Working with YQL-Stream in a C++ UDF

YQL-Stream is a single-pass iterator. In YQL, it can be obtained at the PROCESS or REDUCE output (see the YQL documentation).

In C++, YQL-Stream is a TBoxedValue subclass with the overridden Fetch() method.

class TMyStreamProvider: public TBoxedValue {
public:
    TMyStreamProvider(...) {
        ...
    }

    TMyStreamProvider() = delete;
    TMyStreamProvider(const TMyStreamProvider&) = delete;
    virtual ~TMyStreamProvider() = default;
    void operator=(const TMyStreamProvider&) = delete;
    TMyStreamProvider(TMyStreamProvider&& other) = default;
    TMyStreamProvider& operator=(TMyStreamProvider&& other) = default;

    EFetchStatus Fetch(TUnboxedValue& result) override;
};

Fetch() can return three statuses:

• EFetchStatus::Ok

  Indicates successful writing of a value to the result.

• EFetchStatus::Finish

  Indicates the end of the stream, the result remains unchanged.

• EFetchStatus::Yield

  Special status. If a UDF has no YQL-Stream at the input, your stream will never generate this status. If the EFetchStatus::Yield status appears when reading the input stream, you can write 0 or more values to the output stream but the EFetchStatus::Yield status should still be generated and returned.

Remember that after the UDF returns a stream in C++ code, whatever was in the Run() body is destroyed by the destructor. So you need to pass everything through the constructor to the Stream class to make it the owner of all resources.