read_until (15 of 16 overloads)

Read data into a streambuf until a function object indicates a match.

template<
    typename SyncReadStream,
    typename Allocator,
    typename MatchCondition>
std::size_t read_until(
    SyncReadStream & s,
    boost::asio::basic_streambuf< Allocator > & b,
    MatchCondition match_condition,
    typename enable_if< is_match_condition< MatchCondition >::value >::type *  = 0);

This function is used to 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. The call will block 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 read_some function. If the match condition function object already indicates a match, the function returns immediately.

Parameters

s

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

Return Value

The number of bytes in the streambuf's get area that have been fully consumed by the match function.

Exceptions

boost::system::system_error: Thrown on failure.

Remarks

After a successful 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 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 read data into a streambuf until whitespace is encountered:

typedef boost::asio::buffers_iterator<
    boost::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);
}
...
boost::asio::streambuf b;
boost::asio::read_until(s, b, match_whitespace);

To 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
...
boost::asio::streambuf b;
boost::asio::read_until(s, b, match_char('a'));