Using the default constructor QDir() defaults to the current working directory, which can be anything, depending from where the user or the system started the application. For a UI application, that's a path one should usually ignore completely.
To access data next to your application binary, use QCoreApplication::applicationDirPath(). This is usually used for global read-only data installed with the program.
For user-writable configuration settings and cached data, use QStandardPaths (Qt 5) or QDesktopServices::storageLocation() (Qt 4).