#### async_read_until (8 of 12 overloads)

Start an asynchronous operation to read data into a streambuf until a function object indicates a match.

    template<
        typename AsyncReadStream,
        typename Allocator,
        typename MatchCondition,
        typename ReadToken = DEFAULT>
    DEDUCED async_read_until(
        AsyncReadStream & s,
        asio::basic_streambuf< Allocator > & b,
        MatchCondition match_condition,
        ReadToken && token = DEFAULT,
        typename constraint< is_match_condition< MatchCondition >::value >::type  = 0);

This function is used to asynchronously read data into the specified streambuf until a user-defined match condition function object, when applied to the data contained in the streambuf, indicates a successful match. It is an initiating function for an asynchronous operation, and always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- The match condition function object returns a std::pair where the second element evaluates to true.
- An error occurred.

This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. If the match condition function object already indicates a match, this asynchronous operation completes immediately. The program must ensure that the stream performs no other read operations (such as async_read, async_read_until, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.

###### Parameters

s

  The stream from which the data is to be read. The type must support the AsyncReadStream concept.

b

  A streambuf object into which the data will be read.

match_condition

  The function object to be called to determine whether a match exists. The signature of the function object must be:

      pair<iterator, bool> match_condition(iterator begin, iterator end);

  where `iterator` represents the type:

      buffers_iterator<basic_streambuf<Allocator>::const_buffers_type>

  The iterator parameters `begin` and `end` define the range of bytes to be scanned to determine whether there is a match. The `first` member of the return value is an iterator marking one-past-the-end of the bytes that have been consumed by the match function. This iterator is used to calculate the `begin` parameter for any subsequent invocation of the match condition. The `second` member of the return value is true if a match has been found, false otherwise.

token

  The completion token that will be used to produce a completion handler, which will be called when the read completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        // Result of operation.
        const asio::error_code& error,

        // The number of bytes in the streambuf's get
        // area that have been fully consumed by the
        // match function. O if an error occurred.
        std::size_t bytes_transferred
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `post`.

###### Remarks

After a successful async_read_until operation, the streambuf may contain additional data beyond that which matched the function object. An application will typically leave that data in the streambuf for a subsequent async_read_until operation to examine.

###### Completion Signature

    void(asio::error_code, std::size_t)

###### Remarks

The default implementation of the `is_match_condition` type trait evaluates to true for function pointers and function objects with a `result_type` typedef. It must be specialised for other user-defined function objects.

###### Examples

To asynchronously read data into a streambuf until whitespace is encountered:

    typedef asio::buffers_iterator<
        asio::streambuf::const_buffers_type> iterator;

    std::pair<iterator, bool>
    match_whitespace(iterator begin, iterator end)
    {
      iterator i = begin;
      while (i != end)
        if (std::isspace(*i++))
          return std::make_pair(i, true);
      return std::make_pair(i, false);
    }
    ...
    void handler(const asio::error_code& e, std::size_t size);
    ...
    asio::streambuf b;
    asio::async_read_until(s, b, match_whitespace, handler);

To asynchronously read data into a streambuf until a matching character is found:

    class match_char
    {
    public:
      explicit match_char(char c) : c_(c) {}

      template <typename Iterator>
      std::pair<Iterator, bool> operator()(
          Iterator begin, Iterator end) const
      {
        Iterator i = begin;
        while (i != end)
          if (c_ == *i++)
            return std::make_pair(i, true);
        return std::make_pair(i, false);
      }

    private:
      char c_;
    };

    namespace asio {
      template <> struct is_match_condition<match_char>
        : public boost::true_type {};
    } // namespace asio
    ...
    void handler(const asio::error_code& e, std::size_t size);
    ...
    asio::streambuf b;
    asio::async_read_until(s, b, match_char('a'), handler);

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`

if they are also supported by the `AsyncReadStream` type's `async_read_some` operation.