...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 ReadHandler> void async_read_until( AsyncReadStream & s, boost::asio::basic_streambuf< Allocator > & b, MatchCondition match_condition, ReadHandler handler, typename boost::enable_if< is_match_condition< MatchCondition > >::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. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:
This operation is implemented in terms of zero or more calls to the stream's async_read_some function. If the match condition function object already indicates a match, the operation completes immediately.
The stream from which the data is to be read. The type must support the AsyncReadStream concept.
A streambuf object into which the data will be read.
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.
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 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 boost::asio::io_service::post().
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.
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.
To asynchronously 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); } ... void handler(const boost::system::error_code& e, std::size_t size); ... boost::asio::streambuf b; boost::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 boost::system::error_code& e, std::size_t size); ... boost::asio::streambuf b; boost::asio::async_read_until(s, b, match_char('a'), handler);