow')

It's driving me potty as the slot definitely does exist (it's in the moc_)

class MainWindow : public QMainWindow
{
Q_OBJECT

// SNIP private typedefs

public:
    MainWindow(QWidget *parent = 0, Qt::WFlags flags = 0);
    ~MainWindow();
// SNIP public methods

signals:
   void testSendQuuid(const QUuid &qcid);

public slots:
   void on_comp_connected(const QUuid &qcid);

private:
// SNIP private parts

QOpenAcnController *acnInt;  // This is where the signal comes from

};

At the end of the MainWindow constructor (the line 143 mentioned) I have:

connect(acnInt, SIGNAL(callback_comp_connected(QUuid)),
        this, SLOT(on_comp_connected(QUuid)));

Given that the slot is definitely there in the moc_mainwindow.cpp (I checked, it's slot #1), what on earth could be stopping the connection happening?

If I try to connect the testSendQuuid(QUuid) signal to the slot, I get no such signal and no such slot as well.

I cannot for the life of me figure out why Qt is denying the existence of a slot that is most definitely there!

Check whether whether that moc_mainwindow.cpp is in your Build Path. Or you are using some other moc_window.cpp file. Because, for ex: In QtCreator, it build the source to a new build directory. And also it uses old moc_cpp file(s) if you try to open the source in a different location.

What I am trying to say is the moc file which you checked may contain those slot definition, but compiler might be using some other moc file which was created earlier.

I solved problem by adding Q_OBJECT macro in mainwindow class.

You need

connect(acnInt, SIGNAL(callback_comp_connected(QUuid)),         this, SLOT(on_comp_connected(const QUuid&)));

Pass by value shouldn't match pass a const reference.

But I tried it and it does. I created a minimal project with QtCreator 2.4.1 using Qt 4.7.4 on Windows. I added a single label to the main window and modified MainWindow.cpp as follows

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(
      this, SIGNAL(testSendQuuid(QUuid)),
      this, SLOT(on_comp_connected(QUuid))
    );
    QUuid x = QUuid::createUuid();
    emit testSendQuuid(x);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_comp_connected(const QUuid &qcid)
{
    ui->label->setText(qcid.toString());
}

And I get a uuid on my main window.

I also tried with the two QUuid arguments in the connect changed to const QUuid& and that worked too.

So your problem must be build related.

来源：https://stackoverflow.com/questions/10656510/qt-connect-no-such-slot-when-slot-definitely-does-exist

标签

signals-slots