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- [/
- / Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
- /
- / Distributed under the Boost Software License, Version 1.0. (See accompanying
- / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
- /]
- [section:line_based Line-Based Operations]
- Many commonly-used internet protocols are line-based, which means that they
- have protocol elements that are delimited by the character sequence `"\r\n"`.
- Examples include HTTP, SMTP and FTP. To more easily permit the implementation
- of line-based protocols, as well as other protocols that use delimiters, Boost.Asio
- includes the functions `read_until()` and `async_read_until()`.
- The following example illustrates the use of `async_read_until()` in an HTTP
- server, to receive the first line of an HTTP request from a client:
- class http_connection
- {
- ...
- void start()
- {
- boost::asio::async_read_until(socket_, data_, "\r\n",
- boost::bind(&http_connection::handle_request_line, this, _1));
- }
- void handle_request_line(boost::system::error_code ec)
- {
- if (!ec)
- {
- std::string method, uri, version;
- char sp1, sp2, cr, lf;
- std::istream is(&data_);
- is.unsetf(std::ios_base::skipws);
- is >> method >> sp1 >> uri >> sp2 >> version >> cr >> lf;
- ...
- }
- }
- ...
- boost::asio::ip::tcp::socket socket_;
- boost::asio::streambuf data_;
- };
- The `streambuf` data member serves as a place to store the data that has been
- read from the socket before it is searched for the delimiter. It is important
- to remember that there may be additional data ['after] the delimiter. This
- surplus data should be left in the `streambuf` so that it may be inspected by a
- subsequent call to `read_until()` or `async_read_until()`.
- The delimiters may be specified as a single `char`, a `std::string` or a
- `boost::regex`. The `read_until()` and `async_read_until()` functions also
- include overloads that accept a user-defined function object called a match
- condition. For example, 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 boost { namespace asio {
- template <> struct is_match_condition<match_char>
- : public boost::true_type {};
- } } // namespace boost::asio
- ...
- boost::asio::streambuf b;
- boost::asio::read_until(s, b, match_char('a'));
- The `is_match_condition<>` type trait automatically evaluates to true for
- functions, and for function objects with a nested `result_type` typedef. For
- other types the trait must be explicitly specialised, as shown above.
- [heading See Also]
- [link boost_asio.reference.async_read_until async_read_until()],
- [link boost_asio.reference.is_match_condition is_match_condition],
- [link boost_asio.reference.read_until read_until()],
- [link boost_asio.reference.streambuf streambuf],
- [link boost_asio.examples.cpp03_examples.http_client HTTP client example].
- [endsect]
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