C++ Gurus

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  Solved Strange LNK 2019 error in qt
  • Shaharyaar

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  I am currently using Qt 5.6.3

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  Unsolved unique_ptr question
  • adutzu89

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  @VRonin Oh, I was not taking unique_ptr into account as I removed it from my code.

  Thanks for your input though!

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  Unsolved instance of a struct
  • Digitale_GFacchini

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  @JonB said in istance of a struct:

  Even when you get this bit working, like I said I suspect your whole approach like now is not going to achieve what you want in the way of making these values available to every module, so you're going to have to do it a different way like I said anyway...

  Good evening, I'm sorry for my absence.
  It's true! The problem is a C ++ concept that I have been studying for a short time, together with QT. But those who use QT, if I understand correctly, also know C ++. For that I asked the question here. Apart from that, I think it's right that what you said also on the approach. I think that my intent must act in another way. the question is which one?

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  Unsolved Information about Ethernet over USB (RNDIS) in Qt
  qt creator c++ qt usb qt ethernet rndis • • cnmM

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  Sounds rather like a job for libusb

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  Solved struct and passing array parameters
  • Digitale_GFacchini

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  Thank SGaist

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  Solved Signal and Slots are not Working in my project
  • Shaharyaar

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  I think you misunderstand how signals/slots work.

  in your connect statement the first param is an object emitting a signal and the second one is a method (signal) of that object's class. Class QPushButton doesn't have a method (signal) named sayHello. It's a method (signal) in MainWindow class.

  QPushButton has a method (signal) named clicked(), so you can connect that. If you want to call pushed() when the button "Hi" is clicked then you connect like this:

  connect(ui->pushButton_2, &QPushButton::clicked, this, &MainWindow::pushed);

  If you want the main window to emit sayHello when the button "Hi" is pressed and then in response to that you want to call replyHi you make two connections:

  connect(ui->pushButton_2, &QPushButton::clicked, this, &MainWindow::sayHello); // emits sayHello when the button is clicked connect(this, &MainWindow::sayHello, this, &MainWindow::replyHi); // calls replyHi when sayHello is emitted
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  This topic is deleted!
  • mad-hatter

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  Solved compiler error on inclusion of string library
  • mzimmers

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  @mzimmers said in compiler error on inclusion of string library:

  Yes, and therein lies the dilemma - the extern "C" turns off the function decoration that template function names require. I'd gone through and used extern "C" on all my classes, and that got me into trouble.

  That's because C does not support templates (or classes!). The API wrapped in extern "C" must be C-compatible.

  There is a way (I'd call it a hack) to add member functions to extern "C" structs though: https://stackoverflow.com/a/1950150/1144539

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  Solved Does the linker care about namespaces?
  • VRonin

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  @VRonin said in Does the linker care about namespaces?:

  Can I then add USE_NAMESPACE to the definition of an application that links to that library?

  Nope. The linker very much cares about namespaces, to answer your title. The namespace identifier will be part of the fully qualified name of the symbol, so you need to link with code compiled with a compatible header.

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  Solved need help with pointer to member function
  • mzimmers

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  Well, that's some pretty fancy stuff. Thanks for the explanation(s)...this will be very useful.

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  Solved How to check object is valid.
  • Taz742

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  @Taz742 said in How to check object is valid.:

  qDebug() << b1->x << b2->x; //its print 421312312 -2131231231

  Yes, it prints garbage because nobody initialises x.

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  Pointer to C++ function
  • mrdebug

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  Many thanks for your post but this thread is very outdated.
  Regards.

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  Solved Linker Error in Qt v5.6
  • Shaharyaar

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  Thanks for solution!!!

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  Solved error: 'to_string' is not a member of 'std' ^
  • column

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  this has nothing to do with Qt.
  std::to_string is C++11, you have to make sure you have a recent enough compiler and to enable C++11 features. If you use Qt Creator you should add CONFIG += c++11 in your .pro file and re-run qmake

  Alternatively you can #include <sstream> and use std::stringstream gstream; gstream << 4; std::string g=gstream.str();

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  Solved How to get rid of changed code lingering in the moc?
  • landslyde

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  Actually, while I'd deleted the action code from mainwindow.cpp, I hadn't deleted its prototype in mainwindow.h. That's what the compiler was complaining about. So I deleted it in the .h and then ran Build -> Clean All. Doing that removed the reference to the deleted action in the moc file. Problem solved. Thanks again for your help.

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  Solved How to manipulate data on one.cpp using two.cpp?
  • landslyde

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  Hi jsulm,

  Thank you for your response. I understand exactly what you're talking about. This is what I needed, I just didn't know how to go about it. I'll get busy with this and see if I can accomplish what you laid out for me. Thanks again. Very much appreciated.

