Creating a UDAF written in C++

Note: Unlike a UDSF or UDTF, which can use the instantiate method to initialize class variables for use during the entire invocation period of the function (and across multiple rows of data), the class variables of a UDAF are localized to each phase. The best way for a UDAF to achieve a similar effect is for it to use something like Extended Scratchpad or the Aggregate States itself to share common variables.