pointer paramters c++

ChrisW67

@Kent-Dorfman said in pointer paramters c++:

I dont remebmer if pointers are registered with the MOC metadata system as allowable params in signals,

Signals sending pointers exist in the standard API, e.g. QApplication::focusChanged(), so it has to be possible to create a matching slot.

@hye2 You should be seeing a run time message regarding failure to make the connection.

If you wish to marshal a pointer then surely it should look like this:

// old style
connect(
  button, SIGNAL(clicked(CurrentState*,char)), 
  this,   SLOT(on_button_clicked(CurrentState*,char))
);

Of course, "button" has to have a signal declared with a pointer and emit that signal with a pointer.

hye2

@ChrisW67

Thanks for help..

I have to change this too? >>void MainWindow::on_button_clicked(CurrentState *state, char number) <<

J.Hilk

@hye2 said in pointer paramters c++:

on_button_clicked

you should also change the function name or you will get conflicts/problems with the connect function by name feature of Qt.

hye2

@ChrisW67

Sorry it doesn't work...
The build works, but the value doesn't connect..

jsulm

@hye2 said in pointer paramters c++:

The build works, but the value doesn't connect.

Then there should be warnings in terminal at runtime.
But you should switch to new Qt5 connect syntax, so you will get compiler error if your connect is wrong.

J.Hilk

@hye2 what type is button ? does it actually have a signal clicked with arguments ?

hye2

mainwindow.cpp

mainwindow::mainwindow(Qwidget *parent) : QmainWindow(parent), ui(new Ui::MainWindow)
{
ui ->setupUi(this);
QpushButton *button = new Qpushbutton("button");
connect(button, SIGNAL(clicked(CurrentState*,char)), this,   SLOT(on_button_clicked(CurrentState*,char)));
}

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

void MainWindow::on_button_clicked(CurrentState *state,char number)
{
ui->button->setText(state->value);
}

mainwindow.h

~~
public slots:
void on_button_clicked(CurrentState *state,char number);

Here is my code,,

jsulm

@hye2 said in pointer paramters c++:

QpushButton

QPushButton has no such signal!
Please read documentation: https://doc.qt.io/qt-5/qabstractbutton.html#clicked

hye2

@jsulm said in pointer paramters c++:

QPushButton has no such signal!

oh.. you mean..
I have to use void QAbstractButton::clicked(bool checked = false) this..? sorry

jsulm

@hye2 You can only use signals which exist. If you want to pass additional parameters to your slot switch to Qt5 connect syntax and use a lambda as slot.

hye2

@jsulm

I see. Can you help me a little?... I've never used Lambda before...

JonB

@hye2
Search https://wiki.qt.io/New_Signal_Slot_Syntax and https://doc.qt.io/qt-5/signalsandslots.html for lambda to see a couple of examples. Otherwise lambdas are a feature of C++ nothing to do with Qt, you can look them up in a C++ reference/tutorial. And while you are here stop using your old style SIGNAL/SLOT() macros and switch over the new style connect()s.

hye2

@JonB

Thank you! :)

hye2

@JonB

[](CurrentState *state, char number){ //something }

it is right..?

JonB

@hye2 said in pointer paramters c++:

[](CurrentState *state, char number){ //something }

Yes, that is the right syntax:

Inside the [...] you pass any context/local variables from where you are defining the lambda that you want the lambda to have access to, if any.
Inside the (...) you declare the parameters which the signal passes to the slot, if any.

For example, a slot for QLineEdit::textChanged(const QString &text) signal applied to a whole bunch of QLineEdits where we want to know which one was changed, as well as the new text:

for (QLineEdit *lineEdit : bunchOfLineEdits)
    connect(lineEdit, &QLineEdit::textChanged, [lineEdit](const QString &text) { qDebug() << lineEdit->objectName() << text; });

hye2

@JonB

Ok like this..?

mainwindow.cpp

QpushButton *button = new Qpushbutton("button");
connect(button, Qpushbutton::clicked,[button](CurrentState *state, char number){ //somthing } );

SimonSchroeder

@hye2 said in pointer paramters c++:

[](CurrentState *state, char number){ //something }
it is right..?

Well, the syntax is right for a lambda. However, in the context of what you are trying to achieve it will not help. QPushButton has a signal clicked(bool). So, your 'slot' (which will be the lambda in this case) has to have a bool as parameter or no parameter at all (you can drop parameters from the end of the function call when using slots).

I assume that state and number are member variables of your class. In this case you need to capture this inside the lambda to have access to these. The lambda you are most likely looking for is [this](){ on_button_clicked(state, number); }.

J.Hilk

I'm getting the feeling, you're trying to run before you can crawl.

why don't you simply subclass QPushButton, add the signal you want and emit it where and when you want it, with the parameters you specified.

JonB

@hye2

connect(button, Qpushbutton::clicked,[button](CurrentState *state, char number){ //somthing } );

You only need to pass [button] if you will wish to access button in your { ... } lambda body code, else it's not needed.
QPushbutton::clicked, [...](CurrentState *state, char number) cannot be right because the signature for the signal is void QAbstractButton::clicked(bool checked = false), and you cannot alter that fact. You can add further parameters to the slot in the lambda's [...], but you cannot alter what is inside the (...) unless you define your own signal which passes extra/different parameters. So where exactly do your CurrentState *state, char number come from?

hye2

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