[/
/ Copyright (c) 2003-2025 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, 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()
{
asio::async_read_until(socket_, data_, "\r\n",
boost::bind(&http_connection::handle_request_line, this, _1));
}
void handle_request_line(asio::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;
...
}
}
...
asio::ip::tcp::socket socket_;
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 asio::buffers_iterator<
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);
}
...
asio::streambuf b;
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
...
asio::streambuf b;
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 asio.reference.async_read_until async_read_until()],
[link asio.reference.is_match_condition is_match_condition],
[link asio.reference.read_until read_until()],
[link asio.reference.streambuf streambuf],
[link asio.examples.cpp11_examples.http_client HTTP client example].
[endsect]
