Programming PerlQt

Germain Garand

This document describes a set of Perl bindings for the Qt toolkit. Contact the author at <>


PerlQt-3 is Ashley Winters' full featured object oriented interface to Trolltech's C++ Qt toolkit v3.0.

It is based on the SMOKE library, a language independent low-level wrapper generated from Qt headers by Richard Dale's kalyptus thanks to David Faure's module.

This document describes the principles of PerlQt programming. It assumes you have some basic Perl Object Oriented programming knowledge.

Some C++ knowledge is recommended but not required. It would mostly help you to find your way through Qt's excellent documentation which is our ultimate and only reference. If Qt is installed on your system, then you most probably also have its documentation. Try the $QTDIR/bin/assistant program.



To compile and use PerlQt, you'll need :

Perl and Qt's installation is out of the scope of this document. Please refer to those projects' documentation.


PerlQt uses GNU's Autoconf framework. However, the standard ./configure script is preferably driven by the Makefile.PL wrapper. All options are forwarded to ./configure :

 perl Makefile.PL

If SMOKE is missing, configure will generate its sources. Then :

 make install

This will install PerlQt, Puic and Smoke (if needed), as well as the pqtsh and pqtapi utilities.

The preferred install location for SMOKE and Puic is in the KDE3 file system. If you don't have KDE3 installed, specify a location with configure's --prefix option. e.g:

 perl Makefile.PL --prefix=/usr

Troubleshooting and Configure Options

If Smoke's linking fails or your Qt library was built with very specific options, run Makefile.PL again with:

 perl Makefile.PL --with-threshold=0

When building smoke, configure will check for OpenGL and try to compile support for it if it is properly installed and supported by Qt.

You may disable this checking with:


Also, default behaviour is to prefer the Mesa GL library over a proprietary implementation. If your system features a proprietary OpenGL library, and you'd like to use it, specify:


How to install PerlQt with user rights

To install PerlQt without super-user rights, simply follow this procedure:

Anatomy of PerlQt

A typical Qt program using GUI components is based on an event loop.

This basically means that such a program is no more envisioned as a straight flow where you would need to handle yourself every single events (such as a mouse click or a key press).

Instead, you just create an Application object, create the GUI components it uses, define what objects methods need to be called when an event occurs, and then start the main event loop.

That's all! Qt will handle all events and dispatch them to the correct subroutine.

Lets see how this process is implemented in a minimal PerlQt program.

Hello World

 1: use Qt;
 2: my $a = Qt::Application(\@ARGV);
 3: my $hello = Qt::PushButton("Hello World!", undef);
 4: $hello->resize(160, 25);
 5: $a->setMainWidget($hello);
 6: $hello->show;
 7: exit $a->exec;

This program first loads the Qt interface [line 1] and creates the application object, passing it a reference to the command line arguments array @ARGV [l.2]. This application object is unique, and may later be accessed from anywhere through the Qt::app() pointer.

At line 3, we create a PushButton, which has no parent (i.e : it won't be contained nor owned by another widget). Therefore, we pass to the constructor an undef value for the parent argument, which is PerlQt's way of passing a Null pointer.

After some layouting at [l.4], we tell the application object that our main widget is this PushButton [l.5]... that way, it will know that closing the window associated with this widget means : quit the application.

Now the last steps are to make this widget visible (as opposed to hidden, which is the default) by calling the show method on it [l.6] and to start the application loop [l.7].

Syntax elements summary :

  1. All Qt classes are accessed through the prefix Qt::, which replaces the initial Q of Qt classes. When browsing the Qt documentation, you simply need to change the name of classes so that QFoo reads Qt::Foo.

  2. An object is created by calling the constructor of the class. It has the same name as the class itself.

    You don't need to say new Qt::Foo or Qt::Foo->new() as most Perl programmers would have expected.

    Instead, you just say :

     my $object = Qt::<classname>(arg_1, ..., arg_n);

    If you don't need to pass any argument to the constructor, simply say :

     my $object = Qt::<classname>;

  3. Whenever you need to pass a Null pointer as an argument, use Perl's undef keyword. Do not pass zero. Beware: this is by far the most common error in PerlQt programs.

