Feature request: optional_node

[Lawrencemm]

Hello, my flow graph usage requires some nodes to conditionally send messages to their successors.

Currently I achieve this by using a single-output multifunction_node and conditionally sending to the output port.

I think this could more elegantly be handled with a new node type. Its usage would be identical to function_node with one difference: the return type for body would be an optional<output_type>.

If the body returns nullopt then a message is not sent to successors.

[Gerrard-YNWA]

Also want this feature. Is there any progress about this?

[knoepfel]

I've run into needing this feature as well, and am planning on using the same solution that @Lawrencemm proposed above in the meantime. If I were to create a PR that implemented this (I'd probably call it something like flow::filter_node<Input, Output>), would the oneTBB developers be interested in adopting it?