A number of guides and documents are available that cover porting issues, from detailed coverage of API differences between Qt 3 and Qt 4 to platform and tool-specific documentation.
Porting advice for authors of new and existing Qt 3 applications.
Information about changes to the UI file format in Qt 4.
Hints and tips to assist with porting canvas applications to the Graphics View framework.
An overview of issues and techniques to consider when porting from Qt 3 to Qt 4.
An overview of the porting process for applications that use drag and drop.
An overview of changes to virtual functions in Qt 4.
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