    Pointers are arguments preceded by an * character in Qt's documentation (e.g: ``QWidget * widget'').

Inheritance and Objects

Before we can discuss how Perl subroutines can be called back from Qt, we need to introduce PerlQt's inheritance mechanism.

PerlQt was designed to couple as tightly as possible Qt's simplicity and Perl's power and flexibility.

In order to achieve that goal, the classical Object Oriented Perl paradigm had to be extended, much in the same way than Qt itself had to extend C++'s paradigm with metaobjects.

A Custom Widget

Lets rewrite the ``Hello World!'' program, this time using a custom version of PushButton:

  1: use strict;
  3: package Button;
  4: use Qt;
  5: use Qt::isa qw(Qt::PushButton);
  7: sub NEW
  8: {
  9:   shift->SUPER::NEW(@_[0..2]);
 10:   resize(130, 40);
 11: }
 13: 1;
 15: package main;
 17: use Qt;
 18: use Button;
 20: my $a = Qt::Application(\@ARGV);
 21: my $w = Button("Hello World!", undef);
 22: $a->setMainWidget($w);
 23: $w->show;
 24: exit $a->exec;

Here, we want to create our own version of the PushButton widget. Therefore, we create a new package for it [l.3] and import Qt [l.4].

We now want to declare our widget as subclassing PushButton. This is done through the use of the Qt::isa pragma [l.5], which accepts a list of one or more parent Qt classes.

It is now time to create a constructor for our new widget. This is done by creating a subroutine called NEW (note the capitalized form, which differentate it from the usual ``new'' constructor. PerlQt's NEW constructor is called implicitly as can be seen on line 21).

Since we want our widget to call its parent's constructor first, we call the superclass's constructor (here: Qt::PushButton) on line 9, passing it all arguments we received.

At this time, a class instance has been created and stored into a special object holder named this (not $this but really just this).

Each time you invoke a method from within your package, you may now indifferently say method() or this->method();

Using Attributes

When building a new composite widget, you may just create its different parts inside my variables, since widgets are only deleted by their parents and not necessarily when their container goes out of scope.

In other words, PerlQt performs clever reference counting to prevent indesirable deletion of objects.

Now, you'll often want to keep an access to those parts from anywhere inside your package. For this purpose, you may use the this object's blessed hash, as is usual in Perl, but that isn't really convenient and you don't have any compile time checking...

Here come Attributes. Attributes are data holders where you can store any kind of properties for your object.

Declaring new attributes is done through the use Qt::attributes pragma, as is demonstrated in the following package implementation :

  1: use strict;
  3: package Button;
  4: use Qt;
  5: use Qt::isa qw(Qt::PushButton);
  6: use Qt::attributes qw(
  7:     itsTime
  8:     pData
  9: );
 11: sub NEW
 12: {
 13:   shift->SUPER::NEW(@_[0..2]);
 14:   itsTime = Qt::Time;
 15:   itsTime->start;   
 16:   pData = " Foo ";
 17: }
 19: sub resizeEvent
 20: {
 21:    setText( "w: ". width() ." h: ". height() .
 22:             "\nt: ". itsTime->elapsed . pData );
 23: }
 25: 1;

An attribute itsTime is declared at line 7, and loaded with a Qt::Time object at line 14.

Since we reimplement the virtual function ``resizeEvent'' [l.19]. each time the main widget is resized, this function will be triggered and our Button's text updated with values coming from the object [l.21] and from the attributes we defined [l.22].


Signals and Slots

We'll now learn how Qt objects can communicate with each other, allowing an event occuring, for instance, in a given widget to trigger the execution of one or several subroutines anywhere inside your program.

Most other toolkits use callbacks for that purpose, but Qt has a much more powerful and flexible mechanism called Signals and Slots.

Signals and slots are used for communication between objects.

This can be thought off as something similar to the wiring between several Hi-fI components : an amplificator, for instance, has a set of output signals, wich are emitted wether a listening device is connected to them or not. Also, a tape recorder deck can start to record when it receives a signal wired to it's input slot, and it doesn't need to know that this signal is also received by a CD recorder device, or listened through headphones.

