custom slot/signal

Hi Im new here and Id like to ask a few questions please,

In my program there is a QLineEdit and a QLabel. What I want to do is when the user enter something in the LineEdit, I want to do some processing with it then displaying the result in QLabel. I was thinking of something like

Quote:

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connect(lineEdit, SIGNAL(returnPressed()), qLabel, SLOT(setText("asdf")))

---

but after some reading i found out that you cannot pass values in connect and besides with the above code i cannot process the input from lineedit. I read the documentation about signals and slots and I understood it somehow but I just didnt understand it enough to write my own slot/signal.

so can somebody please help me to understand and write my own slot/signal. thank you.

Let's say you have a main window containing those widgets. Then the code could look something like this:

header:

Code:

---

...
class MainWindow: public QMainWindow
{
    Q_OBJECT // <- this macro is important
 
 public:
    MainWindow(...);
    ~MainWindow();
    ...
 
// your own custom slot declaration looks like this:
private slots:
    void processInput();
};

---

implementation:

Code:

---

MainWindow::MainWindow(...) : ...
{
    // connection between line edit and your custom slot
    connect(lineEdit, SIGNAL(returnPressed()), this, SLOT(processInput()));
}
 
// custom slot implementation
void MainWindow::processInput()
{
    // process the line edit text
    QString text = process(lineEdit->text());
 
    // set the result
    qLabel->setText(text);
}

---

thank you for the speedy reply, now here is the code

Code:

---

#include<QApplication>
#include<QLabel>
#include<QLineEdit>
#include<QString>
 
class myWidget:public QWidget{
    Q_OBJECT
 
    public:
        myWidget(QWidget *parent=0);
    private slots:
        void processInput();
};
 
myWidget::myWidget(QWidget *parent)
    :QWidget(parent)
{
    QLabel *display=new QLabel(this);
 
    QLineEdit *lineEdit=new QLineEdit;
 
    connect(lineEdit, SIGNAL(returnPressed()), this, SLOT(processInput()));
 
}
 
void myWidget::processInput(){
    QString text=lineEdit->text();
    display->setText(text);
}
 
int main(int argc, char **argv){
    QApplication app(argc, argv);
    myWidget window;
 
    window.show();
    return app.exec();
}

---

and i got undeclared error of lineEdit and display

The problem is that you declare lineEdit as a local variable in the constructor. If you want to access it in a different method, you have to declare it as a member variable.

you have declared the objects in the constructor, so they are available only there.

you should do something like that

Code:

---

class myWidget:public QWidget{
    Q_OBJECT
    public:
        myWidget(QWidget *parent=0);
    private slots:
        void processInput();
    private:
        QLabel display;
        QLineEdit lineEdit;
};

---

now i got new error, undefined reference to vtable for myWidget

Code:

---

#include<QApplication>
#include<QLabel>
#include<QLineEdit>
#include<QString>
 
class myWidget:public QWidget{
    Q_OBJECT
 
    public:
        myWidget(QWidget *parent=0);
    private slots:
        void processInput();
    private:
        QLabel *display;
        QLineEdit *lineEdit;
};
 
myWidget::myWidget(QWidget *parent)
    :QWidget(parent)
{
    display=new QLabel;
    lineEdit=new QLineEdit;
 
    connect(lineEdit, SIGNAL(returnPressed()), this, SLOT(processInput()));
 
}
 
void myWidget::processInput(){
    QString text=lineEdit->text();
    display->setText(text);
}
 
int main(int argc, char **argv){
    QApplication app(argc, argv);
    myWidget window;
 
    window.show();
    return app.exec();
}

---

Try adding line:

Code:

---

#include "main.moc"

---

somewhere in your main.cpp.

You will have to include the moc file by hand in case you don't have separate class header files.

so i just add the include line to main.cpp right? i did that but the compiler said that there is no such file or directory :confused:

Quote:

---

Originally Posted by awnjoor

i did that but the compiler said that there is no such file or directory

---

How do you compile your application?

kay, i deleted everything in the folder, except main.cpp. Then i did qmake -project then qmake and then make. Now the error is in the moc file

here is my moc file

Code:

---

/****************************************************************************
** Meta object code from reading C++ file 'main.cpp'
**
** Created: Mon Jul 17 06:56:41 2006
**      by: The Qt Meta Object Compiler version 59 (Qt 4.1.4)
**
** WARNING! All changes made in this file will be lost!
*****************************************************************************/
 
