I have a buttongroup defined with two radiobuttons
buttonGroupFFTDimension = new QButtonGroup(this);
buttonGroupFFTDimension->addButton(ui->radioButton1D, 1);
buttonGroupFFTDimension->addButton(ui->radioButton2D, 2);
buttonGroupFFTDimension->setExclusive(true);
ui->radioButton1D->setChecked(true);
The connect also compiles
connect(this->buttonGroupFFTDimension, static_cast<void(QButtonGroup::*)(int)>(&QButtonGroup::buttonClicked),
this, &MainWindow::on_buttonGroupFFTDimension_buttonClicked);
but it throws and error at runtime
QMetaObject::connectSlotsByName: No matching signal for on_buttonGroupFFTDimension_buttonClicked(int)
I admit that I am not familiar with the new connect syntax, but also do not see the obvious error. What is wrong?
The message shown is because you are using Qt Designer and it uses the connectSlotsByName method to connect various elements, it recognizes the format on_somesender_somesignal, and in your case matches your slot.
First solution: It iss unnecessary to use the connect function, this will automatically do it. Also I think that the slot does not have as parameter the type int that requires.
In your case the slot should be as follows:
private slots:
void on_buttonGroupFFTDimension_buttonClicked (int val);
Another possible solution is to rename the slot, after that you run make clean and qmake.
Related
I'm designed a QTableWidget with QPushButton, I would like to connect these buttons with a slot to hide some rows.
I'm using a lambda expression to pass a number of a row. But the compiler doesn't recognized this expression :
connect(this->ui->tableWidget->cellWidget(i,0),&QPushButton::clicked,[this,i]{hideRows(i);});
I have this error:
error: no matching function for call to 'SoftwareUdpater::MainWidget::connect(QWidget*, void (QAbstractButton::*)(bool), SoftwareUdpater::MainWidget::displayTable()::<lambda(int)>)'
The function hideRows(int) is declared as a function. And, as a slot, it doesn't work,
CONFIG += c++11 is added in pro file,
My class MainWidget inherits from QWidget,
Q_OBJECT is added in the header.
So I don't udnerstand why connect() is not recognized by Qt 5.9.1 MinGw 32bit.
Edit: [this,i]() instead of [this](const int i) for the lambda expression
Your connection is wrong. You can't connect a function that doesn't take parameters (clicked()) with a function that takes parameters (your lambda). To verify that this is the case, just do this:
connect(this->ui->tableWidget->cellWidget(i,0),&QPushButton::clicked,[this](){});
And see that it will compile. You have to make your design in such a way that signals and slots are compatible.
Also avoid using lambdas in signals and slots. Read the caveats here.
I was reading your comments on the accepted answer and noticed the root of the problem: This error is being thrown because the effective type of the object — as supplied to QObject::connect; i.e QWidget in your case — does not define the referenced signal QPushButton::clicked.
What likely happened is that the QPushButton pointer was cast into a QWidget and then that pointer was given to connect instead of the original which defines the signal.
Cast the pointer back to a QPushButton * and the error should go away.
I have three pushbutton widgets in my class and i want to connect the clicked signals from all of the three to one slot. Here is what my syntax looks like:
QObject::connect(PtrLineedit1, SIGNAL(Clicked()), this, SLOT(myslot()));
QObject::connect(PtrLineedit2, SIGNAL(Clicked()), this, SLOT(myslot()));
QObject::connect(PtrLineedit3, SIGNAL(Clicked()), this, SLOT(myslot()));
But the above syntax is not working. And the above syntax i am using in one of my WizardPage class. this Wizard has three pages, and in one of the page(class) has three pushbuttons and in this class constructor i am trying to achieve. I have used "clicked()" too, but no luck
First of all, there is no signal "Clicked", but there is "clicked" - the lowercase one.
Second - it's better not to use the macros SIGNAL and SLOT - they are error prone, like in your case. If you look inside them, they are generating a string, which could not be checked by compiler. Instead better to use following syntax:
QObject::connect(PtrLineedit1, &LineEditClass::clicked, this, &ThisClass::myslot);
Cause if you will write then:
QObject::connect(PtrLineedit1, &LineEditClass::Clicked, this, &ThisClass::myslot);
It would produce the error during compilation, in case when LineEditClass::Clicked is not defined. This would protect you from typos.
