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PORepeated std::move on an boost::asio socket object in C++11
 

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    <p>I am exploring using boost::asio along with C++11 features. In particular, I am focusing on an example called "async_tcp_echo_server.cpp", located here (code is also shown at the end of my question):</p> <p><a href="http://www.boost.org/doc/libs/1_54_0/doc/html/boost_asio/example/cpp11/echo/async_tcp_echo_server.cpp" rel="noreferrer">http://www.boost.org/doc/libs/1_54_0/doc/html/boost_asio/example/cpp11/echo/async_tcp_echo_server.cpp</a></p> <p>My question involves the <code>tcp::socket</code> member <code>socket_</code> of the <code>server</code> class. In the <code>do_accept()</code> method of the <code>server</code> class, <code>socket_</code> is passed to <code>async_accept()</code>. (According to the asio documentation, <code>async_accept()</code> requires, as its first parameter, the <code>socket</code> to accept the connection into.) So far, so good.</p> <p>The next parameter, the callback for the asynchronous accept operation, is a lambda function. The body of the lambda constructs a new <code>session</code> object, whose constructor also needs the same <code>socket</code>. Interestingly, <code>socket</code> objects cannot be copied; so in the example, the <code>socket_</code> object, which is a member of the <code>server</code> object, is passed using <code>std::move()</code>.</p> <p>I understand that the "one and only" <code>socket_</code> object (which is a "permanent" member of the <code>server</code> object) is "moved" into the <code>session</code> object. Fine -- <code>socket</code> object is not copied, but moved -- everybody's happy.</p> <p>But what happens on the next call to <code>async_accept()</code>? Is the same <code>socket_</code> (member of <code>server</code>), that was previously moved, passed in again? When we "move" a member, what is left behind? Is there a magical fountain of unlimited <code>socket</code> objects?</p> <p>Or is something really less-than-obvious happening here? When the <code>socket</code> is moved into the <code>session</code>, is the contents of the "left behind/moved from" object (<code>socket_</code> member of <code>server</code>) <em>swapped</em> with the contents of the "new" <code>session</code> object's own "not-yet-constructed" <code>socket_</code> member? Am I even making sense?</p> <h1>Summary</h1> <p>Code is below. Program flow is fairly simple. <code>main()</code> constructs a single <code>server</code> object. The <code>server</code> makes repeated calls to <code>async_accept()</code>. Each <code>async_accept()</code> callback creates a new <code>session</code> object, each constructed with a (fresh?) <code>socket</code>. Where do all the "fresh" <code>socket</code> objects come from, if they are simply (repeatedly) "moved" from the same <code>socket_</code> member in the (single) <code>server</code>?</p> <pre><code>#include &lt;cstdlib&gt; #include &lt;iostream&gt; #include &lt;memory&gt; #include &lt;utility&gt; #include &lt;boost/asio.hpp&gt; using boost::asio::ip::tcp; class session : public std::enable_shared_from_this&lt;session&gt; { public: session( tcp::socket socket ) : socket_( std::move( socket ) ) {} void start() { do_read(); } private: void do_read() { auto self( shared_from_this() ); socket_.async_read_some( boost::asio::buffer( data_, max_length ), [this, self]( boost::system::error_code ec, std::size_t length ) { if( !ec ) { do_write( length ); } } ); } void do_write( std::size_t length ) { auto self( shared_from_this() ); boost::asio::async_write( socket_, boost::asio::buffer( data_, length ), [this, self]( boost::system::error_code ec, std::size_t /*length*/ ) { if( !ec ) { do_read(); } } ); } tcp::socket socket_; enum { max_length = 1024 }; char data_[max_length]; }; class server { public: server( boost::asio::io_service&amp; io_service, short port ) : acceptor_( io_service, tcp::endpoint( tcp::v4(), port ) ) , socket_( io_service ) { do_accept(); } private: void do_accept() { acceptor_.async_accept( socket_, [this]( boost::system::error_code ec ) { if( !ec ) { std::make_shared&lt;session&gt;( std::move( socket_ ) )-&gt;start(); // is this a *swap* of socket_ ??? } do_accept(); } ); } tcp::acceptor acceptor_; tcp::socket socket_; }; int main( int argc, char* argv[] ) { try { if( argc != 2 ) { std::cerr &lt;&lt; "Usage: async_tcp_echo_server &lt;port&gt;\n"; return 1; } boost::asio::io_service io_service; server s( io_service, std::atoi( argv[1] ) ); io_service.run(); } catch( std::exception&amp; e ) { std::cerr &lt;&lt; "Exception: " &lt;&lt; e.what() &lt;&lt; "\n"; } return 0; } </code></pre>
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