Install Qt5 on Raspberry Pi/Raspbian and Hello World

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Updated@2016-02-27: 
For Raspbian Jessie, qt5-default (Qt 5 development defaults package) is included in default repository. Refer the updated post "Install Qt5/Qt Creator for Raspberry Pi 2/Raspbian Jessie".

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A member mentioned in the post on raspberrypi.org forums that there are "backports" packages of Qt5 you can install.

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Edit the file: /etc/apt/sources.list and add:
deb http://twolife.be/raspbian/ wheezy main backports
deb-src http://twolife.be/raspbian/ wheezy main backports

Install the required key:
$sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key 2578B775

Then get Qt5 and esoteric requirements installed:
$ sudo apt-get update
$ sudo apt-get install qt5-default qt5-qmake libegl1-mesa libgles2-mesa

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Hello World Qt5 on Raspberry Pi/Raspbian (console application)

- Create a directory, for example, named helloworld. And change to it.
- Create a cpp file, named helloworld.cpp.

#include 
#include 
#include 

int main(int argc, char *argv[])
{
    printf("Qt Version %s\n", QT_VERSION_STR);
    QCoreApplication app(argc, argv);
    qDebug() << "Hello Qt5"; 
    return app.exec();
 
}

Make it with Qt5:
$ qmake -project
$ qmake
$ make

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Hello World Qt5 on Raspberry Pi/Raspbian (with GUI widgets)

- Create a directory, for example, named helloqt5. And change to it.
- Create a cpp file, named helloqt5.cpp.

#include 
#include 
#include 

int main(int argc, char *argv[])
{
    printf("Qt Version %s\n", QT_VERSION_STR);
 
    QApplication app(argc, argv);
    QLabel *label = new QLabel("Hello Qt5");
    label->show();
    return app.exec();
}

Make it with Qt5:
$ qmake -project
$ qmake
$ make

Now you have some error reported. Like this:

helloqt5.cpp:(.text.startup+0x1c): undefined reference to `QApplication::QApplication(int&, char**, int)'
helloqt5.cpp:(.text.startup+0x50): undefined reference to `QLabel::QLabel(QString const&, QWidget*, QFlags)'
helloqt5.cpp:(.text.startup+0x60): undefined reference to `QWidget::show()'
helloqt5.cpp:(.text.startup+0x64): undefined reference to `QApplication::exec()'
helloqt5.cpp:(.text.startup+0x70): undefined reference to `QApplication::~QApplication()'
helloqt5.cpp:(.text.startup+0x84): undefined reference to `QApplication::~QApplication()'

Now open and edit helloqt5.pro, add the line "QT += widgets":

TEMPLATE = app
TARGET = helloqt5
INCLUDEPATH += .

QT += widgets

# Input
SOURCES += helloqt5.cpp

Now re-make the project:
$ qmake
$ make