C++ Gurus

• 

  Solved Template wizardry for QObject::connect-like method needed.
  • kshegunov

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  After dissecting Qt's QObject::connect implementation thoroughly I can say I have now a much better understanding of what is going on. It's a mess out there folks, the amount of instantiations for each one connect call is just mind-boggling. Anyway, the problem is now solved. One needs 4 distinct implementations to support what I want to have, one each for pointer to members, global/static functions with context object, functors (lambdas and such) with context object and one for function-like objects without context.

  In the end I had to do some special substitutions to fool the compatibility checks but ultimately the code compiles and matches correctly now. Just for completeness here's what I came up:

  // Bind to pointer-to-members template <class MessageType, typename Slot> typename QMetaObject::Connection bind(const typename QtPrivate::FunctionPointer<Slot>::Object * receiver, Slot slot, Qt::ConnectionType type = Qt::AutoConnection) { typedef QtPrivate::FunctionPointer<decltype(&QMpiMessageNotifier::received)> SignalType; typedef QtPrivate::FunctionPointer<Slot> SlotType; typedef QtPrivate::List<const MessageType &> SignalArguments; // Check the compatibility of arguments (from Qt's source). Q_STATIC_ASSERT_X(int(SignalType::ArgumentCount) >= int(SlotType::ArgumentCount), "The slot requires more arguments than the signal provides."); Q_STATIC_ASSERT_X((QtPrivate::CheckCompatibleArguments<SignalArguments, typename SlotType::Arguments>::value), "Signal and slot arguments are not compatible."); Q_STATIC_ASSERT_X((QtPrivate::AreArgumentsCompatible<typename SlotType::ReturnType, SignalType::ReturnType>::value), "Return type of the slot is not compatible with the return type of the signal."); return QObject::connect(notifier(QtMpi::messageType<MessageType>()), &QMpiMessageNotifier::received, receiver, [slot] (const QSharedPointer<QMpiMessage> & message) -> void { QMpiMessage * msg = message.data(); SlotType::template call<typename SlotType::Arguments, typename SlotType::ReturnType>(slot, nullptr, reinterpret_cast<void **>(&msg)); } , type); } // Bind to static/global functions with a context object template <class MessageType, typename Functor> typename std::enable_if<QtPrivate::FunctionPointer<Functor>::ArgumentCount >= 0 && !QtPrivate::FunctionPointer<Functor>::IsPointerToMemberFunction, QMetaObject::Connection>::type bind(const QObject * context, Functor slot, Qt::ConnectionType type = Qt::AutoConnection) { typedef QtPrivate::FunctionPointer<decltype(&QMpiMessageNotifier::received)> SignalType; typedef QtPrivate::FunctionPointer<Functor> SlotType; typedef QtPrivate::List<const MessageType &> SignalArguments; // Check the compatibility of arguments (from Qt's source). Q_STATIC_ASSERT_X(int(SignalType::ArgumentCount) >= int(SlotType::ArgumentCount), "The slot requires more arguments than the signal provides."); Q_STATIC_ASSERT_X((QtPrivate::CheckCompatibleArguments<SignalArguments, typename SlotType::Arguments>::value), "Signal and slot arguments are not compatible."); Q_STATIC_ASSERT_X((QtPrivate::AreArgumentsCompatible<typename SlotType::ReturnType, typename SignalType::ReturnType>::value), "Return type of the slot is not compatible with the return type of the signal."); return QObject::connect(notifier(QtMpi::messageType<MessageType>()), &QMpiMessageNotifier::received, context, [slot] (const QSharedPointer<QMpiMessage> & message) -> void { QMpiMessage * msg = message.data(); QtPrivate::FunctionPointer<Functor>::template call<typename QtPrivate::FunctionPointer<Functor>::Arguments, void>(slot, nullptr, reinterpret_cast<void **>(&msg)); }, type); } // Bind to functors (lambdas included) with a context object template <class MessageType, typename Functor> typename std::enable_if<QtPrivate::FunctionPointer<Functor>::ArgumentCount == -1, QMetaObject::Connection>::type bind(const QObject * context, Functor slot, Qt::ConnectionType type = Qt::AutoConnection) { typedef QtPrivate::List<const MessageType &> SignalArguments; const int FunctorArgumentCount = QtPrivate::ComputeFunctorArgumentCount<Functor, SignalArguments>::Value; Q_STATIC_ASSERT_X((FunctorArgumentCount >= 0), "Signal and slot arguments are not compatible."); const int SlotArgumentCount = (FunctorArgumentCount >= 0) ? FunctorArgumentCount : 0; auto functor = new QtPrivate::QFunctorSlotObject<Functor, SlotArgumentCount, typename QtPrivate::List_Left<SignalArguments, SlotArgumentCount>::Value, void>(slot); return QObject::connect(notifier(QtMpi::messageType<MessageType>()), &QMpiMessageNotifier::received, context, [functor] (const QSharedPointer<QMpiMessage> & message) -> void { QMpiMessage * msg = message.data(); functor->call(nullptr, reinterpret_cast<void **>(&msg)); }, type); } // Bind to functors or functions (no context object) template <class MessageType, typename Functor> typename QMetaObject::Connection bind(Functor slot) { return bind<MessageType, Functor>(this, slot, Qt::DirectConnection); }