UPD: As eventually I've found another your question which faces issue relevant to this one, I've updated this answer with more details, which might be helpful for others.
In case of overloaded methods, you must explicitly tell compiler which one should be used - for example we have following:
void QComboBox::currentIndexChanged(int index)
void QComboBox::currentIndexChanged(const QString &text)
thus the &QComboBox::currentIndexChanged could be resolved ambigously, and you will get the compilation error. To fix it you need to explicitly tell what are the arguments of the function you want to connect to:
QObject::connect(comboBox, static_cast<void(QComboBox::*)(int)>(&QComboBox::currentIndexChanged), this, &ThisClass::myComboBoxSlotWithIntArg);
or
QObject::connect(comboBox, static_cast<void(QComboBox::*)(const QString &)>(&QComboBox::currentIndexChanged), this, &ThisClass::myComboBoxSlotWithStringArg);
I have one c++ class and have one signal in it and want to connect that signal with slot another C++ class. Here is my code
class Data : public QObject
{
Q_OBJECT
public:
static Data *instance(void);
signals:
void sendUserID(const QString& userId);
private:
static Data *s_instance;
};
here is my Slot in another class
void DataDetails::seTUserID(const QString user_id)
{
QAndroidJniObject user_id = QAndroidJniObject::fromString(user_id);
QAndroidJniObject::callStaticMethod<void>("com/user/data/userActivity",
"setUserID",
"(Ljava/lang/String;)V",
user_id.object<jstring>());
}
The idea is to access the value of user_id from Data class to DataDetails class
The connection is trying is
QObject::connect(&s_instance, SIGNAL(sendUserID(uid), this, SIGNAL(setUserID(uid))
any other id to get uid to other class is also fine ..
Generally speaking, when you encounter an issue with QObject::connect...
Make sure you have your declarations in order:
Both classes need the Q_OBJECT macro in their declaration.
Make sure your slot is actually declared as a slot (i.e. is part of a public slot: section).
Because connecting signals and slots just uses character strings evaluated at run-time, it's possible to write absolute nonsense and have it compile. In my experience, errors caused by typos are pretty common.
Always test in an environment where you can see your application's console output. Failed connect calls will usually trigger an error message printed to stderr.
Double-check your method names and signatures. Code will still compile even if you've made a typo!
For debugging, use assertions (e.g. bool c = connect(...); Q_ASSERT(c);) to catch missed connections early.
Alternatively, you can use the QMetaMethod-based version of QObject::connect, introduced in Qt 4.8, to avoid some of these issues.
In your particular case:
You've got a typo in the function declaration: it's called seTUserID but you're using setUserID in the connect call.
You're using variable names, not function signatures, in your signal and slot names. Qt expects to see QObject::connect(&s_instance, SIGNAL(sendUserID(const QString), this, SLOT(setUserID(const QString))
You've got a signal connected to another signal, which is valid but doesn't do what you want (it's usually used to chain stuff like this: SomeChildWidget's signal -> MyClass1's signal -> MyClass2's slot).
Check that seTUserID definition is marked as slot. You are probably calling connect in some Data method (because you are using private member directly). Are you trying to use a ref to a pointer to s_instance O_o? Write sendUserID(const Qstring) (the signature of a signal) rather then sendUserID(uid). The same situation with setUserID. You are trying to connect to this pointer and want to send info to another class!? Use new style of Qt connecting signals and slots (pointers to members), with was introduced in Qt 5. Check for setUserID has written right.
In my project, I have 40 QPushButtons all put into a QButtonGroup like this:
QButtonGroup* group = new QButtonGroup(this);
group->addButton(ui->slot_0);
group->addButton(ui->slot_1);
//...
group->addButton(ui->slot_38);
group->addButton(ui->slot_39);
Each button is a QPushButton that I made checkable. That way only one button can be checked at a time. All works great, but how can I "make a slot" when one of the buttons becomes checked? I don't want to have 40 different slots, one for each button all to end up doing essentially the same thing. Is there any way I can just use the QButtonGroup I put them in?