#if !defined(Q_MOC_OUTPUT_REVISION)
#error "The header file 'main.cpp' doesn't include <QObject>."
#elif Q_MOC_OUTPUT_REVISION != 59
#error "This file was generated using the moc from 4.1.4. It"
#error "cannot be used with the include files from this version of Qt."
#error "(The moc has changed too much.)"
#endif
 
static const uint qt_meta_data_myWidget[] = {
 
 // content:
       1,       // revision
       0,       // classname
       0,    0, // classinfo
       1,   10, // methods
       0,    0, // properties
       0,    0, // enums/sets
 
 // slots: signature, parameters, type, tag, flags
      10,    9,    9,    9, 0x08,
 
       0        // eod
};
 
static const char qt_meta_stringdata_myWidget[] = {
    "myWidget\0\0processInput()\0"
};
 
const QMetaObject myWidget::staticMetaObject = {
    { &QWidget::staticMetaObject, qt_meta_stringdata_myWidget,
      qt_meta_data_myWidget, 0 }
};
 
const QMetaObject *myWidget::metaObject() const
{
    return &staticMetaObject;
}
 
void *myWidget::qt_metacast(const char *_clname)
{
    if (!_clname) return 0;
    if (!strcmp(_clname, qt_meta_stringdata_myWidget))
	return static_cast<void*>(const_cast<myWidget*>(this));
    return QWidget::qt_metacast(_clname);
}
 
int myWidget::qt_metacall(QMetaObject::Call _c, int _id, void **_a)
{
    _id = QWidget::qt_metacall(_c, _id, _a);
    if (_id < 0)
        return _id;
    if (_c == QMetaObject::InvokeMetaMethod) {
        switch (_id) {
        case 0: processInput(); break;
        }
        _id -= 1;
    }
    return _id;
}

---

i dont know how to output the errors to a file so here is the image

http://us.share.geocities.com/dymlg/1.bmp

Quote:

---

Originally Posted by awnjoor

kay, i deleted everything in the folder, except main.cpp. Then i did qmake -project then qmake and then make.

---

Then it should work, unless your project is located in "qt" directory (in such case you must add "TARGET += something" to your .pro file).

Quote:

---

Originally Posted by awnjoor

Now the error is in the moc file

---

All moc files correct. The error must be in some other place.

Quote:

---

Originally Posted by awnjoor

i dont know how to output the errors to a file so here is the image

---

Just cut & paste them. If you are using windows console, you can find cut & paste tools after you left-click the icon in the top-left corner of the window.

Quote:

---

Originally Posted by awnjoor

http://us.share.geocities.com/dymlg/1.bmp

---

"This page is not available."

You can post images here --- just click "Manage Attachements" button below the editor, but please don't post BMPs (even MS Paint can save images as PNGs or GIFs).

>>Then it should work, unless your project is located in "qt" directory (in such case you must add "TARGET += something" to your .pro file)

no, my project is not located in qt directory.

here is the error

Quote:

---

In file included from main.cpp:6:
release/main.moc:38: error: `myWidget' has not been declared
release/main.moc:43: error: `myWidget' has not been declared
release/main.moc:44: error: non-member function `const QMetaObject* metaObject()
' cannot have `const' method qualifier
release/main.moc:48: error: `myWidget' has not been declared
release/main.moc: In function `void* qt_metacast(const char*)':
release/main.moc:52: error: `myWidget' has not been declared
release/main.moc:52: error: expected `>' before '*' token
release/main.moc:52: error: expected `(' before '*' token
release/main.moc:52: error: expected primary-expression before '>' token
release/main.moc:52: error: invalid use of `this' in non-member function
release/main.moc:52: error: expected `)' before ';' token
release/main.moc:53: error: cannot call member function `virtual void* QWidget::
qt_metacast(const char*)' without object
release/main.moc: At global scope:
release/main.moc:56: error: `myWidget' has not been declared
release/main.moc: In function `int qt_metacall(QMetaObject::Call, int, void**)':

release/main.moc:58: error: cannot call member function `virtual int QWidget::qt
_metacall(QMetaObject::Call, int, void**)' without object
release/main.moc:63: error: `processInput' undeclared (first use this function)
release/main.moc:63: error: (Each undeclared identifier is reported only once fo
r each function it appears in.)
mingw32-make[1]: *** [release\main.o] Error 1
mingw32-make[1]: Leaving directory `C:/Thesis/temp'
mingw32-make: *** [release-all] Error 2

---

Quote:

---

Originally Posted by awnjoor

In file included from main.cpp:6:

---

Move #include "main.moc" directive to the end of file.