  A big thanks to Luca for playing along, and I hope this may be useful to someone in the future.

  If you have any questions that I can answer, don't hesitate to ask.

• 

  Unsolved show qobject always infront (directx) and grap mouse with shortkey
  • QT-static-prgm

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  No one has replied

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  Unsolved How to retriew data from one ui to other ui in the same project.
  • moyin

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  @moyin As @yuvaram already said use signals/slot to communicate between different UI parts. See http://doc.qt.io/qt-5.9/signalsandslots.html
  Your view should not know anything about main window.

• 

  Solved How to set current cell color in a tableview and how to set text centerAligned
  • moyin

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  @Chris-Kawa
  thanks chris,
  i got it thanks a lot.

• 

  Unsolved How to use class and function both in .h file
  • qt_ankit_developer

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  Why do you want to do this?
  In short: If you think you need to do this you're most probably doing it wrong. Just put the definitions in a .cpp file where they belong.

  Placing a body of the function in a header and then including that header in more than one place violates the one definition rule. Basically it means that your code gets compiled into every object file it is included in and the linker doesn't know which one to use when it links them together to produce the output executable.

  There are couple of exceptions to this rule, notably function templates, because they are instantiated at the place they are used, not defined, and inlined functions, because they are replaced with their bodies at the place they are used.

• 

  Solved static getter/setter
  • user4592357

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  @user4592357 said in static getter/setter:

  @aha_1980
  good idea. but i don't need to do checks before assigning, should i still use static getter + static setter?

  Isn't the reason to use classes that you can hide your data? And even if you don't need a check now, this can change in half a year. Further, it avoids the problems @JohanSolo mentioned.

  it'll be a QString, so a reference or value?

  In this case I'd use void mySetter(const QString &str); like all the Qt API do it.

• 

  Unsolved pThreadCreate with main gui thread
  • qt_ankit_developer

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  @qt_ankit_developer Did you check http://wiringpi.com/examples/ ?
  You're asking how to use Wiring Pi in a Qt forum.

• 

  Solved Relocation against undefined hidden symbol
  • kshegunov

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  I have found the proper solution to this problem. It's not the moc per se, it's how the generated cpp file is linked in the binary.
  Anyway, without going into the ugly details, to solve this, one must simply include the generated moc_xxx.cpp file into the regular class's implementation .cpp file.
  In my case I had to include moc_aedmpimessages.cpp in aedmpimessages.cpp to make it work correctly.

• 

  Solved basic variables problems with Qt or C++
  • arsinte_andrei

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  @arsinte_andrei said in basic variables problems with Qt or C++:

  200 commands

  Hi 200 commands its not so simply ....
  I have generated association from 240 language..
  ID from google language to make associate in QT5.9 QLocale...
  search "DATALANGUAGEPROVIDEQT591_H" in file:
  https://raw.githubusercontent.com/pehohlva/QCLD2/master/translate/datalanguageprovider.h

  I love struct data in C++..

• 

  Unsolved How do i display an image using QImageReader?
  • Kevin M Chipili

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  Hi! You can use a QLabel to display images:

  QImageReader reader("/path/to/file.png"); ui->label->setPixmap(QPixmap::fromImageReader(&reader));
• 

  Solved mutex/unique_lock/try_lock_for
  • 6thC

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  I think I understand the double lock now.

  std::unique_lock<std::timed_mutex> lock(tMutex); // RAII lock attempt lock.try_lock_for(std::chrono::milliseconds(100)); // another lock request == resource deadlock

  This defer lock commands the unique lock to not lock on construction, so then I can command locking behavior myself.

