How to make AnalyticPartitionWriter produce arbitrary fixed-point numeric data?

You should be able to create a VNumeric and pass the pointer in the C++ SDK:
VNumeric * getColPtrForWrite (size_t idx)

Thanks. I'm struggling to get that function to work though, getting strange values like 1402984621720.88. Probably I'm using it wrong. Do you have an example of how it should be used? Attaching a reproducer.

dbadmin@pc=>with nums as (select 12.5::numeric(8,2) as num) select *, numeric_test(num) over () from nums;
  num  |     ?column?
-------+------------------
 12.50 | 1402984621720.88
(1 row)

Ok, figured it out with the help of InvertedIndex.cpp example from the SDK.

class numeric_test : public AnalyticFunction
{
    virtual void processPartition(ServerInterface &srvInterface, AnalyticPartitionReader &inputReader,
                                    AnalyticPartitionWriter &outputWriter)
    {   
        const SizedColumnTypes &inTypes = inputReader.getTypeMetaData();
        vector<size_t> argCols;
        inTypes.getArgumentColumns(argCols);

        const SizedColumnTypes &outTypes = outputWriter.getTypeMetaData();
        vector<size_t> outCols;
        outTypes.getArgumentColumns(outCols);

        do {
            const VNumeric* inVal = inputReader.getNumericPtr(argCols.at(0));
            char* buff = new char[64];
            inVal->toString(buff, 64);
            srvInterface.log("string value of inVal: %s", buff);

            VNumeric &outNum = outputWriter.getNumericRef(outCols.at(0));
            outNum.copy(inVal);

            outputWriter.next();
        } while (inputReader.next());
    }

    virtual void setup(ServerInterface &srvInterface, const SizedColumnTypes
                        &argTypes)
    { }

    virtual void destroy(ServerInterface &srvInterface, const SizedColumnTypes
                        &argTypes)
    { }
};

Just to point out, conversions to/from string are outrageously expensive.
I remember solving problem, by creating var of required datatype, for example NUMERIC(10,2). Then, overriding 8-byte array with my value. That performs quite reasonable.