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#### async_read_until (2 of 4 overloads)

Start an asynchronous operation to read data into a streambuf until it contains a specified delimiter.

    template<
        typename AsyncReadStream,
        typename Allocator,
        typename ReadHandler>
    void async_read_until(
        AsyncReadStream & s,
        boost::asio::basic_streambuf< Allocator > & b,
        const std::string & delim,
        ReadHandler handler);

This function is used to asynchronously read data into the specified streambuf until the streambuf's get area contains the specified delimiter. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- The get area of the streambuf contains the specified delimiter.
- 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 streambuf's get area already contains the delimiter, 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. Ownership of the streambuf is retained by the caller, which must guarantee that it remains valid until the handler is called.

delim

  The delimiter string.

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 boost::system::error_code& error,

        // The number of bytes in the streambuf's get
        // area up to and including the delimiter.
        // 0 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 `boost::asio::io_service::post()`.

###### Remarks

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

###### Example

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

    boost::asio::streambuf b;
    ...
    void handler(const boost::system::error_code& e, std::size_t size)
    {
      if (!e)
      {
        std::istream is(&b);
        std::string line;
        std::getline(is, line);
        ...
      }
    }
    ...
    boost::asio::async_read_until(s, b, "\r\n", handler);

After the `async_read_until` operation completes successfully, the buffer `b` contains the delimiter:

    { 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }

The call to `std::getline` then extracts the data up to and including the delimiter, so that the string `line` contains:

    { 'a', 'b', ..., 'c', '\r', '\n' }

The remaining data is left in the buffer `b` as follows:

    { 'd', 'e', ... }

This data may be the start of a new line, to be extracted by a subsequent `async_read_until` operation.