As an optimisation, user-defined completion handlers may provide move constructors, and Asio's implementation will use a handler's move constructor in preference to its copy constructor. In certain circumstances, Asio may be able to eliminate all calls to a handler's copy constructor. However, handler types are still required to be copy constructible.

When move support is enabled, asynchronous that are documented as follows:

template <typename Handler>
void async_XYZ(..., Handler handler);

are actually declared as:

template <typename Handler>
void async_XYZ(..., Handler&& handler);

The handler argument is perfectly forwarded and the move construction occurs within the body of async_XYZ(). This ensures that all other function arguments are evaluated prior to the move. This is critical when the other arguments to async_XYZ() are members of the handler. For example:

struct my_operation
{
  shared_ptr<tcp::socket> socket;
  shared_ptr<vector<char>> buffer;
  ...
  void operator(error_code ec, size_t length)
  {
    ...
    socket->async_read_some(asio::buffer(*buffer), std::move(*this));
    ...
  }
};

Move support is automatically enabled for g++ 4.5 and later, when the -std=c++0x or -std=gnu++0x compiler options are used. It may be disabled by defining ASIO_DISABLE_MOVE, or explicitly enabled for other compilers by defining ASIO_HAS_MOVE. Note that these macros also affect the availability of movable I/O objects.