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copied!<p>The Boost.Asio documentation states:</p> <blockquote> <p>A buffer object represents a contiguous region of memory as a 2-tuple consisting of a pointer and size in bytes. A tuple of the form {void*, size_t} specifies a mutable (modifiable) region of memory.</p> </blockquote> <p>This means that in order for a call to <code>async_read</code> to write data to a buffer, it must be (in the underlying buffer object) a contiguous block of memory. Additionally, the buffer object must be able to write to that block of memory. </p> <p><code>std::string</code> does not allow arbitrary writes into its buffer, so <code>async_read</code> cannot write chunks of memory into a string's buffer (note that <code>std::string</code> <em>does</em> give the caller read-only access to the underlying buffer via the <code>data()</code> method, which guarantees that the returned pointer will be valid until the next call to a non-const member function. For this reason, Asio can easily create a <code>const_buffer</code> wrapping an <code>std::string</code>, and you can use it with <code>async_write</code>).</p> <p>The Asio documentation has example code for a simple "chat" program (see <a href="http://www.boost.org/doc/libs/1_43_0/doc/html/boost_asio/examples.html#boost_asio.examples.chat" rel="noreferrer">http://www.boost.org/doc/libs/1_43_0/doc/html/boost_asio/examples.html#boost_asio.examples.chat</a>) that has a good method of overcoming this problem. Basically, you need to have the sending TCP send along the size of a message first, in a "header" of sorts, and your read handler must interpret the header to allocate a buffer of a fixed size suitable for reading the actual data.</p> <p>As far as the need for using <code>shared_from_this()</code> in <code>async_read</code> and <code>async_write</code>, the reason is that it guarantees that the method wrapped by <code>boost::bind</code> will always refer to a live object. Consider the following situation:</p> <ol> <li>Your <code>handle_accept</code> method calls <code>async_read</code> and sends a handler "into the reactor" - basically you've asked the <code>io_service</code> to invoke <code>Connection::handle_user_read</code> when it finishes reading data from the socket. The <code>io_service</code> stores this functor and continues its loop, waiting for the asynchronous read operation to complete.</li> <li>After your call to <code>async_read</code>, the <code>Connection</code> object is deallocated for some reason (program termination, an error condition, etc.)</li> <li>Suppose the <code>io_service</code> now determines that the asynchronous read is complete, <em>after</em> the <code>Connection</code> object has been deallocated but <em>before</em> the <code>io_service</code> is destroyed (this can occur, for example, if <code>io_service::run</code> is running in a separate thread, as is typical). Now, the <code>io_service</code> attempts to invoke the handler, and it has an invalid reference to a <code>Connection</code> object.</li> </ol> <p>The solution is to allocate <code>Connection</code> via a <code>shared_ptr</code> and use <code>shared_from_this()</code> instead of <code>this</code> when sending a handler "into the reactor" - this allows <code>io_service</code> to store a shared reference to the object, and <code>shared_ptr</code> guarantees that it won't be deallocated until the last reference expires.</p> <p>So, your code should probably look something like:</p> <pre><code>class Connection : public boost::enable_shared_from_this<Connection> { public: Connection(tcp::acceptor &acceptor) : acceptor_(acceptor), socket_(acceptor.get_io_service(), tcp::v4()) { } void start() { acceptor_.get_io_service().post( boost::bind(&Connection::start_accept, shared_from_this())); } private: void start_accept() { acceptor_.async_accept(socket_, boost::bind(&Connection::handle_accept, shared_from_this(), placeholders::error)); } void handle_accept(const boost::system::error_code& err) { if (err) { disconnect(); } else { async_read(socket_, boost::asio::buffer(user_), boost::bind(&Connection::handle_user_read, shared_from_this(), placeholders::error, placeholders::bytes_transferred)); } } //... }; </code></pre> <p>Note that you now must make sure that each <code>Connection</code> object is allocated via a <code>shared_ptr</code>, e.g.:</p> <pre><code>boost::shared_ptr<Connection> new_conn(new Connection(...)); </code></pre> <p>Hope this helps!</p>

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