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  Solved How to unit test race conditions
  • VRonin

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  @kshegunov said in How to unit test race conditions:

  It can be modified outside the context of the class

  Good spot but in this small example I was just assuming everything goes through ThreadUnsafeClass

  @ambershark said in How to unit test race conditions:

  just thrown stuff at my threads as fast as possible via a test to try to have the race condition pop up.

  That's what I'm doing at the moment but I felt like I was doing a dumb thing because I didn't know the correct solution

  Thanks everyone. At least I feel a little less unconfortable by not being able to reliably data-test this

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  Solved Segfault core dumped
  segmentation segfault • • Qjay

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  @sierdzio said in Segfault core dumped:

  Anyway, the bug is in line 61 in = &inFile; - you assign a variable created on a stack to variable in. In the next line, the inFile goes out of scope and is deleted. So when you try to access in later in the code, it segfaults.

  Thanks !! . I know it's not Qt problem but that's why i posted it in C++ section

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  This topic is deleted!
  • Kofr

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  Unsolved C++ / Qt5.1.1 - Call virtual function in another class
  virtual • • mad-hatter

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  @mad-hatter You can't define a destructor of a base class in a derived class! So, adding ~Plug_Entity() {} in derived is invalid. Simply add ~derived() {}.
  See https://www.geeksforgeeks.org/pure-virtual-destructor-c/

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  Unsolved No taskbar entry for QDialog using console application
  • Neshom

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  @JonB said in No taskbar entry for QDialog using console application:

  @Neshom
  I would guess that is indeed the reason if none of those work! It always helps if you tell us which OS you are under, especially if it's not Windows! I would imagine this is a desktop-thing, so more than Lubuntu what is probably relevant is which desktop manager you are using. And it may be that your manager does not allow you not to have an icon....

  I am using Linux - Lubuntu which has LXDE. Is there anyway to fix this? Is there any other way in QT to show a frame-less window/dialog/message/label/etc without a task-bar entry?

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  Unsolved Build error while importing a simple project from a makefile
  • kevinmeal

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  I solved it temporarily by compiling the object files for the subdirectories (using make directly in the shell) then linking them as libraries in the .pro file.
  Then I added main_code.cpp as SOURCES .

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  Solved How to include Qwt library to current project
  • thippu

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  @jsulm yes , I'm trying

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  Solved Need help in understanding Qt documentation
  • thippu

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  As I already wrote earlier: no.

  QDial is a QAbstractSlider and there's no value painted. Hence my earlier suggestion to use QwtDial which provides more customisable dial control that should fit your needs.

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  Unsolved Error message: QStandardPaths: wrong ownership. (1000 instead of 0) How do I fix this, I have no idea where the Qt program came form.
  • RexduPont

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  You shouldn't need to use sudo to build anything.

  Are you getting errors because the application is trying to access some devices ?

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  Unsolved Once you create a project via Qt Creator, how do you run it outside of Creator
  • Mike456

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  @Mike456 said in Once you create a project via Qt Creator, how do you run it outside of Creator:

  Can you specifically explain to me why Qt 5 is better than Qt4? Are you talking about Qt service?

  Qt 4 reached End-of-Life status in December 2015. This means it is no longer supported -- it will not receive any new features, bug fixes, or security fixes.

  If someone finds a security vulnerability in Qt 4, that means they could attack any computer that runs Qt 4 software. The user of the software will have no protection because the flaw will never be fixed. In contrast, if such a vulnerability is found in a supported version of Qt 5, the vulnerability will be fixed urgently so that users cannot be attacked.

  If you find a bug in Qt 4 that affects your project, there is not much you can do about it -- you'll need to find your own workaround to the bug or patch Qt yourself. In contrast, if you find a bug in a supported version of Qt 5, you can file a bug report and there's a good chance it will be fixed.

  In any case, better have some more information about your project otherwise it's only going to be speculations.

  What specific kind of information?

  For starters: What is your development PC's OS? What is your deployment target's OS? Will the final product be released to customers, or is it for internal use only? How do you plan to maintain the project? Which features of Qt do you plan to use?

  Also, remember to start reading http://doc.qt.io/qt-5/deployment.html

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  Solved Connecting to a slot in a QThread
  • Christianvs

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  @Christianvs said in Connecting to a slot in a QThread:

  Or am i wrong about this?

  You are, sorry. You are free to connect signals and slots in different threads as long as both are running an event loop. Qt will take care of making the connection safe.

  i'd might be able to make a public slot in the ros_comm class

  You can but if ros_comm is a QThread subclass the slots will be executed in the thread that created the ros_comm, not in the thread created by ros_comm

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  Solved Invalid use of void expression, when trying to connect signal/slot
  • Christianvs

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  @Pablo-J.-Rogina

  That was it! Thanks a lot :)

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  Unsolved error not a type
  • Qjay

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  hey , i have pasted the errors here : https://ghostbin.com/paste/4xebg

  i used cpp.sh to compile the above code ( settings : c++11)

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  Solved Trouble writing to file
  • Christianvs

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  Thanks for all the replies folks!

