alexprivalov Boost.Asio / standalone Asio
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
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).
Common mistakes
| 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_writeper 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;
selfcaptured in every handler andco_spawn. - Framing / delimited reads use composed
async_read/async_read_until. - Errors are handled, not swallowed:
as_tuple(use_awaitable)destructured, or the callback'secchecked, on every op. -
operation_aborteddistinguished 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,
-fcoroutinesfor GCC (C++20 only),BOOST_ERROR_CODE_HEADER_ONLYin one place, andBoost::coroutineonly if using stackfulspawn. - 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. Paths are relative to this skill directory; if only the skill was installed, they are at https://github.com/alexprivalov/boost-asio-skill/tree/main/examples.
../../examples/market-data-feed/— C++20 coroutines (Boost 1.77+; verified 1.83–1.90)../../examples/market-data-feed-precpp20/— callbacks, C++11-clean (verified Boost 1.74+, incl. Windows/MSVC)../../examples/market-data-feed-classic/— classicio_service(verified back to Boost 1.62 / Debian 9)