::bows down::

thank you all. now jacek would you care to explain why i have to move the moc directive down

Quote:

---

Originally Posted by awnjoor

why i have to move the moc directive down

---

Because main.moc contains implementation of myWidget methods (which were added by Q_OBJECT macro) and in C++ you have to place method implementation after corresponding class definition.

sigh, im trying to do the custom slot/signal again but it is not working. here is the code

Code:

---

#include<QTableWidgetItem>
#include<QApplication>
#include<QWidget>
#include<QPushButton>
#include<QLabel>
#include<QFont>
#include<QLineEdit>
#include<QTableWidget>
#include<QVBoxLayout>
#include<QHBoxLayout>
#include<QStringList>
#include<QtSql/QSqlDatabase>
#include<QtSql/QSqlQuery>
#include<QString>
#include<QObject>
 
class myWidget:public QWidget{
    Q_OBJECT
 
    public:
        myWidget(QWidget *parent=0);
 
    private slots:
        void processInput();
 
    private:
        QLabel *displayItem;
        QLineEdit *itemnumber;
};
 
myWidget::myWidget(QWidget *parent)
    :QWidget(parent)
{
    setWindowState(Qt::WindowFullScreen);
 
    QLabel *title = new QLabel;
    title->setFont(QFont("Verdana", 30, 0));
    title->setText("title");
    title->setAlignment(Qt::AlignHCenter);
 
    QLabel *item=new QLabel;
    item->setFont(QFont("verdana", 15, QFont::Bold));
    item->setText("Item");
 
    displayItem=new QLabel;
    //displayItem->setFont(QFont("verdana", 15, QFont::Bold));
    //displayItem->setText("asdf");
 
    itemnumber=new QLineEdit;
    itemnumber->setMaxLength(10);
 
    QLabel *quantity=new QLabel;
    quantity->setFont(QFont("verdana", 15, QFont::Bold));
    quantity->setText("Quantity");
 
    QLineEdit *quantitynumber=new QLineEdit;
    quantitynumber->setMaxLength(10);
 
    QStringList columnLabels;
    columnLabels << "Item" << "#" << "TOTAL";
 
    QTableWidget *tableWidget=new QTableWidget;
    tableWidget->setRowCount(1);
    tableWidget->setColumnCount(3);
    tableWidget->setHorizontalHeaderLabels(columnLabels);
    tableWidget->setColumnWidth(0, 250);
    tableWidget->setColumnWidth(1, 30);
    tableWidget->setColumnWidth(2, 100);
 
    QHBoxLayout *hLeft1=new QHBoxLayout;
    hLeft1->addWidget(item);
    hLeft1->addWidget(itemnumber);
 
    QHBoxLayout *hLeft2=new QHBoxLayout;
    hLeft1->addWidget(quantity);
    hLeft1->addWidget(quantitynumber);
 
    QVBoxLayout *vLeft=new QVBoxLayout;
    vLeft->addLayout(hLeft1);
    vLeft->addLayout(hLeft2);
 
    QHBoxLayout *hLeft=new QHBoxLayout;
    hLeft->addLayout(vLeft);
    hLeft->addStretch(0);
 
    QVBoxLayout *mainLeft=new QVBoxLayout;
    mainLeft->addWidget(displayItem);
    mainLeft->addStretch(0);
    mainLeft->addLayout(hLeft);
    mainLeft->addStretch(0);
 
    QHBoxLayout *mainBottom=new QHBoxLayout;
    mainBottom->addLayout(mainLeft);
    mainBottom->addWidget(tableWidget);
 
    QVBoxLayout *mainLayout=new QVBoxLayout(this);
    mainLayout->addWidget(title);
    mainLayout->addSpacing(80);
    mainLayout->addLayout(mainBottom);
    setLayout(mainLayout);
 
    itemnumber->setFocus();
    //itemnumber->setText("asdf");
 
    connect(itemnumber, SIGNAL(returnPressed()), this, SIGNAL(processInput()));
 
}
 
void myWidget::processInput(){
    //QString text=itemnumber->text();
    itemnumber->setText("asfd");
}
 
int main(int argc, char *argv[]){
    QApplication app(argc, argv);
    myWidget window;
 
    window.show();
    return app.exec();
}
 
#include "client.moc"

---

everything compiles fine, it just doesnt do the processInput() when i press enter in itemnumber lineedit.

EDIT:
it seems that item number is not emitting the returnPressed() signal because i tried replacing it with

Code:

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connect(itemnumber, SIGNAL(returnPressed()), qApp, SIGNAL(quit()));

---

and when i pressed enter nothing happened.

The latter should be SLOT:

Quote:

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connect(itemnumber, SIGNAL(returnPressed()), this, SLOT(processInput()));

---

and i wonder how did i miss that one .. :o

thank you