  I feel quite stupid for not seeing that .size() actually solves the entire problem, but I am still quite green when it comes to programming.

  Thanks for the tip with iterators @mranger90 @VRonin ! I'll look into that

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  Solved Error while compiling Qt mySQL driver
  • denko

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  @chrabaszcz.patryk said in Error while compiling Qt mySQL driver:

  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/"

  I try to solve this problem for the whole night,your solution saved me.Solve the problem perfectly. Thanks very much.Thanks .

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  Unsolved Hi,I'm viewing 2 camera video simultanesly and recording them on linux system, but both camera lags(slow) after 3sec and I tried to put recording in Qtconcurrent::run(). but also both camera lags(slow)
  • thippu

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  @jsulm Thank u so much

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  Solved Help with QTimer scope
  • Christianvs

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  @Christianvs said in Help with QTimer scope:

  Is there any downside to just calling this as parent?

  No. The other solution is worse, because it is easy to forget about calling delete. If you use Qt's parent-child system, then Qt will automatically delete objects when their parent is deleted.

  There is a possibility that it is not the behaviour you want, but that is not something we can determine from the code above.

  Other options for managing object life:

  QSharedPointer stl pointers
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  Unsolved comaprision of the file values
  • anuj nogja

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  Just to demonstrate what @JohanSolo and @jsulm suggested:

  const QSettings file1("file1.txt", QSettings::IniFormat); qDebug() << "file1 keys" << file1.allKeys(); const QSettings file2("file2.txt", QSettings::IniFormat); qDebug() << "file2 keys" << file2.allKeys(); foreach (const QString &key, file1.allKeys()) { if (!file2.contains(key)) qDebug() << "in file1 only:" << key; } foreach (const QString &key, file2.allKeys()) { if (!file1.contains(key)) qDebug() << "in file2 only:" << key; } foreach (const QString &key, file1.allKeys()) { if ((file2.contains(key)) && (file1.value(key) != file2.value(key))) qDebug() << "file1 and file2 differ:" << key << file1.value(key) << file2.value(key); }

  Output (with your sample files above):

  file1 keys ("s1", "s2", "s3") file2 keys ("s1", "s2") in file1 only: "s3" file1 and file2 differ: "s1" QVariant(QString, "ABCDE") QVariant(QString, "ABCD23") file1 and file2 differ: "s2" QVariant(QString, "ERTY 1") QVariant(QString, "EEEE")

  Cheers.

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  Solved Critique Tutorial Of 31 Videos
  • Mike456

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  The big things missing just looking at the titles:

  QML/QtQuick Networking (QNetworkAccessManager/QWebEngine) Sockets/Comunication (QTcpSocket/QSerialPort) Threads (QtConurrent/QThread) Containers (QList/QMap) Database (QSqlDatabase/QSqlQuery) XML/JSON input/output Model/Views (QAbstractItemModel/QAbstractItemView) Graphics (QGraphicsScene/QGraphicsItem) Qt3D
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  Solved Books/Videos/Tutorials on Qt
  • Mike456

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  Hi and welcome to devnet,

  The question about Qt related books comes regularly. You should search the forum a bit for them.

  You should also consider taking a look at Qt's own documentation. There are a lot of information there to help you get started.

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  Unsolved Adding Eigen library to Qt Creator
  • iMarcK

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  I use Eigen on a daily basis, there's no real setting up to use it - it's a template library, meaning only headers. You only add its location to the INCLUDEPATH variable in your project file and you're good to go (see @JKSH's link and here). Since I use linux and I already have eigen in my system path, I don't even do that step, I just code.

  PS.
  A piece of advice for after you get it working:
  Don't use auto with Eigen! People have been bitten by it, and there's even a dedicated page in the docs why.

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  This topic is deleted!
  • Tegan

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  Solved Need help with a segfault problem
  c++11 segfault c++ • • Qjay

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  Hi @jsulm and @Paul-Colby,

  if (segs) { delete[] segs; segs = nullptr; }

  This is an anti-pattern! Just make sure segs is nullptr before you call new and set it to nullptr after delete. No need to check for nullptr before delete:

  delete segs; segs = nullptr;

  Quoting the C++ Reference:

  "If this is a null-pointer, the function does nothing."

  Regards

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  Unsolved Speech to text command integration in my QT5.5 application
  • saurabhverma

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  Hi and welcome to devnet,

  The QtSpeech module has a WIP branch using PocketSphinx for ASR. Otherwise you'll have to use the native Windows Speech Recognition API.