DDSPSMC11_ — Define relationship between Query and QueryCondition

The current DDS-PSM-Cxx spec distinguishes between Queries and QueryConditions. The former can be used in a Selector, the latter can be used in a WaitSet. However, from the end-user perspective it would make sense that if I want my WaitSet to trigger on incoming data matching my QueryCondition, that I then want to consume the data matching my Query in some way.

According to the current specification I would need to instantiate both a Query and a QueryCondition based on the same expression. If I then want to modify my Query parameters, I would need to do so consistently for both my Query and my QueryCondition.

I propose to make QueryCondition extend from Query, and make Query extend from Reference instead of Value. That allows us to store a reference to our internal query state (e.g. to the parse-tree) into the Query object instead of having to redo the whole thing over and over again.