I have downloaded and installed Qt and I'm learning how to use it.
So, I created a new project myfristqt (empty project). I then added a main.cpp file with this code:
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
return app.exec();
}
First thing I noticed #include <QApplication> line is hightlighted with a red line as if QApplication was an unknown identifier. So, I compiled it to see what happens and here's the error I got:
(internal):1: error : Using OS scope before setting MAKEFILE_GENERATOR
Any idea why this is happening ? I'm using Windows XP
MAKEFILE_GENERATOR is a qmake variable.
This variable contains the name of the Makefile generator to use when generating a Makefile. The value of this variable is typically handled internally by qmake and rarely needs to be modified.
It define in QTDIR/mkspecs/PLATFORM/qmake.conf. Where PLATFORM is maybe cygwin-g++, win32-msvc200x on your Windows XP.
Related
I have a Qt Widget project that I created using QtCreator and Qt version 5.15.2 to which I'm trying to add the QtVirtualKeyboard as matchbox-keyboard I've already tried using stays under the application when it's in fullscreen mode.
However I'm having trouble getting it to work as it's not appearing at all at the moment. This is how I've tried adding it
Main.cpp
#include "mainwindow.h"
#include <QApplication>
#include <QtWidgets>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
qputenv("QT_IM_MODULE", QByteArray("qtvirtualkeyboard"));
MainWindow w;
//w.showFullScreen();
w.show();
return a.exec();
}
I've tried adding QT += qtvirtualkeyboard or QT += virtualkeyboard in the .pro file but it just gives me an error saying "unknown module"
How can I add the virtual keyboard to the project?
You will need to make sure that the QtVirtualKeyboard library is selected during the installation process.
Also, I would recommend you that you start using cmake for Qt applications as Qt has officially dropped qmake since the release of Qt 6.
It is not to say that you cannot use it, but you will get better support and results by using an actively developed build system rather than an abandoned.
I have a Qt5 project based on qmake.
main.cpp is the following:
#include <QApplication>
#include <QWidget>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget w;
w.show();
return a.exec();
}
The project seems to build fine. However when I run it, it will crash. That's strange, right?
What could explain that is the bunch of files I added to the project .pro file.
Anyway, if that files are not referenced in the main.cpp, would that cause any trouble? I would not expect.
Context: I'm trying to build a small part of a bigger project and so I added all kinds of things in this project until it finally build.
I already deleted .pro.user and files created during build and tried to rebuild everything again.
And even the following will also crash
int main()
{
return 0;
}
I am trying to locate the memory leaks in my Qt application. I already have used Visual Leak Detector for some other projects, but VLD writes the output to the console window.
My problem now is that when using a QApplication, no console window, and therefore no output from VLD, is shown. I am using Visual Studio 2015 with the Qt VS Tools plugin.
Is there a way to force the application to show the console window? Or maybe a way to write the output generated by VLD to a file?
How I start up my application:
#include "mainwindow.h"
#include <vld.h>
#include <QApplication>
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
As ssbssa pointed out as a comment, the problem could be solved by setting ReportTo and ReportFile in vld.ini found in the installation folder of VLD:
change ReportFile = to ReportFile = memory_leak_report.txt or something like that.
change ReportTo = debugger to ReportTo = file or ReportTo = both.
Now the output produced by VLD will be written to the specified file.
When trying to use some Qt-5 classes I'm experiencing crashes. I first discovered this trying to use QFileSystemModel. Trying to call setRootPath immediately leads to a crash. The callstack isn't of much help (all of it is assembly code) except that QFileIconProvider::icon() is the last function called before the seg fault occurs.
So next I tried using QFileIconProvider manually and -to no surprise- it also crashed the program.
I'm using QtCreator 4 and the type of project is "Qt Quick Application". When I instead create a project of type "Qt Widgets Application", I can use both QFileIconProvider and QFileSystemModel without problems.
Here's where I'm out of ideas. I don't know enough about the Qt environment to know what difference between the two types of projects could lead to the seg fault.
Both projects use the same Kit (same gcc, same Qt 5.6.1) and the default settings as set by QtCreator.
This is my project.pro file:
TEMPLATE = app
QT += qml quick widgets //default .pro file except for widgets
CONFIG += c++11
SOURCES += main.cpp
RESOURCES += qml.qrc
# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH =
# Default rules for deployment.
include(deployment.pri)
This is main.cpp:
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QDir>
#include <QFileSystemModel>
#include <QQmlContext>
#include <QFileIconProvider>
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
//If trying to use QFileSystemMode...
QFileSystemModel model;
model.setRootPath("/somefolder/"); //..the crash happens here
//Attempting to use QFileIconProvider also crashes
//QFileIconProvider fip;
//fip.icon( QFileInfo("/somefolder/somefile") ); //<- here
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}
I'd appreciate any help or pointers as to how to debug that mess.
As confusing as it may sound, QFileSystemModel is part of QtWidgets and therefore requires you to create and instance of QApplication instead of QGuiApplication.
I finished migrating an application from Qt4 to Qt5, it compiles and everything but it crashes at a certain point. I am debugging it and trying to find why but I have reached a dead end:
Here is the stack:
main.cpp line 373:
TouchSwibz w(NULL, NULL, renderMode ? renderMode : AppSettings::RASTERMODE);
When it reaches the breakpoint and I try to go further, it crashes with the usual
"This application has requested the Runtime to terminate it in an
unusual way."
And the aplication output shows
QWidget: Must construct a QApplication before a QWidget
Invalid parameter passed to C runtime function.
Invalid parameter passed to C runtime function.
I have thought maybe its because the widget is being initialized when the main window is being created, but what can be done to solve this? What would be a good workaround? I dont even know if this is the real issue.
I work under Windows 7 x64 using Qt 5.2.1 and compiling with mingw 4.8 32bit, the application is in 32bits also. Everything is compiled with the same kit.
I dont know what other useful information I can provide. I tried stepping inside the QwtSlider constructor but I cant.
I managed to solve it by compiling all the libraries in debug mode, turns out having libraries in release mode while building your application in debug mode will make undefined behaviour happen.
You're most likely having non-local instances of QWidget type. By definition, those will be initialized before main starts executing, so before QApplication gets constructed. The code below reproduces the problem:
#include <QLabel>
#include <QApplication>
QLabel label("Hello, world!");
int main(int argc, char ** argv)
{
QApplication app(argc, argv);
label.show();
return app.exec();
}
The fix is to delay the construction until there is a QApplication object:
#include <QLabel>
#include <QApplication>
// Won't ever be a dangling pointer.
QPointer<QLabel> label;
int main(int argc, char ** argv)
{
QApplication app(argc, argv);
QLabel label_("Hello, world!");
label.reset(&label_);
label->show();
return app.exec();
}
I just solved a similar problem, and here is my detailed situation and solution:
I use VS with Qt add-on;
and Qt version is 5.7 64 bit (but this is not important)
I compiled successfully in both debug and release mode;
I could run it in debug mode but not in release, where caused an massage "Must construct a QApplication before a QWidget".
[IMPORTANT] I first compiled and tested it under DEBUG mode, and then I met some computational thresh-hold that encouraged me to use RELEASE mode.
[IMPORTANT] I used some third-party library, related to Qt GUI component, that requires you to add an external dependency to the project.
[IMPORTANT] I just copy the configures in the Project Property page from DEBUG to RELEASE, like exteral C++ library or External Dependencies.
I finally found the reason. In the project's Property Pages dialog box -> Linker folder -> Input property page -> Additional Dependencies, one of the external library should be replaced as a release version one, which have different name, in my case, QGLViewerd2.lib to QGLViewer2.lib.