Compiling a Qt program fails with error messages looking something like this: undefined reference to `_vt.11QPushButton'

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Answer:

This usually means you have two different versions of the Qt library (libqt.so) installed on your system, and they interfere with each other. Either modify the compiler's library search path so that it finds your new Qt library first, or simply remove the old one.

It may also indicate that your Qt library was compiled with a compiler that is incompatible with the one you are using, e.g. gcc vs. egcs.

If you get this error trying to run a pre-compiled Qt program, it usually means that the program has been compiled with a version of Qt that is newer than the one on your system. Upgrade to the latest version of Qt.