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| 6 | <title>Qt Tutorial - Chapter 1: Hello, World!</title>
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| 18 | <td valign=center>
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| 19 | <a href="index.html">
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| 20 | <font color="#004faf">Home</font></a>
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| 21 | | <a href="classes.html">
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| 22 | <font color="#004faf">All Classes</font></a>
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| 23 | | <a href="mainclasses.html">
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| 24 | <font color="#004faf">Main Classes</font></a>
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| 26 | <font color="#004faf">Annotated</font></a>
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| 28 | <font color="#004faf">Grouped Classes</font></a>
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| 30 | <font color="#004faf">Functions</font></a>
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| 31 | </td>
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| 32 | <td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Qt Tutorial - Chapter 1: Hello, World!</h1>
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| 33 |
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| 34 |
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| 35 | <p> <center><img src="t1.png" alt="Screenshot of tutorial one"></center>
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| 36 | <p> This first program is a simple hello-world example. It contains only
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| 37 | the bare minimum you need to get a Qt application up and running.
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| 38 | The picture above is a snapshot of this program.
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| 39 | <p> <pre>/****************************************************************
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| 40 | **
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| 41 | ** Qt tutorial 1
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| 42 | **
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| 43 | ****************************************************************/
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| 44 |
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| 45 | #include <<a href="qapplication-h.html">qapplication.h</a>>
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| 46 | #include <<a href="qpushbutton-h.html">qpushbutton.h</a>>
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| 47 |
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| 48 |
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| 49 | int main( int argc, char **argv )
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| 50 | {
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| 51 | <a href="qapplication.html">QApplication</a> a( argc, argv );
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| 52 |
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| 53 | <a href="qpushbutton.html">QPushButton</a> hello( "Hello world!", 0 );
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| 54 | hello.<a href="qwidget.html#resize">resize</a>( 100, 30 );
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| 55 |
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| 56 | a.<a href="qapplication.html#setMainWidget">setMainWidget</a>( &hello );
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| 57 | hello.<a href="qwidget.html#show">show</a>();
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| 58 | return a.<a href="qapplication.html#exec">exec</a>();
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| 59 | }
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| 60 | </pre>
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| 61 |
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| 62 |
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| 63 |
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| 64 | <p> <h2> Line-by-line Walkthrough
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| 65 | </h2>
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| 66 | <a name="1"></a><p> <pre> #include <<a href="qapplication-h.html">qapplication.h</a>>
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| 67 | </pre>
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| 68 | <p> This line includes the <a href="qapplication.html">QApplication</a> class definition. There has to be
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| 69 | exactly one QApplication object in every application that uses Qt.
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| 70 | QApplication manages various application-wide resources, such as the
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| 71 | default font and cursor.
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| 72 | <p> <pre> #include <<a href="qpushbutton-h.html">qpushbutton.h</a>>
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| 73 | </pre>
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| 74 | <p> This line includes the <a href="qpushbutton.html">QPushButton</a> class definition. The
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| 75 | <a href="hierarchy.html">reference documentation</a> for each class
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| 76 | mentions at the top which file needs to be included to use that class.
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| 77 | <p> QPushButton is a classical GUI push button that the user can press
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| 78 | and release. It manages its own look and feel, like every other <a href="qwidget.html">QWidget</a>. A widget is a user interface object that can process user
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| 79 | input and draw graphics. The programmer can change both the overall
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| 80 | <a href="qapplication.html#setStyle">look and feel</a> and many minor
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| 81 | properties of it (such as color), as well as the widget's content. A
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| 82 | QPushButton can show either a text or a <a href="qpixmap.html">QPixmap</a>.
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| 83 | <p> <pre> int main( int argc, char **argv )
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| 84 | {
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| 85 | </pre>
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| 86 | <p> The main() function is the entry point to the program. Almost always
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| 87 | when using Qt, main() only needs to perform some kind of initialization
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| 88 | before passing the control to the Qt library, which then tells the
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| 89 | program about the user's actions via events.
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| 90 | <p> <tt>argc</tt> is the number of command-line arguments and <tt>argv</tt> is the
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| 91 | array of command-line arguments. This is a C/C++ feature. It is not
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| 92 | specific to Qt; however, Qt needs to process these arguments (see
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| 93 | following).
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| 94 | <p> <pre> <a href="qapplication.html">QApplication</a> a( argc, argv );
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| 95 | </pre>
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| 96 | <p> <tt>a</tt> is this program's <a href="qapplication.html">QApplication</a>. Here it is created and processes
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| 97 | some of the command-line arguments (such as -display under X Window).
