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# Getting Started with Qt 4

This document covers the most important differences between Qt 3 and Qt 4. Although it is not intended to be a comprehensive porting guide, it tells you about the most important portability issues that you may encounter. It also explains how to turn on Qt 3 compatibility support, and provides a brief overview of some Qt 4 features beyond the five main technologies in this preview.

- Build System
- Include Syntax
- Namespaces
- QObject/QWidget Constructors
- Dynamic Casts
- QPointer<T>
- Paint Events
- Qt 3 Compatibility

## Build System

Unlike previous Qt releases, Qt 4 is a collection of smaller libraries.

- QtCore: core non-GUI functionality
- QtGui: core GUI functionality
- QtNetwork: network module
- QtOpenGL: OpenGL module
- QtSql: SQL module
- QtXml: XML module
- Qt3Compat: Qt 3 compatibility classes (see Qt 3 Compatibility below)

QtCore contains tool classes like QString, QList, and QFile, as well as kernel classes like QObject and QTimer. A few classes have been refactored so that they can be used in non-GUI applications:

- QApplication is split into QCoreApplication (in QtCore) and QApplication (in QtGui).
- QVariant is split into QCoreVariant and QVariant.

This split makes it possible to develop server applications using Qt without linking in any unnecessary GUI-related code and without requiring GUI-related system libraries to be present on the target machine (e.g. Xlib on X11, Carbon on Mac OS X).

If you use qmake to generate your makefiles, qmake will by default link your application against QtCore and QtGui. To remove the dependency upon QtGui, add the line

        QT -= gui

to your .pro file. To enable the other libraries, add the line

        QT += network opengl sql compat

Another change to the build system is that moc now understands preprocessor directives. qmake automatically passes the defines set for your project (using "DEFINES +=") on to moc, which has its own built-in C++ preprocessor.

To compile code that uses .ui files, you will also need this line in the .pro file:

        CONFIG += uic3

## Include Syntax

The syntax for including Qt class definitions has become

        #include <QClassName>

For example:

        #include <QString>
        #include <QApplication>
        #include <QSqlTableModel>

This is guaranteed to work for any public Qt class. The old syntax,

        #include <qclassname.h>

still works, but we encourage you to switch to the new syntax.

If you attempt to include a header file from a library that isn't linked against the application, this will result in a compile-time warning (e.g., "QSqlQuery: No such file or directory"). You can remedy to this problem either by removing the offending include or by specifying the missing library in the QT entry of your `.pro` file (see Build System above).

To include the definitions for all the classes in a library, simply specify the name of that library. For example:

        #include <QtCore>

## Namespaces

Qt 2 introduced a class called Qt for global-like constants (e.g., `Qt::yellow`). The C++ namespace construct was not used because not all compilers understood it when it was released.

With Qt 4, the Qt class has become the Qt namespace. If you want to access a constant that is part of the Qt namespace, prefix it with `Qt:`: (e.g., `Qt::yellow`), or add the directive

        using namespace Qt;

at the top of your source files, after your `#include` directives. If you use the "using namespace" syntax you don't need the prefix (e.g., yellow is sufficient).

When porting Qt 3 applications, you may run into some source compatibility problems with some of these symbols. For example, in Qt 3, it was legal to write `QWidget::yellow` instead of `Qt::yellow`, because QWidget inherited from Qt. This won't work in Qt 4; you must write `Qt::yellow` or add the "using namespace" directive and drop the `Qt:`: prefix.

The qt3to4 porting tool automates this conversion.

## QObject/QWidget Constructors

In Qt 4 we have tried to simplify the constructors of QObject/QWidget subclasses. This makes subclassing easier, at the same time as it helps make the Qt library more efficient.

Constructors no longer take a "const char *name" parameter. If you want to specify a name for a QObject, you must call QObject::setObjectName() after construction. The object name is now a QString.

QWidget's WFlags data type has been split in two: Qt::WFlags specifies low-level window flags (the type of window and the frame style), whereas Qt::WidgetAttribute specifies various higher-level attributes about the widget (e.g., WA_StaticContents). Widget attributes can be set at any time using QWidget::setAttribute(); low-level window flags can be passed to the QWidget constructor or set later using QWidget::setParent(). As a consequence, the constructors of most QWidget subclasses don't need to provide a WFlags parameter.

The parent parameter of all QObject classes in Qt defaults to a null pointer, as it used to do in Qt 1. This enables a style of programming where widgets are created without parents and then inserted in a layout, at which point the layout automatically reparents them.

## Dynamic Casts

Qt 4 provides a qt_cast<>() function that performs a dynamic cast based on the meta-information generated by moc for QObject subclasses. Unlike the standard C++ dynamic_cast<>() construct, qt_cast<>() works even when RTTI is disabled, and it works correctly across DLL boundaries.

Here's the Qt 3 idiom to cast a type to a subtype:

        // DEPRECATED
        if (obj->inherits("QPushButton")) {
            QPushButton *pushButton = (QPushButton *)obj;
            ...
        }

The Qt 4 idiom is both cleaner and safer, because typos will always result in compiler errors:

        QPushButton *pushButton = qt_cast<QPushButton *>(obj);
        if (pushButton) {
            ...
        }

## QPointer<T>

The QPointer<T> class provides a pointer to type T (where T inherits from QObject) that is automatically set to 0 when the referenced object is destroyed. Guarded pointers are useful whenever you want to store a pointer to an object you do not own.

Example:

        QLabel *label = new QLabel;
        QPointer<QLabel> safeLabel = label;
        safeLabel->setText("Hello world!");
        delete label;
        // safeLabel is now 0, whereas label is a dangling pointer

QPointer<T> is more or less the same as the old QGuardedPtr<T> class, except that it is now implemented in a much more lightweight manner than before. The cost of one QPointer<T> object is now approximately the same as that of a signal--slot connection.

## Paint Events

Qt 4 supports double buffering transparently on all platforms. This feature can be turned off on a per-widget basis by calling QWidget::setAttribute(Qt::WA_PaintOnScreen).

A consequence of this is that all painting must now be done from the paintEvent() function. This is also required by the HIView API on Mac OS X. In practice, this is seldom a problem, since you can call update() from anywhere in your code to create a paint event, with the region to update as the argument.

To help porting, QWidget supports a Qt::WA_PaintOutsidePaintEvent attribute that can be set to make it possible to paint outside paintEvent() on Windows, X11, and Qt/Embedded.

## Qt 3 Compatibility

Qt 4 provides an extension library that applications based on Qt 3 can link against. This allows for more compatibility than ever before, without bloating Qt.

- Classes that have been replaced by a different class with the same name, such as QListView, and classes that no longer exist in Qt 4 are available with a `3` in their name (e.g., Q3ListView, Q3Accel).
- Other classes provide compatibility functions. Most of these are implemented inline, so that they don't bloat the Qt libraries.

To enable the compatibility classes and functions, add the line

        QT += compat

to your `.pro` file.

On Visual C++ 7 and GCC 3.2+, using compatibility function results in a compiler warning (e.g., "'find' is deprecated"). If you want to turn off that warning, add the line

        DEFINES += QT_COMPAT

to your `.pro` file.

If you want to use compatibility functions but don't want to link against the compatibility library, add the line

        DEFINES += QT_COMPAT_WARNINGS

or

        DEFINES += QT_COMPAT

to your `.pro` file, depending on whether you want compatibility function calls to generate compiler warnings or not.

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