• 

  Unsolved ADDING library in QT c++
  • shashi prasad

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  What was the issue ?

  And since you have it working now, please mark the thread as solved using the "Topic Tools" button so that other forum users may know a solution has been found :)

• 

  Unsolved Displaying BMP in QT c++
  • shashi prasad

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  Hi,

  QPixmap works with local file path not with remote URLs. First get the files using e.g. QNetworkAccessManager and then load them in a QPixmap.

• 

  Unsolved curl probleam in QT
  • shashi prasad

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  @jsulm you're right, this is 0% Qt-related :-)

  Anyway, 10.97.32.18 is very likely a proxy, that's why it answers, and 192.168.55.3 is a private network address, perhaps not reachable.

• 

  Unsolved Call a function from child
  • MSDQuick

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  Hi,

  To add to @raven-worx, what you are planning to do is bad design more specifically tight coupling. Why should the child know anything about its parent and furthermore call a function of it ?

  If you want your MainWindow to react to something happening in the child then you should use signals and slots.

• 

  Solved Template dependant argument
  • Kofr

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  Since both variables are supposed to be of the same type it can even be shortened as

  MyStruct<int> myData; decltype(myData) myDataTyped;
• 

  Unsolved Warning in C++ QT creator Linux
  • shashi prasad

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  @SGaist

  More over it's just a warning, most of the cases we can ignore Warnings.

  Thanks for sharing the link and my intention in mention the above words are not to totally ignore the warnings there are cases which can be ignored and which can't be ignored. I was unable to explain perfectly in post any way thanks for correcting my mistake.

• 

  Solved Error while compiling Qt mySQL driver
  • denko

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  Hi I had the same problem with qtsqldrivers-config.pri

  I leave it here, maybe it will be helpful

  Go to :
  /Qt_new/5.9.1/Src/qtbase/src/plugins/sqldrivers
  Run qmake:
  ../../../../../gcc_64/bin/qmake sqldrivers.pro
  After that I got:

  Configure summary:

  Qt Sql:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. yes
  OCI (Oracle) ........................... no
  ODBC ................................... no
  PostgreSQL ............................. no
  SQLite2 ................................ no
  SQLite ................................. yes
  Using system provided SQLite ......... no
  TDS (Sybase) ........................... no

  Then go to:
  /Qt_new/5.9.1/Src/qtbase/src/plugins/sqldrivers/mysql
  Run qmake again:
  ../../../../../../gcc_64/bin/qmake mysql.pro
  Run make:
  make
  You will see as a part of an output message that it was compiled and moved into:
  "mv -f libqsqlmysql.so ../plugins/sqldrivers/"

• 

  Unsolved getting warning in Qt creator c++
  • shashi prasad

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  The compiler is just warning you that order of initialization will be different than what you requested.
  You need to initialize the variables in the constructor's initializer in the order they're declared in the class. E.g:

  class X { X() : a(0), b(0) //< In the same order they're declared in the class, if you use b(0), a(0) it will generate the warning. { } int a; int b; }
• 

  Solved Array - for storing the first value in Qt C++
  • Jerwinprabu

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  @Daniil-Chernyshev Because Each and every time value will update from hardware, so for every 5 milli second will get new value. In that same thread only I need first value and real time value.

• 

  Unsolved virtual functions understanding
  • guru007

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  Hi,

  If we remove virtual keyword in:

  class C : virtual public A
  then It creates ambiguity.

  How it works internally.

  Class D is derived from both Class B and Class C , in which both Class B and Class C are derived from same class i.e Class A ,
  so that's the reason ambiguity is created ,
  when you remove virtual keyboard in Class C,

  Thanks
  Akshay.

• 

  Unsolved C++11 std::initializer_list for container
  • Maarten Verhage

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  @Maarten-Verhage said in C++11 std::initializer_list for container:

  QStringRef

  Sounds risky to use as your key.

  Warning: A QStringRef is only valid as long as the referenced string exists. If the original string is deleted, the string reference points to an invalid memory location.

  Would you find a map better for this application? Map sorts your key and everything... I use the std:: containers mostly because range loops are so nice and concise.

