C++ tbb flow graph, multifunction_node giving incomplete type error

Far as I understand, incomplete type error happens when there's forward declaration without full specification of type, then instantiate that type since the compiler can't tell the size of such type. In my case I don't understand how this code produces such a problem. For simplicity I've removed unrelated parts.

class FilterOnNode {
   struct PartialResult { int n1,n2; long e; };
   typedef PartialResult* InputResult;
   typedef tbb::flow::multifunction_node<InputResult, InputResult> FilterNodeType;

   void BuildGraph(tbb::flow::graph &g, tbb::flow::graph_node &src) {
       auto sampleNode = FilterNodeType(
                g,
                tbb::flow::concurrency::unlimited,
                [&](const InputResult &input, typename FilterNodeType::output_ports_type &op) { //error points to here
                    //some logic here... might not send message.
                    std::get<0>(op).try_put(input);
                });
       tbb::flow::make_edge(src, sampleNode);
       //create couple other nodes like previous then connect edges
       tbb::flow::make_edge(tbb::flow::output_port<0>(sampleNode), otherNode);
   }

compiler is giving error of incomplete type at the lambda expression. Exact error says:

/usr/local/include/tbb/flow_graph.h:1568:7: error: incomplete type ‘std::tuple_size<FilterOnNode::PartialResult*>’ used in nested name specifier class multifunction_node

Isn't the type PartialResult fully specified already? Tried taking it out of the class but still same problem.

Solution

Isn't the type PartialResult fully specified already?

Yes, but that is not what the error message complains about. The error message should be read as "error: incomplete type ‘[name of the incomplete type]’ used [place where it is used]". The incomplete type is std::tuple_size<FilterOnNode::PartialResult*>, not PartialResult. There is no definition of std::tuple_size<T> when T is a pointer (e.g. PartialResult*).

The location of the use of the incomplete type has something to do with class multifunction_node. I looked around for the header defining this and found the following. (I don't know why this information was not in the API documentation that I found.)

tbb/flow_graph.h, beginning of the definition of multifunction_node:

//! implements a function node that supports Input -> (set of outputs)
// Output is a tuple of output types.
template<typename Input, typename Output, typename Policy = queueing>
class multifunction_node :

So the Output parameter has to be a tuple of output types, yet you supplied a raw output type. Try changing your definition of FilterNodeType to the following.

typedef tbb::flow::multifunction_node<InputResult, std::tuple<InputResult>> FilterNodeType;
//                                                 ^^^^^^^^^^^           ^

It might be possible to replace std::tuple with tbb::flow::tuple, but I did not look into how the TBB tuple is to be used.