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| 98 | Note that all command-line arguments recognized by Qt are removed from
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| 99 | <tt>argv</tt> (and <tt>argc</tt> is decremented accordingly). See the <a href="qapplication.html#argv">QApplication::argv</a>() documentation for details.
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| 100 | <p> <strong>Note:</strong> It is essential that the QApplication object be
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| 101 | created before any window-system parts of Qt are used.
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| 102 | <p> <pre> <a href="qpushbutton.html">QPushButton</a> hello( "Hello world!", 0 );
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| 103 | </pre>
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| 104 | <p> Here, <em>after</em> the QApplication, comes the first window-system code: A
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| 105 | push button is created.
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| 106 | <p> The button is set up to display the text "Hello world!" and be a
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| 107 | window of its own (because the constructor specifies 0 for the parent
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| 108 | window, inside which the button should be located).
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| 109 | <p> <pre> <a name="x2285"></a> hello.<a href="qwidget.html#resize">resize</a>( 100, 30 );
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| 110 | </pre>
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| 111 | <p> The button is set up to be 100 pixels wide and 30 pixels high (plus the
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| 112 | window system frame). In this case we don't care about the button's
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| 113 | position, and we accept the default value.
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| 114 | <p> <pre> <a name="x2284"></a> a.<a href="qapplication.html#setMainWidget">setMainWidget</a>( &hello );
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| 115 | </pre>
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| 116 | <p> The push button is chosen as the main widget for the application. If
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| 117 | the user closes a main widget, the application exits.
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| 118 | <p> You don't have to have a main widget, but most programs do have one.
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| 119 | <p> <pre> <a name="x2286"></a> hello.<a href="qwidget.html#show">show</a>();
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| 120 | </pre>
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| 121 | <p> A widget is never visible when you create it. You must call show() to
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| 122 | make it visible.
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| 123 | <p> <pre> <a name="x2283"></a> return a.<a href="qapplication.html#exec">exec</a>();
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| 124 | </pre>
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| 125 | <p> This is where main() passes control to Qt, and exec() will return when
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| 126 | the application exits.
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| 127 | <p> In exec(), Qt receives and processes user and system events and passes
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| 128 | these on to the appropriate widgets.
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| 129 | <p> <pre> }
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| 130 | </pre>
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| 131 | <p> You should now try to compile and run this program.
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| 132 | <p> <a name="compiling"></a>
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| 133 | <h2> Compiling
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| 134 | </h2>
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| 135 | <a name="2"></a><p> To compile a C++ application you need to create a makefile. The
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| 136 | easiest way to create a makefile for Qt is to use the <a href="qmake-manual.html">qmake</a> build tool supplied with Qt. If you've
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| 137 | saved <tt>main.cpp</tt> in its own directory, all you have to do is:
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| 138 | <pre>
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| 139 | qmake -project
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| 140 | qmake
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| 141 | </pre>
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| 142 |
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| 143 | <p> The first command tells <a href="qmake-manual.html">qmake</a> to
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| 144 | create a <tt>.pro</tt> (project) file. The second command tells it to create
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| 145 | a (platform-specific) makefile based on the project file. You should
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| 146 | now be able to type <tt>make</tt> (or <tt>nmake</tt> if you're using Visual
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| 147 | Studio) and then run your first Qt application!
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| 148 | <p> <h2> Behavior
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| 149 | </h2>
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| 150 | <a name="3"></a><p> When you run it, you will see a small window filled with a single
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| 151 | button, and on it you can read the famous words, Hello World!
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| 152 | <p> <h2> Exercises
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| 153 | </h2>
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| 154 | <a name="4"></a><p> Try to resize the window. Press the button. If you're running X
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| 155 | Window, try running the program with the -geometry option
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| 156 | (for example, <tt>-geometry 100x200+10+20</tt>).
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| 157 | <p> You're now ready for <a href="tutorial1-02.html">Chapter 2.</a>
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| 158 | <p> [<a href="tutorial1-02.html">Next tutorial</a>]
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| 159 | [<a href="tutorial.html">Main tutorial page</a>]
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| 160 | <p>
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| 161 | <!-- eof -->
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| 162 | <p><address><hr><div align=center>
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| 163 | <table width=100% cellspacing=0 border=0><tr>
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| 164 | <td>Copyright © 2007
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| 165 | <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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| 166 | <td align=right><div align=right>Qt 3.3.8</div>
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| 167 | </table></div></address></body>
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| 168 | </html>
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