  #include <QGuiApplication> #include <QQmlApplicationEngine> #include <QDebug> int main(int argc, char *argv[]) { qputenv("QT_IM_MODULE", QByteArray("qtvirtualkeyboard")); QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling); QGuiApplication app(argc, argv); const std::map<std::string, int> StateIndexMap { { "Arizona" , 6 }, { "Montana" , 2 }, { "Nevada" , 5 }, { "Texas" , 1 }, { "North Carolina" , 3 }, { "New Mexico" , 4 }, }; const QMap<QString, int> StateIndexQMap { { "Arizona" , 6 }, { "Montana" , 2 }, { "Nevada" , 5 }, { "Texas" , 1 }, { "North Carolina" , 3 }, { "New Mexico" , 4 }, }; const std::map<int, std::string> IndexStateMap { { 6, "Arizona" }, { 2, "Montana" }, { 5, "Nevada" }, { 1, "Texas" }, { 3, "North Carolina"}, { 4, "New Mexico" }, }; qDebug()<<"std::map<std::string, int> StateIndexMap"; for(const auto& StateIter : StateIndexMap){ qDebug()<<"StateIter.first: "<<StateIter.first.c_str()<<", StateIter.second: "<<StateIter.second; printf("%s, %d", StateIter.first.c_str(), StateIter.second ); } qDebug()<<"\r\nQMap<QString, int> StateIndexQMap"; QMap<QString, int>::const_iterator iterState = StateIndexQMap.constBegin(); while (iterState != StateIndexQMap.constEnd()) { qDebug()<<" key(): "<<iterState.key().toLatin1().data()<<", value(): "<< iterState.value(); printf("%s, %d", iterState.key().toLatin1().data(), iterState.value() ); ++iterState; } qDebug()<<"\r\nstd::map<int, std::string> IndexStateMap"; for(const auto& Iter : IndexStateMap){ qDebug()<<"Iter.first: "<<Iter.first<<", Iter.second: "<<Iter.second.c_str(); printf("%d, %s", Iter.first, Iter.second.c_str() ); } QQmlApplicationEngine engine; engine.load(QUrl(QLatin1String("qrc:/main.qml"))); return app.exec(); } Console output: std::map<std::string, int> StateIndexMap StateIter.first: Arizona , StateIter.second: 6 StateIter.first: Montana , StateIter.second: 2 StateIter.first: Nevada , StateIter.second: 5 StateIter.first: New Mexico , StateIter.second: 4 StateIter.first: North Carolina , StateIter.second: 3 StateIter.first: Texas , StateIter.second: 1 QMap<QString, int> StateIndexQMap key(): Arizona , value(): 6 key(): Montana , value(): 2 key(): Nevada , value(): 5 key(): New Mexico , value(): 4 key(): North Carolina , value(): 3 key(): Texas , value(): 1 std::map<int, std::string> IndexStateMap Iter.first: 1 , Iter.second: Texas Iter.first: 2 , Iter.second: Montana Iter.first: 3 , Iter.second: North Carolina Iter.first: 4 , Iter.second: New Mexico Iter.first: 5 , Iter.second: Nevada Iter.first: 6 , Iter.second: Arizona
• 

  Unsolved Converting float datatype values as array of bytes
  • Pradeep Kumar

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  @Pradeep-Kumar said in Converting float datatype values as array of bytes:

  had to do serialize of datatype values

  What is the purpose of your serialized data?

  If you want to store your serialized data in a file, and then use your same application to read the stored data, then I recommend using QDataStream to serialize/deserialize it. You can find examples at http://doc.qt.io/qt-5/qdatastream.html

  However, if you want to send your serialized data to a 3rd-party application, then you need to read the other application's documentation. There are countless different ways to serialize data.

• 

  Unsolved Stack small array in big array
  • rockon209

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  @rockon209 said in Stack small array in big array:

  But i am getting error

  And the error says what?

• 

  Unsolved casts, if's and array-access.
  • J.Hilk

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  Hi,

  Shouldn't it rather be:

  QVector<int> paramVector = vindex.at(i); value = dStorage.getvalueMethod_1(paramVector.at(0), paramVector.at(1), paramVector.at(2));

  to ensure the compiler can optimise access a bit ?

• 

  Unsolved QNetworkConfigurationManager does not recognise active wireless network when Cisco AnyConnect secure mobility client is enabled
  • DLHTS

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  Hi,

  Please don't post the exact same question on multiple sub-forum. One is enough.

  Duplicate

  Closing this one.

• 

  Unsolved Newbie question: How do i add PDcurses in Qt Creator?
  • Teisted

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  Okay, i'll use it. I guess the starting period is weird.

