从Qt4.x 迁移到Qt5问题汇总

若是想把Qt4.x上面开发的软件在Qt5上面正常运行，若是什么都不作的话估计会出现各作各样的错误，笔者也是经历过这种迷茫痛，后面才发现官方已有系统发文一一说起到诸多迁移会遇到的问题以及解决办法。附上官网地址，以供参考http://qt-project.org/wiki/Transition_from_Qt_4.x_to_Qt5

为了提升开发者的开发效率，我对这个文档大体作一个翻译，也算为你们提供中文版的解决方案吧，若有异议，请参考原文。

1. QtWidgets 是一个独立的模块

例如，编译出现以下错误

    error: QMainWindow: No such file or directory

    error: QToolButton: No such file or directory

    error: QWidget: No such file or directory

解决方法：

在工程的 *.pro文件中添加下面内容：

    QT += widgets

同时把头文件

    #include <QtGui>

更改成

    #include <QtWidgets>

这样代码就能够正常运行了，不过有时候须要添加更详细的头文件，例如

    #include <QtWidgets/QtoolButton>

2.QtWebKitWidgets也是一个独立的模块

例如，编译的时候出现以下错误:

    error: invalid use of incomplete type 'class QWebFrame'

    error: forward declaration of 'class QWebFrame'

解决方法：

在工程的 *.pro 文件中添加：

    QT += webkitwidgets

备注：当已经有QT += webkitwidgets ，就不须要添加 QT += widgets

同时把工程文件里面添加的全部

    #include <QtWebKit>

更改成

    #include <QtWebKitWidgets>

3.QPrinter 不起做用

若是你已经添加了以下两行

    #include <QPrinter>

    #include <QPrintDialog>

在工程的 *.pro 文件中添加：

    QT += printsupport

若是仍是不能工做，把添加的

    #include <QPrinter>

    #include <QprintDialog>

更改成

    #include <QtPrintSupport/QPrinter>

    #include <QtPrintSupport/QPrintDialog>

4.没有toAscii() 和 fromAscii()方法

把

    fromAscii()

    toAscii()

替换成

    fromLatin1()

    toLatin1()

例如，在 Qt 4 上的代码

    QByteArray configfileti = TMP_Config.toAscii();

更改为

    QByteArray configfileti = TMP_Config.toLatin1();

5.去掉了QCoreApplication::UnicodeUTF8 

由于Qt5都是采用 UnicodeUTF8的编码方式，因此 UnicodeUTF8这个枚举类型就显得有点画蛇添足了，因此删掉了全部的关于QCoreApplication::UnicodeUTF8参数的传递。

例如：

    Href_Gui->setWindowTitle(QApplication::translate("Href_Gui", "Url / www", 0, QApplication::UnicodeUTF8));

    label->setText(QApplication::translate("Href_Gui", "Text:", 0, QApplication::UnicodeUTF8));

    label_2->setText(QApplication::translate("Href_Gui", "Url:", 0, QApplication::UnicodeUTF8));

    label_3->setText(QApplication::translate("Href_Gui", "Target / Name:", 0, QApplication::UnicodeUTF8));

更改成：

    Href_Gui->setWindowTitle(QApplication::translate("Href_Gui", "Url / www", 0));

    label->setText(QApplication::translate("Href_Gui", "Text:", 0));

    label_2->setText(QApplication::translate("Href_Gui", "Url:", 0));

    label_3->setText(QApplication::translate("Href_Gui", "Target / Name:", 0));

6.去掉了QWorkspace类

在Qt4.3中，这个类就已经被 QMdiArea类替换掉了，在Qt5中把QWorkspace类正式删除掉了。QMdiArea类和QWorkspace类有相似的API接口，只是须要更改下一些方法，信号和槽的接口名字便可。

替换

    #include <QWorkspace>

为

    #include <QMdiArea>

7.QDrag类问题

应用的拖拽功能或许须要一些微调，例如

    QDrag *drag = new QDrag(event->widget());

在 Qt 5 会有以下错误

    error: no matching function for call to 'QDrag::QDrag(QWidget*)'

加上以下头文件或许能够解决如上错误

    #include <QWidget>

8.删掉了qFindChildren

若是遇到以下错误

    error: 'qFindChildren' was not declared in this scope

解决办法

用findChildren 方法替代qFindChildren ，须要注意的是，调用方法的方式略有改变。参考以下例子：

    toString(const QObject* obj, int indentLevel) const {

    [...]

        /* Query over QObjects */

        if (m_children) {

            QList<QObject*> childlist = qFindChildren<QObject*>(obj, QString());

    [...]

把上述例子中的以下代码

    QList<QObject*> childlist = qFindChildren<QObject*>(obj, QString());

替换成以下代码

    QList<QObject*> childlist = obj->findChildren<QObject*>(QString());

9.删掉了qVariantValue 

编译时报以下错误

    error: 'qVariantValue' was not declared in this scope

这个函数等同于QVariant::value<T>(value)。所以能够把以下代码

    QTime t = qVariantValue<QTime>(val);

更改成

    QTime t = val.value<QTime>();

This QTime enclosed in the angled brackets lets the compiler know what QVariant will return. However, if the variable is not a QVariable the type enclosed in the angled brackets should not be used(doing so will result in a vague compile time error). So given that m_color is of type QColor you will rewrite

    s.setValue("color/favorite", qVariantValue<QColor>(m_color));

to

    s.setValue("color/favorite", m_color.value());

10.删除了qVariantCanConvert

替换

    Q_ASSERT(qVariantCanConvert<QString>(variant));

