libunifex Structured Concurrency Implementation Analysis

This article provides an in-depth analysis of the structured concurrency implementation in libunifex, a C++ asynchronous programming library. The content covers the DSL-based design of execution framework and its three main components: Sender Factory, Sender Adapter, and Receiver.

The article begins with an introduction to structured concurrency using a pipeline-style example: schedule(tcontext.get_scheduler()) | then([&] { ++count; }) | sync_wait(); . This demonstrates how execution uses a DSL approach where the pipeline consists of Sender Factory { '|' Sender Adapter } '|' Receiver .

Section two focuses on Sender Factory implementation, using just() as an example. A Sender Factory CPO produces Senders that trigger completion notifications via set_value , set_error , or set_done receiver CPOs. The implementation includes three parts: the CPO entry point ( just_cpo::operator() ), the sender type definition with value_types and error_types, and the OperationState that stores values and invokes set_value() on the connected Receiver during start() .

Section three explores Sender Adapter implementation using then() as an example. A Sender Adapter wraps a preceding Sender to transform its async results before passing them to subsequent nodes. The implementation requires: an entry CPO (then()), an Internal Sender that stores the predecessor Sender and function, and an Internal Receiver that intercepts set_value() from the predecessor, applies the transformation function, and forwards results via set_value() to the next node.

Section four examines sync_wait() and sync_wait_r() , which serve as terminal nodes to synchronously wait for async operation results. These utilities create a custom Receiver that captures results through set_value / set_error / set_done , then use connect() and start() to execute the async pipeline and return results wrapped in std::optional .

The article concludes that understanding structured concurrency is essential for comprehending libunifex's node implementations and provides a foundation for building custom business-specific nodes.