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        A buffered handshake handler must meet the requirements for a handler.
        A value h of a buffered handshake
        handler class should work correctly in the expression h(ec,
        s),
        where ec is an lvalue of
        type const error_code
        and s is an lvalue of type
        const size_t.
      
A free function as a buffered handshake handler:
void handshake_handler( const asio::error_code& ec, std::size_t bytes_transferred) { ... }
A buffered handshake handler function object:
struct handshake_handler { ... void operator()( const asio::error_code& ec, std::size_t bytes_transferred) { ... } ... };
        A non-static class member function adapted to a buffered handshake handler
        using bind():
      
void my_class::handshake_handler( const asio::error_code& ec, std::size_t bytes_transferred) { ... } ... socket.async_handshake(..., boost::bind(&my_class::handshake_handler, this, asio::placeholders::error, asio::placeholders::bytes_transferred));