Error QApplication: no such file or directory

In Qt 5 you now have to add widgets to the QT qmake variable (in your MyProject.pro file).

 QT += widgets

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>

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.


Looks like you don't have the development libraries installed. Install them using: 

sudo apt-get install libqt4-dev

As you said int the comments that you have them installed, just re-install it. Now. to update the locate's database, issue this command $sudo updatedb

Then $locate QApplication to check that you now have the header file installed.

Now, goto the the folder where you have the code & type these commands

qmake -project
qmake
make

Then you can find the binary created.

Alternatively, you can use Qt Creator if you want the GUI.

  • See the official documentation for more info. http://developer.qt.nokia.com/doc/qt-4.8/gettingstartedqt.html

  • To learn how to use Qt Creator, use http://doc.qt.nokia.com/qtcreator-2.2/creator-qml-application.html

Tags:

C++

Qt

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