co_spawn (3 of 6 overloads)

Spawn a new coroutined-based thread of execution.

template<
    typename ExecutionContext,
    typename T,
    typename AwaitableExecutor,
    typename CompletionToken = DEFAULT>
DEDUCED co_spawn(
    ExecutionContext & ctx,
    awaitable< T, AwaitableExecutor > a,
    CompletionToken && token = DEFAULT,
    constraint_t< is_convertible< ExecutionContext &, execution_context & >::value &&is_convertible< typename ExecutionContext::executor_type, AwaitableExecutor >::value >  = 0);

Parameters

ctx

An execution context that will provide the executor to be used to schedule the new thread of execution.

a

The awaitable object that is the result of calling the coroutine's entry point function.

token

The completion token that will handle the notification that the thread of execution has completed. The function signature of the completion handler must be:

void handler(std::exception_ptr);

Completion Signature

void(std::exception_ptr, T)

Example

boost::asio::awaitable<std::size_t> echo(tcp::socket socket)
{
  std::size_t bytes_transferred = 0;

  try
  {
    char data[1024];
    for (;;)
    {
      std::size_t n = co_await socket.async_read_some(
          boost::asio::buffer(data), boost::asio::use_awaitable);

      co_await boost::asio::async_write(socket,
          boost::asio::buffer(data, n), boost::asio::use_awaitable);

      bytes_transferred += n;
    }
  }
  catch (const std::exception&)
  {
  }

  co_return bytes_transferred;
}

// ...

boost::asio::co_spawn(my_io_context,
  echo(std::move(my_tcp_socket)),
  [](std::exception_ptr e, std::size_t n)
  {
    std::cout << "transferred " << n << "\n";
  });

Per-Operation Cancellation

The new thread of execution is created with a cancellation state that supports cancellation_type::terminal values only. To change the cancellation state, call this_coro::reset_cancellation_state.