stackful coroutines. It is based on the Boost.Coroutine library.
Here is an example of its use:
asio::spawn(my_strand, do_echo);
// ...
void do_echo(asio::yield_context yield)
{
try
{
char data[128];
for (;;)
{
std::size_t length =
my_socket.async_read_some(
asio::buffer(data), yield);
asio::async_write(my_socket,
asio::buffer(data, length), yield);
}
}
catch (std::exception& e)
{
// ...
}
}
The first argument to asio::spawn() may be a strand, io_service or
completion handler. This argument 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 a function object with signature (**):
void coroutine(asio::yield_context yield);
that specifies the code to be run as part of the coroutine. The
parameter yield may be passed to an asynchronous operation in place of
the completion handler, as in:
std::size_t length =
my_socket.async_read_some(
asio::buffer(data), yield);
This starts the asynchronous operation and suspends the coroutine. The
coroutine will be resumed automatically when the asynchronous operation
completes.
Where a completion handler signature has the form:
void handler(error_code ec, result_type result);
the initiating function returns the result_type. In the async_read_some
example above, this is std::size_t. If the asynchronous operation fails,
the error_code is converted into a system_error exception and thrown.
Where a completion handler signature has the form:
void handler(error_code ec);
the initiating function returns void. As above, an error is passed back
in the future as a system_error exception.
To collect the error_code from an operation, rather than have it throw
an exception, associate the output variable with the yield_context as
follows:
error_code ec;
std::size_t length =
my_socket.async_read_some(
asio::buffer(data), yield[ec]);
**Note: if asio::spawn() is used with a custom completion handler of
type Handler, the function object signature is actually:
void coroutine(asio::basic_yield_context<Handler> yield);
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