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

Use this skill whenever async C++ networking code is being written or reviewed — and especially when the target toolchain is old, where coroutine examples simply will not compile. The three worked implementations it references are CI-verified from Boost 1.62 (2016) through 1.90.

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.

Boost	C++ std	Style	Read
≥ 1.77	C++20	Coroutines (co_await + awaitable<T>) — preferred	references/coroutines.md
≥ 1.74	C++11–17	Completion handlers (callbacks) — the portable baseline	references/pre-cpp20.md
≥ 1.80	C++11–17	Stackful asio::spawn + yield_context (links Boost.Coroutine — not header-only)	references/pre-cpp20.md
1.62–1.65	C++11	Classic io_service / strand.wrap / expires_from_now	references/classic-boost.md

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:

Feature	Floor
experimental/awaitable_operators.hpp (the || / && operators)	Boost ≥ 1.77 / Asio ≥ 1.20
as_tuple completion token	Boost ≥ 1.79 / Asio ≥ 1.21
co_composed (custom composed ops)	Boost ≥ 1.85 / Asio ≥ 1.30
3-arg asio::spawn(ex, fn, token)	Boost ≥ 1.80 (older Boost has only spawn(ex, fn))
any_io_executor (strand<any_io_executor>, tcp::socket's default executor)	Boost ≥ 1.74 — the floor for the callback style; below it, use legacy io_context::strand
io_context, make_strand, expires_after	Boost ≥ 1.66 — below it, classic io_service

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).

Anti-Patterns

Mistake	Fix
Buffer dangling (local goes out of scope during async op)	Ensure buffer lifetime ≥ operation lifetime; coroutine locals or members, not callback locals
Forgetting io.run()	No handlers dispatch without run() / run_one()
Concurrent socket access without strand	Wrap in strand<> or serialize via one coroutine chain
Assuming a strand prevents interleaved writes	Add a write queue — see Step 3
Using use_awaitable where deferred suffices	Omit the token (default is deferred) unless using || / &&
Ignoring short reads/writes	Use composed async_read / async_write / async_read_until, not async_read_some
Not setting reuse_address on the acceptor	Set before bind/listen or restarts hit "address in use"
SSL operations without a strand	All ssl::stream ops need strand synchronization
Blocking inside a handler	Never block in a completion handler
Accepting a socket with the wrong executor type	See async_accept(make_strand(...)) in Step 3
Requiring the Boost::system component	Header-only since 1.74: Boost::headers + BOOST_ERROR_CODE_HEADER_ONLY. Only classic (pre-1.66) needs the link
Missing -fcoroutines on GCC	Build fails — add $<$<CXX_COMPILER_ID:GNU>:-fcoroutines>
Writing coroutine code for a Boost that predates it	Do Step 1 first

Boost.Asio vs standalone Asio

Same author, same API — namespace and includes differ.

Aspect	Boost.Asio	Standalone Asio
Namespace / include	boost::asio / <boost/asio.hpp>	asio / <asio.hpp>
Error code	boost::system::error_code	asio::error_code (or std::error_code)
Install (brew)	brew install boost	brew install asio
CMake	Boost::headers	manual include path
Version (2025)	1.87–1.90 (with Boost)	1.30–1.36 (independent)
Macro prefix	BOOST_ASIO_	ASIO_

Support both with a shim, then use net:: throughout:

#ifdef USE_STANDALONE_ASIO
  #include <asio.hpp>
  namespace net = asio;
  using error_code = asio::error_code;
#else
  #include <boost/asio.hpp>
  namespace net = boost::asio;
  using error_code = boost::system::error_code;
#endif
namespace ssl = net::ssl;
using tcp = net::ip::tcp;

Before you call it done

Check the code you just wrote against this list:

• Style matches the target Boost version and C++ standard (Step 1), and every API used clears its floor (Step 2).
• Every buffer passed to an async op outlives that op — no callback locals, no dangling string_view.
• At most one async_write per socket in flight, enforced by a queue + flag, if anything writes concurrently with reading.
• Every async chain on a shared object runs on the same strand; self captured in every handler and co_spawn.
• Framing / delimited reads use composed async_read / async_read_until.
• Errors are handled, not swallowed: as_tuple(use_awaitable) destructured, or the callback's ec checked, on every op.
• operation_aborted distinguished from real errors wherever a timer is re-armed or an op is cancelled.
• Acceptor sets reuse_address; shutdown path closes the acceptor and drains sessions.
• CMake has the standard, -fcoroutines for GCC (C++20 only), BOOST_ERROR_CODE_HEADER_ONLY in one place, and Boost::coroutine only if using stackful spawn.
• It compiles. Build it — most of the mistakes above are compile-time, and the version floors are only real once tested.

Worked examples

Three CI-verified implementations of the same full-duplex framed-protocol server, one per style — copy from the one matching Step 1. All three live in the upstream repository and are built by CI on every push.

• market-data-feed — C++20 coroutines (Boost 1.77+; verified 1.83–1.90)
• market-data-feed-precpp20 — callbacks, C++11-clean (verified Boost 1.74+, incl. Windows/MSVC)
• market-data-feed-classic — classic io_service (verified back to Boost 1.62 / Debian 9)

Official documentation

• Overview: https://www.boost.org/doc/libs/latest/doc/html/boost_asio/overview.html
• Reference: https://www.boost.org/doc/libs/latest/doc/html/boost_asio/reference.html
• Examples: https://www.boost.org/doc/libs/latest/doc/html/boost_asio/examples.html

Cross-References

• engineering/docker-development — the old-Boost verification lanes this skill's floors come from are containerised builds (Debian 9 / bookworm, Fedora).
• engineering/chaos-engineering — for exercising the failure paths this skill tells you to handle: half-open sockets, idle timeouts, partial frames.
• engineering-team/playwright-pro — the client-side counterpart when the server built here is driven from browser-based integration tests.