Concepts Reference

Named requirements for Asio types and functions.

Stream Concepts

AsyncReadStream

A type that supports async read operations.

Requirements:

  • async_read_some(buffers, token) — Initiates a read operation

Models:

  • tcp::socket

  • ssl::stream<tcp::socket>

  • buffered_read_stream<>

Usage with coroutines:

template<typename AsyncReadStream>
awaitable<std::size_t> read_some(AsyncReadStream& stream)
{
    char buf[1024];
    co_return co_await stream.async_read_some(
        asio::buffer(buf), use_awaitable);
}

AsyncWriteStream

A type that supports async write operations.

Requirements:

  • async_write_some(buffers, token) — Initiates a write operation

Models:

  • tcp::socket

  • ssl::stream<tcp::socket>

  • buffered_write_stream<>

AsyncStream

A type that is both AsyncReadStream and AsyncWriteStream.

Buffer Concepts

ConstBufferSequence

A sequence of read-only buffers.

Requirements:

  • Iterable with begin() / end()

  • Iterator dereferences to something convertible to const_buffer

Models:

  • const_buffer

  • std::array<const_buffer, N>

  • std::vector<const_buffer>

  • Result of buffer("string")

MutableBufferSequence

A sequence of writable buffers.

Requirements:

  • Iterable with begin() / end()

  • Iterator dereferences to something convertible to mutable_buffer

Models:

  • mutable_buffer

  • std::array<mutable_buffer, N>

  • std::vector<mutable_buffer>

  • Result of buffer(array)

DynamicBuffer

A buffer that can grow to accommodate data.

Requirements:

  • size() — Current size

  • max_size() — Maximum allowed size

  • capacity() — Current capacity

  • data() — Get const buffer sequence

  • prepare(n) — Get mutable buffer for writing n bytes

  • commit(n) — Mark n bytes as written

  • consume(n) — Remove n bytes from front

Models:

  • dynamic_buffer(std::string&)

  • dynamic_buffer(std::vector<char>&)

  • streambuf

Protocol Concepts

Protocol

Defines a communication protocol (TCP, UDP, etc.).

Requirements:

  • endpoint — Nested endpoint type

  • socket — Nested socket type (or via basic_socket)

Models:

  • tcp

  • udp

  • icmp

  • local::stream_protocol

  • local::datagram_protocol

Endpoint

An endpoint in a protocol (address + port).

Requirements:

  • protocol() — Return the protocol

  • Default constructible

  • Copyable

Models:

  • tcp::endpoint

  • udp::endpoint

  • local::stream_protocol::endpoint

InternetProtocol

A Protocol for IP-based communication.

Requirements:

  • All Protocol requirements

  • v4() — Return IPv4 variant

  • v6() — Return IPv6 variant

  • resolver — Nested resolver type

Models:

  • tcp

  • udp

  • icmp

Executor Concepts

Executor

A handle to an execution context.

Requirements:

  • execute(f) — Execute a function

  • Equality comparable

  • Copyable

Models:

  • io_context::executor_type

  • thread_pool::executor_type

  • strand<E>

  • any_io_executor

ExecutionContext

A context that can create executors.

Requirements:

  • get_executor() — Return an executor

Models:

  • io_context

  • thread_pool

  • system_context

Completion Token Concepts

CompletionToken

Determines how async operation results are delivered.

Models:

  • use_awaitable_t — Return awaitable<T> for coroutines

  • detached_t — Discard result

  • Function objects — Called with result

CompletionCondition

Controls when composed read/write operations complete.

Requirements:

  • operator()(error_code, bytes_transferred) — Return bytes remaining

Models:

  • transfer_all() — Until buffer full/empty

  • transfer_at_least(n) — Until n bytes transferred

  • transfer_exactly(n) — Exactly n bytes

Service Concepts

Service

A singleton attached to an execution context.

Requirements:

  • key_type — Nested type identifying the service

  • Constructor taking execution_context&

Services allow extending io_context with custom functionality.

Socket Option Concepts

GettableSocketOption

An option that can be read from a socket.

Models:

  • socket_base::reuse_address

  • tcp::no_delay

  • socket_base::receive_buffer_size

SettableSocketOption

An option that can be set on a socket.

Models:

Same as GettableSocketOption—most options are both.

Usage Notes

These concepts are informal—they’re documented requirements, not C++20 concept declarations. Asio uses SFINAE and enable_if for constraint checks.

When writing generic code:

// Works with any AsyncReadStream
template<typename Stream>
awaitable<void> read_and_process(Stream& stream)
{
    char buf[1024];
    auto n = co_await stream.async_read_some(
        asio::buffer(buf), use_awaitable);
    // process buf[0..n)
}

For full details on each concept, see the Boost.Asio reference documentation.

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