#### Movable Handlers

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.