CODEWITHSUNDEEP
c++qtc++11buttonsignals-slots
palmtreesnative
palmtreesnative

Reputation: 2855

Pass a widget in argument of a SLOT

Here's what I want to do: I have a QLineEdit and a QPushButton. When I push the button I want to retrieve the text of the QLineEdit and do stuff with it. Unfortunately, I am struggling to do it because I'm kind of a newbie with Qt.

My mainwindow.cpp (simplified):

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QLineEdit *textInput = new QLineEdit();
    QPushButton *awesomeButton = new QPushButton();

    retrieveButton->setText("Click me");

    connect(awesomeButton, SIGNAL(released()), this, SLOT(awesomeFunction(QLineEdit)));

}

void MainWindow::awesomeFunction(QLineEdit myQLineEdit) {
    QString exploitedText = myQLineEdit.text();
    //DO STUFF
}

My mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

#include <QLineEdit>
#include <QPushButton>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

public slots:
    void awesomeFunction(QLineEdit myQLineEdit);

private:
    QPushButton *awesomeButton;
    QLineEdit *textInput;
    Ui::MainWindow *ui;
};
#endif

There's nothing interesting in main.cpp. I am missing something but I don't know what. When I use this without any parameters to my function, it works fine. But when I try to pass a Widget in parameter it doesn't work. The documentation is well done but I can manage to find what I need over the internet.

Upvotes: 0

Views: 1153

Answers (2)

G.M.
G.M.

Reputation: 12879

There are a couple of problems with the code shown. Firstly QObject, and by implication anything that inherits from it, is non-copyable so you can't pass a QLineEdit by value: you need to pass by reference/pointer.

Secondly the line...

connect(awesomeButton, SIGNAL(released()), this, SLOT(awesomeFunction(QLineEdit)));

should issue a runtime warning on the console because the slot expects an argument -- QLineEdit -- but the QPushButton::released signal doesn't provide one.

Assuming the QLineEdit you're interested in is textInput then you can make use of the new Qt5 signal/slot syntax and a lambda. So change the awesomeButton signature to...

void awesomeFunction(QLineEdit *myQLineEdit);

and then connect to the signal with (untested)...

connect(awesomeButton, &QPushButton::released, this,
        [this, textInput]()
          {
            awesomeFunction(textInput);
          });

Upvotes: 4

Igor Mironchik
Igor Mironchik

Reputation: 804

Use QPushButton::clicked() signal. Have a look at this signal documentation, like release(), it doesn't have parameters. Slot in Qt should be with the same signature as signal. These signals doesn't send any QLineEdit's.

Use:

void MainWindow::awesomeFunction() {
    QString exploitedText = textInput->text();
    //DO STUFF
}

Upvotes: 0

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