Error QApplication: no such file or directory

Question

I have installed C++SDK that have Qt but when I try compiling a code linking QApplication it gives me the error:

Error QApplication: no such file or directory


        

Answer 1

you have to add QT +=widgets in the .pro file before the first execution, if you execute before adding this line its not gonna working, so yo need to start file's creation from the beginning.

Answer 2

In Qt5 you should use QtWidgets instead of QtGui

#include <QtGui/QComboBox>     // incorrect in QT5
#include <QtWidgets/QComboBox>    // correct in QT5

Or

#include <QtGui/QStringListModel>    // incorrect in QT5
#include <QtCore/QStringListModel>    // correct in QT5

Answer 3

I suggest you to update your SDK and start new project and recompile everything you have. It seems you have some inner program errors. Or you are missing package.

And ofc do what Abdijeek said.

Answer 4

Please make sure that the version of qmake you are using corresponds to the version of QT you want to use.

To be sure, you can just run :

$qmake -v

Your problem seems to be a symptom of a version conflict between QT 3 and 4, as can be seen here :

http://lists.trolltech.com/qt4-preview-feedback/2005-11/thread00013-0.html

To fix this, you can either delete your old install of QT, or specifically point to qmake-qt4 in your Makefile.

Answer 5

To start things off, the error QApplication: no such file or directory means your compiler was not able to find this header. It is not related to the linking process as you mentioned in the question.

The -I flag (uppercase i) is used to specify the include (headers) directory (which is what you need to do), while the -L flag is used to specify the libraries directory. The -l flag (lowercase L) is used to link your application with a specified library.

But you can use Qt to your advantage: Qt has a build system named qmake which makes things easier. For instance, when I want to compile main.cpp I create a main.pro file. For educational purposes, let's say this source code is a simple project that uses only QApplication and QDeclarativeView. An appropriate .pro file would be:

TEMPLATE += app
QT += gui declarative
SOURCES += main.cpp

Then, execute the qmake inside that directory to create the Makefile that will be used to compile your application, and finally execute make to get the job done.

On my system this make outputs:

g++ -c -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_DECLARATIVE_LIB -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/opt/qt_47x/mkspecs/linux-g++ -I. -I/opt/qt_47x/include/QtCore -I/opt/qt_47x/include/QtGui -I/opt/qt_47x/include/QtDeclarative -I/opt/qt_47x/include -I/usr/X11R6/include -I. -o main.o main.cpp
g++ -Wl,-O1 -Wl,-rpath,/opt/qt_47x/lib -o main main.o -L/opt/qt_47x/lib -L/usr/X11R6/lib -lQtDeclarative -L/opt/qt_47x/lib -lQtScript -lQtSvg -L/usr/X11R6/lib -lQtSql -lQtXmlPatterns -lQtNetwork -lQtGui -lQtCore -lpthread

Note: I installed Qt in another directory --> /opt/qt_47x

Edit: Qt 5.x and later

Add QT += widgets to the .pro file and solve this problem.

Answer 6

For QT 5

Step1: .pro (in pro file, add these 2 lines)

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

Step2: In main.cpp replace code:

#include <QtGui/QApplication> 

with:

#include <QApplication>