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Support for the Coroutines TS 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:
boost::asio::co_spawn(executor,
[socket = std::move(socket)]() mutable
{
return echo(std::move(socket));
},
boost::asio::detached);
// ...
boost::asio::awaitable<void> echo(tcp::socket socket)
{
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 async_write(socket, boost::asio::buffer(data, n), boost::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 a nullary function object that returns a boost::asio::awaitable<R>,
where 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,
boost::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(boost::asio::buffer(data), boost::asio::use_awaitable);
Where an asynchronous operation's handler signature has the form:
void handler(boost::system::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(boost::system::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.
co_spawn, detached, redirect_error, awaitable, use_awaitable_t, use_awaitable, this_coro::executor, Coroutines TS examples, Stackful Coroutines, Stackless Coroutines.