    Q_ASSERT(qVariantCanConvert<QSize>(variant));

    Q_ASSERT(qVariantCanConvert<QFont>(fontVariant));

为

    Q_ASSERT(variant.canConvert(QMetaType::QString));

    Q_ASSERT(variant.canConvert(QMetaType::QSize));

    Q_ASSERT(fontVariant.canConvert(QMetaType::QFont));

11.删掉了Qt::escape 

    error: 'escape' is not a member of 'Qt'

So you would change the following block:

        if (result == QString())

            result = Qt::escape(val.toString());

        else

            result = Qt::escape(result);

        return result;

to

        if (result == QString())

            result = QString(val.toString()).toHtmlEscaped();

        else

            result = QString(result).toHtmlEscaped();

        return result;

12.删掉了QDesktopServices::storageLocation 

    error: 'storageLocation' is not a member of 'QDesktopServices'

    error: 'DataLocation' is not a member of 'QDesktopServices'

用 QStandardPaths::StandardLocation代替，把以下

    QString path = s.value("db.path", QDesktopServices::storageLocation(QDesktopServices::DataLocation)).toString();

替换为

    QString path = s.value("db.path", QStandardPaths::standardLocations(QStandardPaths::DataLocation)).toString();

13.删掉了CONFIG+=qtestlib 

If you have the above line in your project file the compiler will warn you in the compile window, nonetheless the code will still run as usual:

若是在工程文件中有上面这一行，编译器将在编译窗口中产生警告：

    Project WARNING: CONFIG+=qtestlib is deprecated. Use QT+=testlib instead.

14.QWeakPointer quirks

A code block like

    quint64 decodedPointer = line.toULongLong();

    MetaData* md = reinterpret_cast<MetaData*>(decodedPointer);

    QWeakPointer<MetaData> wp(md);

results in

    error: no matching function for call to 'QWeakPointer<MetaData>::QWeakPointer(MetaData*&)'

To fix this add to the project file:

    DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0

15.QtConcurrent Library is Missing?

    C:\Qt\Qt5.0.2\5.0.2\mingw47_32\include\QtConcurrent\qtconcurrentthreadengine.h:133: error: undefined reference to `_imp___ZN12QtConcurrent16ThreadEngineBaseD2Ev'

In Qt 4, QtConcurrent was part of QtCore, so there was no need to include specific headers. This is no longer the case with Qt 5. If your source code have lines like

    m_current = QtConcurrent::blockingMappedReduced(slices, functor, stitchReduce, QtConcurrent::UnorderedReduce );

You will need to include the header:

    #include <QtConcurrent/QtConcurrent>

and add the following line to your project file:

    LIBS += -lQt5Concurrent

16.Fixing #include<> Headers

A Perl script “fixqt4headers.pl” exists in qtbase/bin/. that should be run on source code using Qt that corrects the #include<> directives for Qt components to also consider the module name.

17.Plugin loading

The Q_EXPORT_PLUGIN,Q_EXPORT_PLUGIN2 macros have been deprecated in favor of the new Q_PLUGIN_METADATA macro. The advantage of the new system is that it allows Qt to query the metadata for the plugin without actually dlopen’ing it. This greatly improves performance and reliability of the plugin system.

The new Q_PLUGIN_METADATA macro is included next to the Q_OBJECT macro in the QObject derived class that is returned when loading the plugin. It contains the plugins IID and a filename pointing to a json file containing the metadata for the plugin. The json file is compiled into the plugin and does not need to be installed.

An example on how to change your plugins can be found by looking at the patch that changes the Gif p_w_picpath format plugin, see http://qt.gitorious.org/qt/qtbase/commit/963b4c1647299fd023ddbe7c4a25ac404e303c5d .

18.Deploying to systems without C++11

When Qt is built from source code on a system with C++11 installed, the Qt libraries/frameworks are linked against the system’s C++11 library (libc++). This means that the Qt libraries/frameworks are not deployable to systems without C++11 installed (such as out-of-the-box Mac OS X 10.6). To be able to deploy to systems that only support the older C++ standard (libstdc++), build Qt from source code with the -no-c++11 configure option.

19.QTimer is no longer accurate to the millisecond by default

QTimer has now 3 accuracy types, with a new default behaviour:

    The new default type is Qt::CoarseTimer which, to reduce power/CPU consumption, allow 5% difference between requested time and actual one, and even allow the timer to fire before the requested time.

    The former one is Qt::PreciseTimer (to the millisecond, never before the requested time).

    A third one is Qt::VeryCoarseTimer and allow a 1 second difference

20.QUrl addQueryItem moved to QUrlQuery

If you have:

    QUrl url;

    // ...

    url.addQueryItem(key, value);

You will want to change it to

    QUrl url;

    QUrlQuery urlQuery;

    // ...

    urlQuery.addQueryItem(key, value);

     

    url.setUrlQuery(urlQuery);

21.QAbstractItemModel changes

    void reset()

    void setRoleNames(const QHash<int, QByteArray> & roleNames)

both have changed and are now protected.

See Compatibility Members for QAbstractItemModel [qt-project.org]

Recommended Reading

    C++ API Changes [qt-project.org]

    The porting guide [qt-project.org]

    Porting Desktop Applications from Qt 4 to Qt 5 [blog.ics.com]

    Porting from Qt 4 to Qt 5 [kdab.com]

    Automated porting from Qt 4 to Qt 5 [kdab.com]

Categories:

    Developing_Qt

    Developing_with_Qt