New signals & slots connection syntax with Qt5

kevina67

Hi,

I have a QLineEdit and a QPushButton. I'd like the button to gain focus when the return key is pressed while typing anything in the QLineEdit.

Therefore I wrote :

@QObject::connect(_ui->findExpressionLineEdit, &QLineEdit::returnPressed, _ui->findPushButton, &QWidget::setFocus);@

But such code doen't want to compile :

/home/kevina/Documents/editor/src/findWidget.cpp:56:14: error: no matching member function for call to 'connect'
QObject::connect(_ui->findExpressionLineEdit, &QLineEdit::returnPressed, _ui->findPushButton, &QWidget::setFocus);
~~~^
/usr/include/qt/QtCore/qobject.h:198:36: note: candidate function not viable: no known conversion from 'void (QLineEdit::*)()' to 'const char *' for 2nd argument
static QMetaObject::Connection connect(const QObject *sender, const char signal,
^
/usr/include/qt/QtCore/qobject.h:201:36: note: candidate function not viable: no known conversion from 'void (QLineEdit::)()' to 'const QMetaMethod' for 2nd argument
static QMetaObject::Connection connect(const QObject sender, const QMetaMethod &signal,
^
/usr/include/qt/QtCore/qobject.h:440:41: note: candidate function not viable: no known conversion from 'void (QLineEdit::)()' to 'const char *' for 2nd argument
inline QMetaObject::Connection QObject::connect(const QObject *asender, const char *asignal,
^
/usr/include/qt/QtCore/qobject.h:214:43: note: candidate template ignored: couldn't infer template argument 'Func2'
static inline QMetaObject::Connection connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal,
^
/usr/include/qt/QtCore/qobject.h:244:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
^
/usr/include/qt/QtCore/qobject.h:267:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
^
1 error generated.

But this code does work :

@QObject::connect(_ui->findExpressionLineEdit, &QLineEdit::returnPressed, [this] () {

    _ui->findPushButton->setFocus();
});@

But I'd like to use the first syntax wich is more logical and concise.

Any clue about that ? Thx ;)

dbzhang800

Hi, the reason is that

@
class QWidget {
//...
public Q_SLOTS:
inline void setFocus() { setFocus(Qt::OtherFocusReason); }
@

setFocus() is an inline function.

kevina67

Ok thanks, but why does it works with the second syntax ?

kevina67

Up please ;)

dbzhang800

The second works, as you are calling

@
QMetaObject::Connection QObject::connect(const QObject * sender, PointerToMemberFunction signal, Functor functor)
@

The first doesn't work, as you are trying to call

@
QMetaObject::Connection QObject::connect(const QObject * sender, PointerToMemberFunction signal, const QObject * receiver, PointerToMemberFunction method, Qt::ConnectionType type)
@
which means that you are trying to get the address of an inline function.

kevina67

Ok, thanks for your explanations 1+1=2.