Error QApplication: no such file or directory

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

How do I link these libraries? I searched into the directories and there is a file named QApplication.h; So I tried to link it with -I (linking the directory) but it was still giving me that error.

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

 QT += widgets

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.

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>