Boost Asio Pro

Boost.Asio / standalone Asio

Overview

Write async C++ networking code that compiles on the user's Boost, not the newest one. Asio's API changed shape three times (classic io_service → io_context → C++20 coroutines) and most Asio code on the internet is from the first era, so pick the style from the toolchain first, then follow that style's reference file.

References: Boost.Asio · standalone Asio

Asio's API changed shape three times, so the same task has three correct answers depending on the Boost version in front of you. This skill makes the agent establish that version first, then apply the rules that are genuinely easy to get wrong — strand versus write serialization, buffer and connection lifetime, composed reads for framing — and finally check its own output against a list before calling it done.

When to Use This Skill

• Use when writing or reviewing async C++ networking code with Boost.Asio or standalone Asio: TCP/UDP servers and clients, SSL/TLS streams, timers, resolvers.
• Use when the code involves io_context, io_service, co_spawn, awaitable, async_read, async_write, strand, asio::spawn, yield_context, or completion-handler callbacks.
• Use when the target toolchain is old: an older Boost or a pre-C++20 standard, where coroutine examples will not compile.
• Use when async code compiles but misbehaves: interleaved writes, dangling buffers, sockets closing early, operation_aborted treated as an error.

Step 1: pick the style (do this before writing code)

Determine the Boost (or Asio) version and the C++ standard actually in use — find_package(Boost) output, dpkg -l libboost-dev, brew info boost, CMAKE_CXX_STANDARD, or ask. Do not assume the newest.

SSL/TLS in any style: references/ssl.md. CMake for any style: references/build.md.

io_context, make_strand, bind_executor, steady_timer, signal_set, async_read/async_write/async_read_until, buffers and resolver are library features — identical in the coroutine and callback styles. Only the suspension mechanism differs.

Step 2: version floors (verified by compiling, not from docs)

Reach for one of these and the build breaks on older distros:

Distro floors that bite: Debian bookworm ships Boost 1.74 (no awaitable_operators.hpp — #include fails outright), Ubuntu 20.04 ships 1.71 (no any_io_executor), Debian 9 ships 1.62.

Language, not library: the chrono literals 250ms / 30s are C++14. For a true C++11 build write std::chrono::milliseconds(250).

Step 3: the rules that are actually easy to get wrong

A strand does not serialize writes. A strand serializes handler execution, not whole composed operations. Two async_writes in flight on the same strand still interleave bytes on the wire. Full-duplex (a read loop plus concurrent pushes/replies) needs a per-connection strand and an outbound queue with an in-flight flag, so at most one async_write exists at a time. This is the single most common wrong answer about Asio.

Buffers do not own memory. asio::buffer() is a view. Storage must outlive the operation: coroutine locals are fine across co_await in the same frame; in callback style the same data must become a member, not a local.

Connections must outlive their handlers. enable_shared_from_this, and capture self in every co_spawn / handler — read loop, write loop, and each timer.

Frame with composed reads. async_read (fills the buffer exactly) for a length prefix and then the body; never async_read_some, which returns short.

Wrap as_tuple. Always as_tuple(use_awaitable). Bare as_tuple resolves against the operation's default token and compiles in some contexts, fails in others.

async_accept(make_strand(...)) changes two things: it forces an explicit completion token back on the call, and the accepted socket is basic_stream_socket<tcp, strand<...>>, not tcp::socket. Take it by value or with auto — binding it to tcp::socket& will not compile.

Re-arming a timer resolves the pending wait with operation_aborted. In an idle-timeout loop that is the signal to keep waiting, not an error.

GCC needs -fcoroutines for the C++20 style, and header-only Boost needs BOOST_ERROR_CODE_HEADER_ONLY defined in exactly one place (CMake).

Common mistakes