'Undefined Reference' while linking

Even if compiling an algorithm / tool may work without error, the linker may complain about 'undefined reference to `LHCb::aClass::aMethod(some parameters)'.

This error message occurs, if the .h-file is found, but the libraries in the path do not contain the binaries for the class. There are two possible reasons:

• The corresponding library is not included in the requirements file. You then have to add to your requirements file something like 'use packageGroup v* package'.
• The corresponding library is somehow corrupt, does not correspond to the actual version of the project, …. There is no easy way out (recompile the library?).