A Qt component behaves just like that. It has several output Signals and several input Slots - and each signal can be connected to an unlimited number of listening slots of the same type, wether they are inside or outside the component.

The general syntax of this connection process is either :

Qt::Object::connect( sender, SIGNAL 'mysignal(arg_type)', receiver, SLOT 'myslot(arg_type)');


myObject->connect( sender, SIGNAL 'mysignal(arg_type)', SLOT 'myslot(arg_type)');

This mechanism can be extended at will by the declaration of custom Signals and Slots, through the use Qt::signals and use Qt::slots pragma (see also the other syntax, later on).

Each declared slot will call the corresponding subroutine in your object, each declared signal can be raised through the emit keyword.

As an example, lets rewrite again our Button package :

  1: use strict;
  3: package Button;
  4: use Qt;
  5: use Qt::isa qw(Qt::PushButton);
  6: use Qt::attributes qw(itsTime);
  7: use Qt::slots 
  8:     wasClicked => [],
  9:     change     => ['int', 'int'];
 10: use Qt::signals
 11:     changeIt   => ['int', 'int'];
 13: sub NEW
 14: {
 15:   shift->SUPER::NEW(@_[0..2]);
 16:   itsTime = Qt::Time;
 17:   itsTime->start;   
 18:   this->connect(this, SIGNAL 'clicked()', SLOT 'wasClicked()');
 19:   this->connect(this, SIGNAL 'changeIt(int,int)', SLOT 'change(int,int)');
 20: }
 22: sub wasClicked
 23: {
 24:    my $w = width();
 25:    my $h = height();
 26:    setText( "w: $w h: $h\nt: ". itsTime->elapsed );
 27:    emit changeIt($w, $h);          
 28: }
 30: sub change
 31: {
 32:    my ($w, $h) = @_;
 33:    print STDERR "w: $w h: $h \n";
 34: }
 36: 1;

In this package, we define two extra slots and one extra signal.

We know from the Qt Documentation that a clicked PushButton emits a clicked() signal, so we connect it to our new slot at line 18.

We also connect our signal changeIt to our own change slot- which is quite stupid, but as an example.

Now, whenever our Button is clicked, the clicked() signal is raised and triggers the wasClicked() slot. wasClicked then proceeds to emit the changeIt(int,int) signal [l.27], hence triggering the change(int,int) slot with two arguments.

Finally, since PerlQt-3.008, an alternative syntax can be used to declare Signals and Slots:

 sub a_slot : SLOT(int, QString)
        $int = shift;
        $string = shift;
        # do something


 sub a_signal : SIGNAL(QString);

This syntax is perfectly compatible with the traditional use Qt::signals and use Qt::slots declarations.

Eventually, it can prove good programming practice to mix both syntaxes, by first declaring Signals/Slots with use Qt::slots/signals, then repeat this declaration in the actual implementation with the second syntax.

Declarations will be checked for consistency at compile time, and any mismatch in arguments would trigger a warning.

RAD prototyping with Qt Designer and Puic


As efficient and intuitive as Qt can be, building a complete GUI from scratch is often a tedious task.

Hopefully, Qt comes with a very sophisticated GUI Builder named Qt Designer, which is close to a complete integrated development environment. It features Project management, drag'n drop GUI building, a complete object browser, graphical interconnection of signals and slots, and much much more.

Qt Designer's output is XML which can be parsed by several command line tools, among whose is puic (the PerlQt User Interface Compiler).

Assuming you have already built an interface file with the Designer, translating it to a PerlQt program is simply a matter of issuing one command :

 puic -x -o program.ui

This will generate the package defined in your ui file and a basic main package for testing purposes.

You may prefer :

 puic -o program.ui

This will only generate the package, which can then be used by a separate program.

Embedding Images

If you need to embed images or icons, it can be done in two ways :

Working With .ui Files

It will often happen that you need to regenerate your user interface -either because you changed your initial design, or you want to extend it. Thus writing your program's code straight in the auto-generated Perl file is quite a bad idea. You'd run constantly the risk of overwriting your handcrafted code, or end up doing lot of copy-paste.

