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Coding Standards
Base class to provide completion handler boilerplate for composed operations.
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, Allocator >
Name |
Description |
---|---|
The type of allocator associated with this object. |
|
The type of executor associated with this object. |
Name |
Description |
---|---|
Invoke the final completion handler, maybe using post. |
|
Invoke the final completion handler. |
|
Returns the allocator associated with this object. |
|
Returns the executor associated with this object. |
|
Returns the handler associated with this object. |
|
Returns ownership of the handler associated with this object. |
|
Constructor. Move Constructor. |
|
Destructor. |
Name |
Description |
---|---|
Allocate a temporary object to hold operation state. |
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:
std::allocator<void>
if the parameter is omitted.
Executor1
will
be the type of executor associated with the composed operation.
net::executor_work_guard
for the instance
of Executor1
shall be
maintained until either the final handler is invoked, or the operation
base is destroyed, whichever comes first.
asio_handler_invoke
,
asio_handler_allocate
,
asio_handler_deallocate
,
and asio_handler_is_continuation
,
which use argument-dependent lookup, will be forwarded to the legacy
customization points associated with the handler.
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:
allocate_stable
may be used
to allocate one or more temporary objects associated with the composed
operation.
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 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(); }
Type |
Description |
---|---|
|
The type of the completion handler to store. This type must meet the requirements of CompletionHandler. |
|
The type of the executor used when the handler has no associated executor. An instance of this type must be provided upon construction. The implementation will maintain an executor work guard and a copy of this instance. |
|
The allocator type to use if the handler does not have an associated
allocator. If this parameter is omitted, then |
stable_async_base::allocate_stable
, async_base
Convenience header <boost/beast/core.hpp>