Qt - Connected but doesn't work

Question

I have problem with one class in my project, after click appears new window with QTableWidget and QPushButton, after clicking the button I should have "test" on stdout, but nothing shows, here are parts of this code:

Header:

class ClientsSelector : public QWidget {
Q_OBJECT

public:
ClientsSelector(InvoiceTab* parent);
QWidget *window;

private:
QPushButton *accept;

public slots:
void loadData();

Constructor:

window = new QWidget();
layout = new QGridLayout();
layout->addWidget(table, 0, 0);

/*code*/

accept = new QPushButton(QString::fromUtf8("Load data"));
connect(accept, SIGNAL(clicked()), this, SLOT(loadData()));
layout->addWidget(accept, 0, 1);

/*code*/

window->setLayout(layout);

window->show();

Method:

void ClientsSelector::loadData() {

QTextStream std(stdout);
std << "test" << endl;

}

I have not even one warning nor error. I have nothing on stdout, it looks like button was connected to wrong object(?)

score 1 · Accepted Answer · answered Sep 13 '12 at 13:50

How do you instantiate ClientsSelector? Isn't it a singleton or global variable by chance? Try moving the connect call to a separate init function which is called after the ClientsSelector constructor. It helped me in similar WTF situations. It has something to do with the fact that each QObject inheritor has a static metadata member and you can't be sure about when it is fully initialized until the constructor finishes. connect won't work without that metadata.

See for example here: http://www.qtcentre.org/threads/9479-connect-in-constructor

If still lost, go through this checklist. Qt signals are so easy to use, everybody sometimes forgets it also has some requirements.

score 0 · Answer 2 · answered Sep 13 '12 at 10:35

0

Seems like you forgot a closing " on the test printout.

Try using

qDebug() << "test";

instead of QTextstream

answered Sep 13 '12 at 10:35

Kotte

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1

I made a typo here in the post, in my editor i have `"test"` Edit: Still nothing – agilob Sep 13 '12 at 10:36
1

The code seems correct to me. Are you sure that connect() line is run? Try adding a printout there as well and check that `accept` is not `NULL` or something like that. – Kotte Sep 13 '12 at 10:39
1

Does this run in a multithreaded program? There are some issues when running multiple threads. You sometimes need to call `exec()` in your thread to activate the event loop. – Kotte Sep 13 '12 at 10:49
Have you started the event loop in your `main()` `QCoreApplication app(argc, argv); app.exec();` – Kotte Sep 13 '12 at 11:11
As I wrote in the top post, this is a child windows which appears after clicking a button from parent window. I have this. – agilob Sep 13 '12 at 11:16

score 0 · Answer 3 · answered Sep 14 '12 at 05:33

You can do following to make sure that the connection was made and slot is called.

connect function returns status of connection. Check if the connection was made properly.
put a breakpoint and see if the loadData() is called when button is pressed.

This might help to find the cause.

Qt - Connected but doesn't work

3 Answers3