Instead, you may :

More development tools

PerlQt comes bundled with two simple programs that can help you to find your way through the Qt API:


pqtapi is a commandline driven introspection tool.

 usage: pqtapi [-r <re>] [<class>]
        -r <re> : find all functions matching regular expression/keyword <re>
        -i : together with -r, performs a case insensitive search
        -v : print PerlQt and Qt versions
        -h : print this help message


 $>pqtapi -ir 'setpoint.* int'
        void QCanvasLine::setPoints(int, int, int, int)
        void QPointArray::setPoint(uint, int, int)


pqtsh is a graphical shell that can be used to test the API interactively. It is fairly self explanatory and includes an interactive example (Help->Example)

Known Limitations

Templated classes aren't available yet (classes derived from templated classes are).


PerlQt-3 is (c) 2002 Ashley Winters (and (c) 2003 Germain Garand)

Kalyptus and the Smoke generation engine are (c) David Faure and Richard Dale

Puic is (c) TrollTech AS., Phil Thompson and Germain Garand,

The mentioned software is released under the GNU Public Licence v.2 or later.

Appendix 1 : C++ conventions and their Perl counterpart

Whenever you want to use a class/method described in Qt's documentation (see also the 'assistant' program bundled with Qt) from PerlQt, you need to follow some simple translation rules.

Enumerations are sort of named aliases for numeric values that would be hard to remember otherwise.

A C++ example would be :

 enum Strange { Apple, Orange, Lemon }

where Strange is the generic enumeration name, and Apple, Orange, Lemon its possible values, which are only aliases for numbers (here 0, 1 and 2).

Access to enumerations values in Perl Qt is very similar to a static function call. In fact, it is a static function call.

Therefore, since you probably want to avoid some readability problems, we recommend the use of the alternate function call syntax : &function.

Lets now go back to our Strange example.

If its definition was encountered in the class QFruits, you'd write from PerlQt :

 $apple_plus_orange = &Qt::Fruit::Apple + &Qt::Fruit::Orange;

Within PerlQt, operators overloading works transparently. If a given operator is overloaded in a Qt class (which means using it triggers a custom method) it will behave identically in PerlQt. Beware though that due to limitations of the Smoke binding library, not all overloaded operators are available in PerlQt. You can check the availability of a given operator by using the pqtapi program. Also, due to outstanding differences between C++'s and Perl's object paradigm, the copy constructor operator (a.k.a '=') has been disabled.

e.g-1: '+=' overload

 $p1 = Qt::Point(10, 10)
 $p2 = Qt::Point(30,40)
 $p2 += $p1; # $p2 becomes (40,50)
e.g-2: '<<' overload
 $f = Qt::File("example");
 $f->open( IO_WriteOnly ); # see 'Constants' below
 $s = Qt::TextStream( $f );
 $s << "What can I do with " << 12 << " apples?";

Qt doesn't use many constants, but there is at least one place where they are used : for setting Input/Output flags on files. In order to avoid the namespace pollution induced by global constants, PerlQt group them in the Qt::constants module. For instance, requesting the importation of all IO constants into the current namespace would be done with:
 use Qt::constants;

You may also import specific symbols:

 use Qt::constants qw( IO_ReadOnly IO_WriteOnly );

Global Functions
Qt has also some utilitarian functions such as bitBlt, qCompress, etc.

Those were global scope functions and have been grouped in a common namespace: Qt::GlobalSpace.

Hence, you shall access this namespace either with a fully qualified call:

 Qt::GlobalSpace::qUncompress( $buffer )

Or directly, after importation in the current namespace:

 use Qt::GlobalSpace;
 qUncompress( $buffer )

Of course, you may selectively import a few functions:

 use Qt::GlobalSpace qw( qUncompress bitBlt )

Note: GlobalSpace has also operators, such has the one performing an addition on two Qt::Point(). Those operators are called automatically.


