#### Futures

The `asio::use_future` completion token provides first-class support for returning a `std::future` from an asynchronous operation's initiating function.

To use `asio::use_future`, pass it to an asynchronous operation instead of a completion handler. For example:

    std::future<std::size_t> length =
      my_socket.async_read_some(my_buffer, asio::use_future);

Where a completion signature has the form:

    void handler(asio::error_code ec, result_type result);

the initiating function returns a `std::future` templated on `result_type`. In the above example, this is `std::size_t`. If the asynchronous operation fails, the `error_code` is converted into a `system_error` exception and passed back to the caller through the future.

Where a completion signature has the form:

    void handler(asio::error_code ec);

the initiating function returns `std::future<void>`. As above, an error is passed back in the future as a `system_error` exception.

###### See Also

use_future, use_future_t, Futures example (C++11).