Some nodes create or use messages that are composites of other messages. The flow::tuple template class is included as part of flow to support this. The nodes that send or receive tuples are join_node, multifunction_node, split_node and or_node (Community Preview Feature).
The tuple class is part of the C++11 standard, and earlier implementations of the Standard Library may have a tuple class. If std::tuple is part of the Standard Library, then flow::tuple is typedefed to std::tuple.
If the Standard Library does not contain tuple, an implementation is used that supports a subset of std::tuple's functionality:
Prior releases of flow injected flow's implementation of tuple into the std:: namespace. Users should change flow-specific references to std::tuple to flow::tuple to ensure compatibility with compilers that do not implement std::tuple as part of the Standard Library.