Support for C++20 Coroutines is provided via the awaitable class template, the use_awaitable completion token, and the co_spawn() function. These facilities allow programs to implement asynchronous logic in a synchronous manner, in conjunction with the co_await keyword, as shown in the following example:

asio::co_spawn(executor, echo(std::move(socket)), asio::detached);

// ...

asio::awaitable<void> echo(tcp::socket socket)
{
  try
  {
    char data[1024];
    for (;;)
    {
      std::size_t n = co_await socket.async_read_some(asio::buffer(data), asio::use_awaitable);
      co_await async_write(socket, asio::buffer(data, n), asio::use_awaitable);
    }
  }
  catch (std::exception& e)
  {
    std::printf("echo Exception: %s\n", e.what());
  }
}

The first argument to co_spawn() is an executor that determines the context in which the coroutine is permitted to execute. For example, a server's per-client object may consist of multiple coroutines; they should all run on the same strand so that no explicit synchronisation is required.

The second argument is an awaitable<R>, that is the result of the coroutine's entry point function, and in the above example is the result of the call to echo. (Alternatively, this argument can be a function object that returns the awaitable<R>.) The template parameter R is the type of return value produced by the coroutine. In the above example, the coroutine returns void.

The third argument is a completion token, and this is used by co_spawn() to produce a completion handler with signature void(std::exception_ptr, R). This completion handler is invoked with the result of the coroutine once it has finished. In the above example we pass a completion token type, asio::detached, which is used to explicitly ignore the result of an asynchronous operation.

In this example the body of the coroutine is implemented in the echo function. When the use_awaitable completion token is passed to an asynchronous operation, the operation's initiating function returns an awaitable that may be used with the co_await keyword:

std::size_t n = co_await socket.async_read_some(asio::buffer(data), asio::use_awaitable);

Where an asynchronous operation's handler signature has the form:

void handler(asio::error_code ec, result_type result);

the resulting type of the co_await expression is result_type. In the async_read_some example above, this is size_t. If the asynchronous operation fails, the error_code is converted into a system_error exception and thrown.

Where a handler signature has the form:

void handler(asio::error_code ec);

the co_await expression produces a void result. As above, an error is passed back to the coroutine as a system_error exception.

Coroutines and Per-Operation Cancellation

All threads of execution created by co_spawn have a cancellation state that records the current state of any cancellation requests made to the coroutine. To access this state, use this_coro::cancellation_state as follows:

asio::awaitable<void> my_coroutine()
{
  asio::cancellation_state cs
    = co_await asio::this_coro::cancellation_state;

  // ...

  if (cs.cancelled() != asio::cancellation_type::none)
    // ...
}

When first created by co_spawn, the thread of execution has a cancellation state that supports cancellation_type::terminal values only. To change the cancellation state, call this_coro::reset_cancellation_state.

By default, continued execution of a cancelled coroutine will trigger an exception from any subsequent co_await of an awaitable<> object. This behaviour can be changed by using this_coro::throw_if_cancelled.

Co-ordinating Parallel Coroutines

	Note
	This is an experimental feature.

The logical operators || and && have been overloaded for awaitable<>, to allow coroutines to be trivially awaited in parallel.

When awaited using &&, the co_await expression waits until both operations have completed successfully. As a "short-circuit" evaluation, if one operation fails with an exception, the other is immediately cancelled. For example:

std::tuple<std::size_t, std::size_t> results =
  co_await (
    async_read(socket, input_buffer, use_awaitable)
      && async_write(socket, output_buffer, use_awaitable)
  );

Following completion of a && operation, the results of all operations are concatenated into a tuple. In the above example, the first size_t represents the non-exceptional component of the async_read result, and the second size_t is the result of the async_write.

When awaited using ||, the co_await expression waits until either operation succeeds. As a "short-circuit" evaluation, if one operation succeeds without throwing an exception, the other is immediately cancelled. For example:

std::variant<std::size_t, std::monostate> results =
  co_await (
    async_read(socket, input_buffer, use_awaitable)
      || timer.async_wait(use_awaitable)
  );

Following completion of a || operation, the result of the first operation to complete non-exceptionally is placed into a std::variant. The active index of the variant reflects which of the operations completed first. In the above example, index 0 corresponds to the async_read operation.

These operators may be enabled by adding the #include:

#include <asio/experimental/awaitable_operators.hpp>

and then bringing the contents of the experimental::awaitable_operators namespace into scope:

using namespace asio::experimental::awaitable_operators;

Coroutines that Await and Yield

	Note
	This is an experimental feature.

The coro type is a C++20 coroutine primitive for resumable functions, with the ability to combine both asynchronous waiting (co_await) and yielding (co_yield) into a single, stateful control flow. For example:

#include <asio.hpp>
#include <asio/experimental/coro.hpp>

using asio::ip::tcp;

asio::experimental::coro<std::string> reader(tcp::socket& sock)
{
  std::string buf;
  while (sock.is_open())
  {
    std::size_t n = co_await asio::async_read_until(
        sock, asio::dynamic_buffer(buf), '\n',
        asio::experimental::use_coro);
    co_yield buf.substr(0, n);
    buf.erase(0, n);
  }
}

asio::awaitable<void> consumer(tcp::socket sock)
{
  auto r = reader(sock);
  auto msg1 = co_await r.async_resume(asio::use_awaitable);
  std::cout << "Message 1: " << msg1.value_or("\n");
  auto msg2 = co_await r.async_resume(asio::use_awaitable);
  std::cout << "Message 2: " << msg2.value_or("\n");
}

asio::awaitable<void> listen(tcp::acceptor& acceptor)
{
  for (;;)
  {
    co_spawn(
        acceptor.get_executor(),
        consumer(co_await acceptor.async_accept(asio::use_awaitable)),
        asio::detached);
  }
}

int main()
{
  asio::io_context ctx;
  tcp::acceptor acceptor(ctx, {tcp::v4(), 54321});
  co_spawn(ctx, listen(acceptor), asio::detached);
  ctx.run();
}

See Also

co_spawn, detached, redirect_error, awaitable, use_awaitable_t, use_awaitable, this_coro::executor, experimental::coro, Coroutines examples, Stackful Coroutines, Stackless Coroutines.