Upgrading from qt4 to qt5

wij2

Recently I tried to recompile my programs (used to work fine with qmake-qt4) with qmake-qt5. I found I have to add a line 'QT+=widgets' in .pro file. Except this, my g++ compiler always issues: error: no match for ‘operator==’ (operand types are ‘QMetaObject::Connection’ and ‘bool’) and a long list of candidates follow for each "if(QObject::connect(...)==false) {/.../}" statement.

Question1: How to fix this problem
Question2: Is there any general HOWTO about changes from compiling qt4 programs with qmake-qt5

SGaist

Hi,

Please show the connect statements you are using.

As for the porting guide, see the related chapter in Qt's documentation.

wij2

q_v4l2.cpp: In constructor ‘MainWidget::MainWidget(const char*, QWidget*, unsigned int, unsigned int)’:
q_v4l2.cpp:64:53: error: no match for ‘operator==’ (operand types are ‘QMetaObject::Connection’ and ‘bool’)
if(QObject::connect(m_qtmr, SIGNAL(timeout()),
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
this, SLOT(update()) )==false) {
~~~~~~~~~~^
In file included from /usr/include/qt5/QtCore/qstring.h:48,
from /usr/include/qt5/QtCore/qobject.h:47,
from /usr/include/qt5/QtCore/qabstractanimation.h:43,
from /usr/include/qt5/QtCore/QtCore:5,
from /usr/include/qt5/QtWidgets/QtWidgetsDepends:3,
from /usr/include/qt5/QtWidgets/QtWidgets:3,
from q_v4l2.cpp:22:
/usr/include/qt5/QtCore/qchar.h:598:30: note: candidate: ‘constexpr bool operator==(QChar, QChar)’
Q_DECL_CONSTEXPR inline bool operator==(QChar c1, QChar c2) Q_DECL_NOTHROW { return c1.ucs == c2.ucs; }
^~
....[snipped] long candidte list

wij2

The statement is like:

     if(QObject::connect(m_qtmr, SIGNAL(timeout()),
                          this, SLOT(update()) )==false) {
      throw Error(ECONNREFUSED) ;
     }

m_qtmr is a QTimer instance
*this is a subclass of QLabel

Christian Ehrlicher

QObject::connect() now returns a QMetaObject::Connection and no boolean. So you have to rewrite your statement a little bit to use the operator bool(): http://doc.qt.io/qt-5/qmetaobject-connection.html#operator-bool

if(!QObject::connect(m_qtmr, SIGNAL(timeout()), this, SLOT(update()) ))

Or even better use the new signal/slot syntax: https://wiki.qt.io/New_Signal_Slot_Syntax/de