 $p1 = Qt::Point(10, 10) + Qt::Point(20, 20)

Appendix 2 : Internationalization

PerlQt handles internationalization by always converting QString back to utf8 in Perl.

Conversions from Perl strings to QStrings are made according to context :

Once a string contains utf8, you can convert it back to any locale by setting up converters :

 $tr1=Qt::TextCodec::codecForLocale(); # this one will use current locale
 $tr2=Qt::TextCodec::codecForName("KOI8-R"); # that one forces a specific locale (Russian)
 print $tr1->fromUnicode(Qt::DateTime::currentDateTime()->toString)."\n\n";
 print $tr2->fromUnicode($my_utf8_string);

Or, with Perl >= 5.8.0, you may use Perl's Encode modules (see perldoc Encode).

disabling utf-8

Developers who don't want to use UTF-8 or want to temporarily disable UTF-8 marshalling for handling legacy programs may use the use bytes pragma (and the corresponding no bytes).

Within the scope of this pragma, QStrings are marshalled back to ISO-Latin1 (default) or to your locale (if use locale has been set).

Frivole use of this pragma is strongly discouraged as it ruins worldwide standardization efforts.

Appendix 3 : Debugging Channels

The Qt::debug module offers various debugging channels/features.

 use Qt::debug;
 use Qt::debug qw|calls autoload verbose|;

With the simple use Qt::debug statement, the verbose and ambiguous channels are activated. If you specify a list of channels within the use statement, then only the specified channels will be enabled.

Available channels :

Appendix 4 : Marshallers

A marshaller is a piece of ``glue code'' translating a given datatype to another.

Within PerlQt, most Qt objects keep their object nature, so that one may invoke methods on them. However, some classes and datatypes map so naturally to some Perl types that keeping their object nature would would feel unnatural and clumsy.

For instance, instead of returning a Qt::StringList object, which would require an iterator to retrieve its content, PerlQt will translate it to an array reference containing all the object's strings.

In the other way, instead of providing a Qt::StringList object as an argument of a method, one would simply provide the reference to an array of Perl strings.

Here is the list of Marshallers as of PerlQt-3.008 :

 float, double                         <=>       Perl real (NV)
 char, uchar, int, uint, enum
 long, ulong, short, ushort            <=>       Perl integer (IV)
 QString, -&, -*                        =>       Perl string (utf8)
 QString, -&, -*                       <=        Perl string (utf8 or iso-latin1 or locale)
 QCString, -&, -*                      <=>       Perl string (utf8 or bytes, according to content or "bytes" pragma)
 QByteArray, -&, -*                    <=>       Perl string (bytes)
 QStringList, -&, -*                    =>       Reference to an array of Perl strings (utf8)
 QString, -&, -*                        =>       Perl string (utf8 or iso-latin1 or locale)
 int&, -*                              <=>       Perl integer (IV)
 bool&, -*                             <=>       Perl boolean
 char*                                 <=>       Perl string (bytes)
 char**                                <=        Reference to an array of Perl strings (bytes)
 uchar*                                <=        Perl string (bytes)
 QRgb*                                 <=        Reference to an array of Perl integers (IV)
 QCOORD*                               <=        Reference to an array of Perl integers (IV)
 void*                                 <=>       Reference to a Perl integer (IV)
 QValueList<int>, - *, - &             <=>       Reference to an array of Perl integers (IV)
 QCanvasItemList, - *, - &              =>       Reference to an array of Qt::CanvasItem
 QWidgetList, - *, - &                 <=>       Reference to an array of Qt::Widget
 QObjectList, - *, - &                 <=>       Reference to an array of Qt::Object
 QFileInfoList, - *, - &               <=>       Reference to an array of Qt::FileInfo
 QPtrList<QTab>, - *, - &              <=>       Reference to an array of Qt::Tab
 QPtrList<QToolBar>, - *, - &          <=>       Reference to an array of Qt::ToolBar
 QPtrList<QNetworkOperation>, - *, - & <=>       Reference to an array of Qt::NetworkOperation
 QPtrList<QDockWindow>, - *, - &       <=>       Reference to an array of Qt::DockWindow