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Concept revision and documentation (API Change):

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The concept type traits are renamed for consistency,
and consolidated into a single header file <beast/core/type_traits.hpp>

A new section, Core Concepts, is added to the documentation describing all
of the core utility classes and functions. This also includes a complete
explanation and sample program describing how to write asynchronous initiation
functions and their associated composed operations.
This commit is contained in:
Vinnie Falco
2017-05-10 12:03:00 -07:00
parent aa8a0a2a4b
commit 51f11f0902
66 changed files with 1822 additions and 944 deletions
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examples/echo_op.cpp
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//
// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#include <beast/core.hpp>
#include <boost/asio.hpp>
#include <cstddef>
#include <memory>
#include <utility>
// This composed operation reads a line of input and echoes it back.
//
template<class AsyncStream, class Handler>
class echo_op
{
// This holds all of the state information required by the operation.
struct state
{
// The stream to read and write to
AsyncStream& stream;
// Indicates what step in the operation's state machine
// to perform next, starting from zero.
int step = 0;
// The buffer used to hold the input and output data.
// Note that we use a custom allocator for performance,
// this allows the implementation of the io_service to
// make efficient re-use of memory allocated by composed
// operations during continuations.
//
boost::asio::basic_streambuf<beast::handler_alloc<char, Handler>> buffer;
// handler_ptr requires that the first parameter to the
// contained object constructor is a reference to the
// managed final completion handler.
//
explicit state(Handler& handler, AsyncStream& stream_)
: stream(stream_)
, buffer((std::numeric_limits<std::size_t>::max)(),
beast::handler_alloc<char, Handler>{handler})
{
}
};
// This smart pointer container allocates our state using the
// memory allocation hooks associated with the final completion
// handler, manages the lifetime of that handler for us, and
// enforces the destroy-before-invocation requirement on memory
// allocated using the hooks.
//
beast::handler_ptr<state, Handler> p_;
public:
// Boost.Asio requires that handlers are CopyConstructible.
// In some cases, it takes advantage of handlers that are
// MoveConstructible. This operation supports both.
//
echo_op(echo_op&&) = default;
echo_op(echo_op const&) = default;
// The constructor simply creates our state variables in
// the smart pointer container.
//
template<class DeducedHandler, class... Args>
echo_op(AsyncStream& stream, DeducedHandler&& handler)
: p_(std::forward<DeducedHandler>(handler), stream)
{
}
// Determines if the next asynchronous operation represents a
// continuation of the asynchronous flow of control associated
// with the final handler. If we are past step one, it means
// we have performed an asynchronous operation therefore any
// subsequent operation would represent a continuation.
// Otherwise, we propagate the handler's associated value of
// is_continuation. Getting this right means the implementation
// may schedule the invokation of the invoked functions more
// efficiently.
//
friend bool asio_handler_is_continuation(echo_op* op)
{
// This next call is structured to permit argument
// dependent lookup to take effect.
using boost::asio::asio_handler_is_continuation;
// Always use std::addressof to pass the pointer to the handler,
// otherwise an unwanted overload of operator& may be called instead.
return op->p_->step > 1 ||
asio_handler_is_continuation(std::addressof(op->p_.handler()));
}
// Handler hook forwarding. These free functions invoke the hooks
// associated with the final completion handler. In effect, they
// make the Asio implementation treat our composed operation the
// same way it would treat the final completion handler for the
// purpose of memory allocation and invocation.
//
// Our implementation just passes through the call to the hook
// associated with the final handler.
friend void* asio_handler_allocate(std::size_t size, echo_op* op)
{
using boost::asio::asio_handler_allocate;
return asio_handler_allocate(size, std::addressof(op->p_.handler()));
}
friend void asio_handler_deallocate(void* p, std::size_t size, echo_op* op)
{
using boost::asio::asio_handler_deallocate;
return asio_handler_deallocate(p, size, std::addressof(op->p_.handler()));
}
template<class Function>
friend void asio_handler_invoke(Function&& f, echo_op* op)
{
using boost::asio::asio_handler_invoke;
return asio_handler_invoke(f, std::addressof(op->p_.handler()));
}
// Our main entry point. This will get called as our
// intermediate operations complete. Definition below.
//
void operator()(beast::error_code ec, std::size_t bytes_transferred);
};
// We are callable with the signature void(error_code, bytes_transferred),
// allowing `*this` to be used as both a ReadHandler and a WriteHandler.
//
template<class AsyncStream, class Handler>
void echo_op<AsyncStream, Handler>::
operator()(beast::error_code ec, std::size_t bytes_transferred)
{
// Store a reference to our state. The address of the state won't
// change, and this solves the problem where dereferencing the
// data member is undefined after a move.
auto& p = *p_;
// Now perform the next step in the state machine
switch(ec ? 2 : p.step)
{
// initial entry
case 0:
// read up to the first newline
p.step = 1;
return boost::asio::async_read_until(p.stream, p.buffer, "\n", std::move(*this));
case 1:
// write everything back
p.step = 2;
return boost::asio::async_write(p.stream, p.buffer.data(), std::move(*this));
case 2:
break;
}
// Invoke the final handler. If we wanted to pass any arguments
// which come from our state, they would have to be moved to the
// stack first, since the `handler_ptr` guarantees that the state
// is destroyed before
// the handler is invoked.
//
p_.invoke(ec);
return;
}
// Read a line and echo it back
//
template<class AsyncStream, class CompletionToken>
beast::async_return_type<CompletionToken, void(beast::error_code)>
async_echo(AsyncStream& stream, CompletionToken&& token)
{
// Make sure stream meets the requirements. We use static_assert
// instead of letting the compiler generate several pages of hard
// to read error messages.
//
static_assert(beast::is_async_stream<AsyncStream>::value,
"AsyncStream requirements not met");
// This helper manages some of the handler's lifetime and
// uses the result and handler specializations associated with
// the completion token to help customize the return value.
//
beast::async_completion<CompletionToken, void(beast::error_code)> init{token};
// Create the composed operation and launch it. This is a constructor
// call followed by invocation of operator(). We use BEAST_HANDLER_TYPE
// to convert the completion token into the correct handler type,
// allowing user defined specializations of the async result template
// to take effect.
//
echo_op<AsyncStream, beast::handler_type<CompletionToken, void(beast::error_code)>>{
stream, init.completion_handler}(beast::error_code{}, 0);
// This hook lets the caller see a return value when appropriate.
// For example this might return std::future<error_code> if
// CompletionToken is boost::asio::use_future, or this might
// return an error code if CompletionToken specifies a coroutine.
//
return init.result.get();
}
int main()
{
using address_type = boost::asio::ip::address;
using socket_type = boost::asio::ip::tcp::socket;
using endpoint_type = boost::asio::ip::tcp::endpoint;
// Create a listening socket, accept a connection, perform
// the echo, and then shut everything down and exit.
boost::asio::io_service ios;
socket_type sock{ios};
{
boost::asio::ip::tcp::acceptor acceptor{ios};
endpoint_type ep{address_type::from_string("0.0.0.0"), 0};
acceptor.open(ep.protocol());
acceptor.bind(ep);
acceptor.listen();
acceptor.accept(sock);
}
async_echo(sock,
[&](beast::error_code ec)
{
});
ios.run();
return 0;
}