As Jamin and Nikos stated: you should create your own slot to handle the signal emitted by QButtonGroup. It could be something like this:
In the header file:
public slots:
void buttonWasClicked(int);
In the *.cpp file:
void MainWindow::buttonWasClicked(int buttonID)
{
cout << "You have clicked button: " << buttonID << endl;
}
And in the code responsible for creation of the MainWindow (i.e. in constructor but not necessairly) there should be this line:
connect(group, SIGNAL(buttonClicked(int)), this, SLOT(buttonWasClicked(int)));
Be aware that since Qt5 the connect syntax has changed. The syntax I used here is from Qt4. It still works but is deprecated now (for more information please refer to New Signal Slot Syntax in Qt 5). Moreover I would suggest going through QButtonGroup class reference as there are other available signals which could suit your needs better than the one I've chosen.
BR
The documentation for QButtonGroup shows a QButtonGroup::buttonClicked() signal - have you already tried that one?
The signal comes in two variants - one that gives the QPushButton as a parameter (as a QAbstractButton), and one that gives the ID of the button in the group.
You can use connect() to setup signal and slot connections in your C++ code.
Sometime during the initialization of your window's class (perhaps in the constructor), call this:
connect(myButtonGroup, SIGNAL(buttonClicked(QAbstractButton*)), this, SLOT(theSlotThatYouWrite(QAbstractButton*));
Where myButtonGroup is probably this->ui->nameOfTheButtonGroup, and theSlotThatYouWrite is a function that you write in your own code, that belongs to your window's class, that returns void and takes a signal QAbstractButton* as a parameter (since that's what this specific signal gives as an argument).
Make sure theSlotThatYouWrite is under the label "private slots:" or "public slots:" in your class's interface.
Here's a screenshot of actual usage of some signals and slots in my own code.
Signals and Slots is something very important to learn, but can be bit of a hill to climb when first trying to understand it!
I am pretty new with Qt.
I want to respond to linkClicked in QWebView.
I tried connect like this:
QObject::connect(ui->webView, SIGNAL(linkClicked(QUrl)),
MainWindow,SLOT(linkClicked(QUrl)));
But I was getting error: C:/Documents and Settings/irfan/My Documents/browser1/mainwindow.cpp:9: error: expected primary-expression before ',' token
When I do this using UI Editing Signals Slots:
I have in header file declaration of slot:
void linkClicked(QUrl &url);
in source cpp file :
void MainWindow::linkClicked(QUrl &url)
{
QMessageBox b;
b.setText(url->toString());
b.exec();
}
When I run this it compiles and runs but got a warning :
Object::connect: No such slot MainWindow::linkClicked(QUrl)
in ui_mainwindow.h:100
What is proper way of doing this event handling?
You state that it now works because you changed QObject::connect to connect. Now I'm not 100% on this but I believe the reason for this is that by calling connect, you are calling the method associated with an object which is part of your application. i.e. it's like doing this->connect(...). That way, it is associated with an existing object - as opposed to calling the static method QObject::connect which doesn't know anything about your application.
Sorry if that's not clear, hopefully I got the point across!
Using QObject::connect() and connect() is same in this context. I believe
QObject::connect(ui->webView,SIGNAL(linkClicked(QUrl)),
MainWindow,SLOT(linkClicked(QUrl)));
was called from a function inside MainWindow class. That is why when you tried
connect(ui->webView,SIGNAL(linkClicked(const QUrl)),
this,SLOT(linkClicked(const QUrl)),Qt::DirectConnection);
it works. Notice the difference that make it work - the third parameter. You used this in the second snippet, where as you used MainWindow in the first snippet.
Read this to know how signals and slots mechanism works and how to properly implement it.
I changed QObject::connect to only connect and it works.
So this code works:
connect(ui->webView,SIGNAL(linkClicked(const QUrl)),this,SLOT(linkClicked(const QUrl)),Qt::DirectConnection);
But I don't know why?