This isn't a hard requirement that we follow in Mixxx (sometimes we actually do want to share references of something between classes, e.g. DAOs) but in a case like this where you are just using the reference to output something from the method, consider only using const references for inputs and pointers for outputs and order the inputs before the outputs in the argument list.

http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Function_Parameter_Ordering#Function_Parameter_Ordering