#### async_read_until (4 of 8 overloads)

Start an asynchronous operation to read data into a dynamic buffer sequence until a function object indicates a match.

    template<
        typename AsyncReadStream,
        typename DynamicBuffer,
        typename MatchCondition,
        typename ReadHandler>
    DEDUCED async_read_until(
        AsyncReadStream & s,
        DynamicBuffer && buffers,
        MatchCondition match_condition,
        ReadHandler && handler,
        typename enable_if< is_match_condition< MatchCondition >::value >::type *  = 0);

This function is used to asynchronously read data into the specified dynamic buffer sequence until a user-defined match condition function object, when applied to the data contained in the dynamic buffer sequence, indicates a successful match. The function call 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.

buffers

  The dynamic buffer sequence into which the data will be read. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.

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<typename DynamicBuffer::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.

handler

  The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

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

        // The number of bytes in the dynamic buffer sequence'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 handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `asio::io_context::post()`.

###### Remarks

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

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 `std::string` until whitespace is encountered:

    typedef asio::buffers_iterator<
        asio::const_buffers_1> 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);
    ...
    std::string data;
    asio::async_read_until(s, data, match_whitespace, handler);

To asynchronously read data into a `std::string` 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);
    ...
    std::string data;
    asio::async_read_until(s, data, match_char('a'), handler);