Some nodes execute user-provided body objects. These objects can be created by instantiating function objects or lambda expressions. The nodes that use body objects include continue_node, function_node, source_node and multifunction_node.
The body objects passed to the flow graph nodes are copied. Therefore updates to member variables will not affect the original object used to construct the node. If the state held within a body object must be inspected from outside of the node, the copy_body function can be used to obtain an updated copy.
The source_node has a one-item buffer which may contain a value obtained from executing the node's body. If the source_node has emitted N items, the state of the body returned by copy_body may indicate it has been called N + 1 times.