Boost C++ Libraries

Base class to provide completion handler boilerplate for composed operations.

Synopsis

Defined in header <boost/beast/core/async_base.hpp>

template<
    class Handler,
    class Executor1,
    class Allocator = std::allocator<void>>
class stable_async_base :
    public async_base< Handler, Executor1, std::allocator< void > >

Types

Member Functions

Friends

Description

A function object submitted to intermediate initiating functions during a composed operation may derive from this type to inherit all of the boilerplate to forward the executor, allocator, and legacy customization points associated with the completion handler invoked at the end of the composed operation.

The composed operation must be typical; that is, associated with one executor of an I/O object, and invoking a caller-provided completion handler when the operation is finished. Classes derived from async_base will acquire these properties:

Data members of composed operations implemented as completion handlers do not have stable addresses, as the composed operation object is move constructed upon each call to an initiating function. For most operations this is not a problem. For complex operations requiring stable temporary storage, the class stable_async_base is provided which offers additional functionality:

Example

The following code demonstrates how stable_async_base may be be used to assist authoring an asynchronous initiating function, by providing all of the boilerplate to manage the final completion handler in a way that maintains the allocator and executor associations. Furthermore, the operation shown allocates temporary memory using beast::allocate_stable for the timer and message, whose addresses must not change between intermediate operations:

// Asynchronously send a message multiple times, once per second
template < class AsyncWriteStream, class T, class WriteHandler>
auto async_write_messages(
    AsyncWriteStream& stream,
    T const & message,
    std::size_t repeat_count,
    WriteHandler&& handler) ->
        typename net::async_result<
            typename std::decay<WriteHandler>::type,
            void(error_code)>::return_type
{
    using handler_type = typename net::async_completion<WriteHandler, void(error_code)>::completion_handler_type;
    using base_type = stable_async_base<handler_type, typename AsyncWriteStream::executor_type>;

    struct op : base_type, boost::asio::coroutine
    {
        // This object must have a stable address
        struct temporary_data
        {
            // Although std::string is in theory movable, most implementations
            // use a "small buffer optimization" which means that we might
            // be submitting a buffer to the write operation and then
            // moving the string, invalidating the buffer. To prevent
            // undefined behavior we store the string object itself at
            // a stable location.
            std::string const message;

            net::steady_timer timer;

            temporary_data(std::string message_, net::io_context& ctx)
                : message(std::move(message_))
                , timer(ctx)
            {
            }
        };

        AsyncWriteStream& stream_;
        std::size_t repeats_;
        temporary_data& data_;

        op(AsyncWriteStream& stream, std::size_t repeats, std::string message, handler_type& handler)
            : base_type(std::move(handler), stream.get_executor())
            , stream_(stream)
            , repeats_(repeats)
            , data_(allocate_stable<temporary_data>(*this, std::move(message), stream.get_executor().context()))
        {
            (*this)(); // start the operation
        }

        // Including this file provides the keywords for macro-based coroutines
        #include <boost/asio/yield.hpp>

        void operator()(error_code ec = {}, std::size_t = 0)
        {
            reenter(* this )
            {
                // If repeats starts at 0 then we must complete immediately. But
                // we can't call the final handler from inside the initiating
                // function, so we post our intermediate handler first. We use
                // net::async_write with an empty buffer instead of calling
                // net::post to avoid an extra function template instantiation, to
                // keep compile times lower and make the resulting executable smaller.
                yield net::async_write(stream_, net::const_buffer{}, std::move(* this ));
                while (! ec && repeats_-- > 0)
                {
                    // Send the string. We construct a `const_buffer` here to guarantee
                    // that we do not create an additional function template instantation
                    // of net::async_write, since we already instantiated it above for
                    // net::const_buffer.

                    yield net::async_write(stream_,
                        net::const_buffer(net::buffer(data_.message)), std::move(* this ));
                    if (ec)
                        break ;

                    // Set the timer and wait
                    data_.timer.expires_after(std::chrono::seconds(1));
                    yield data_.timer.async_wait(std::move(* this ));
                }
            }

            // The base class destroys the temporary data automatically,
            // before invoking the final completion handler
            this->complete_now(ec);
        }

        // Including this file undefines the macros for the coroutines
        #include <boost/asio/unyield.hpp>
    };

    net::async_completion<WriteHandler, void(error_code)> completion(handler);
    std::ostringstream os;
    os << message;
    op(stream, repeat_count, os.str(), completion.completion_handler);
    return completion.result.get();
}

Template Parameters

See Also

allocate_stable, async_base