  E: So i searched and found what i have to do but i have one problem.
  I have to use qmake to make the PDcurses library. The readme files tells me how to do it using nmake or make but i don't know what option to use in qmake.

  what should i fill in "??" qmake -makefile -?? vswin32.mak

• 

  This topic is deleted!
  • Jerwinprabu

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• 

  Unsolved How to Shuffle elements of an Array without repeating the same sequence?
  • Eduardo12l

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  @JohanSolo Actually, now that you say it,
  theres a chance for repetitions in selectSome

  QList<QVector<QChar>> selectSome(int cnt, QList<QVector<QChar>> pool){ QList<QVector<QChar>> list; QVector<int> usedIndex; int i(0); while(i < cnt && usedIndex.size() < pool.size(){ int index = rand()%pool.size(); if(!usedIndex.contains(index)){ usedIndex.append(index); list.append(pool.at(index)); } } return list; }

  this should be better, still not perfect, but at least no repetitions, possible infinite loop when pool.size() > sizeOf(int)? Gets very slow for big cnt!

• 

  Unsolved How to use Qt in C++ development
  • Richard Lang

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  @ambershark
  See http://blog.qt.io/blog/2017/05/31/qt-lite-qt-5-9-lts/
  It can have GUI components, it depends on what you configure.

• 

  Unsolved Boolean variable - One time declaration - definition
  • Konstantinos

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  @jsulm

  Hey sorry about the misconception of the topic and my post.

  i should have rephrased saying definition variables can be across all the classes.
  and not as mentioned in the topic as declared and defined.

  Thanks,

• 

  Solved MSVC & MinGW
  • J.Hilk

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  @J.Hilk said in MSVC & MinGW:

  @JKSH
  I actually used a debugger, the problem was, my program was halted not on the right spot but at random places in my code.
  I halted on *List.clear() .List is a list of pointers and List itself is a class, member and allocated on the stack. So I checked everything that was connected to the list and the objects it pointed to.
  The error itself that I later fixed was 10-20 lines above this entry...

  Then a memory checker is our friend.

  I mentioned Valgrind in my previous post. It's not available on Windows, but Dr. Memory looks promising: http://www.drmemory.org/ (I haven't used this myself, though)

• 

  Unsolved Qpoint Question
  • dragonKlavier

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  Yes, if g wasn't there to get the copy it would already have been destroyed.

  Are you studying stack vs heap memory ?

• 

  Solved How can I Fill a 2D QVector with a Loop and Get the size of each Row?
  • Eduardo12l

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  @SGaist Thx it works great.

• 

  Unsolved Capture image using OPENCV in c++ (Qt creator 5.7)
  • Faisal_Ali

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  @Faisal_Ali Have you tried github

• 

  Unsolved Why it works at Release, but in Debug, it can't run sucessfully
  qml profiler • • Solan

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  @Solan
  It will not work in debug mode because in Debug mode there some extra initializations that happen behind the scenes. If you heavily use the pointers the visual debuggers won't help you out.
  For more information on this topic read about -Wall option in gcc for linux and go to msdn about debugger from microsoft windows.
  If the code you are writing from scratch then please use latest conventions of new and delete.

• 

  Solved AutoTest / QtTest: Any experience here?
  c++ qt unit tests quick2 project files • • 6thC

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  I was just mentioning that's how the video demonstrated.

  Thanks to you I have the files in my test project and they are the files from my actual app, now
  compiling seems to be the issue.

  I had a couple things to do to get it build able again but now I can at least compile my c++ classes. I have one test with one of my objects now so thanks.

• 

  Unsolved why when i want copy (with memcpy)struct array to char array , character array is null??
  • stackprogramer

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  @stackprogramer
  You well understand what memcpy does?

  memcpy(str2, str1, 10);
  The above line copies the first 10 characters of str1 to str2.

• 

  Solved Qt on CentOS 7 not working
  • Dino

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  Dear jsulm,

  In the end, it was indeed a mess with 32-bit and 64-bit libraries.

  To fix my problems I have to uninstall all version of Qt on my machine and all OpenGL libraries. Then I have installed the latest version of Qt and the mesa-libGL-devel-11.2.2-2.20160614.el7.x86_64.rpm package.

  Again thank you for your help.

  Kind Regards,

  Dino

• 

  This topic is deleted!
  • feniks_fire

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