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<title>async_read_until (4 of 8 overloads)</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.async_read_until.overload4"></a><a class="link" href="overload4.html" title="async_read_until (4 of 8 overloads)">async_read_until
(4 of 8 overloads)</a>
</h4></div></div></div>
<p>
Start an asynchronous operation to read data into a dynamic buffer sequence
until a function object indicates a match.
</p>
<pre class="programlisting">template<
typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
typename <a class="link" href="../DynamicBuffer.html" title="Dynamic buffer requirements">DynamicBuffer</a>,
typename MatchCondition,
typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a>>
<a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read_until(
AsyncReadStream & s,
DynamicBuffer && buffers,
MatchCondition match_condition,
ReadHandler && handler,
typename enable_if< is_match_condition< MatchCondition >::value >::type * = 0);
</pre>
<p>
This function is used to asynchronously read data into the specified dynamic
buffer sequence until a user-defined match condition function object, when
applied to the data contained in the dynamic buffer sequence, indicates
a successful match. The function call always returns immediately. The asynchronous
operation will continue until one of the following conditions is true:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The match condition function object returns a std::pair where the second
element evaluates to true.
</li>
<li class="listitem">
An error occurred.
</li>
</ul></div>
<p>
This operation is implemented in terms of zero or more calls to the stream's
async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
If the match condition function object already indicates a match, 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.
</p>
<h6>
<a name="boost_asio.reference.async_read_until.overload4.h0"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_until.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_read_until.overload4.parameters">Parameters</a>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">s</span></dt>
<dd><p>
The stream from which the data is to be read. The type must support
the AsyncReadStream concept.
</p></dd>
<dt><span class="term">buffers</span></dt>
<dd><p>
The dynamic buffer sequence into which the data will be read. Although
the buffers object may be copied as necessary, ownership of the underlying
memory blocks is retained by the caller, which must guarantee that
they remain valid until the handler is called.
</p></dd>
<dt><span class="term">match_condition</span></dt>
<dd>
<p>
The function object to be called to determine whether a match exists.
The signature of the function object must be:
</p>
<pre class="programlisting">pair<iterator, bool> match_condition(iterator begin, iterator end);
</pre>
<p>
where <code class="computeroutput">iterator</code> represents the type:
</p>
<pre class="programlisting">buffers_iterator<typename DynamicBuffer::const_buffers_type>
</pre>
<p>
The iterator parameters <code class="computeroutput">begin</code> and <code class="computeroutput">end</code> define
the range of bytes to be scanned to determine whether there is a
match. The <code class="computeroutput">first</code> 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 <code class="computeroutput">begin</code>
parameter for any subsequent invocation of the match condition. The
<code class="computeroutput">second</code> member of the return value is true if a match
has been found, false otherwise.
</p>
</dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
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:
</p>
<pre class="programlisting">void handler(
// Result of operation.
const boost::system::error_code& error,
// The number of bytes in the dynamic buffer sequence's
// get area that have been fully consumed by the match
// function. O if an error occurred.
std::size_t bytes_transferred
);
</pre>
<p>
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 <code class="computeroutput">boost::asio::io_context::post()</code>.
</p>
</dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.async_read_until.overload4.h1"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_until.overload4.remarks"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_read_until.overload4.remarks">Remarks</a>
</h6>
<p>
After a successful async_read_until operation, the dynamic buffer sequence
may contain additional data beyond that which matched the function object.
An application will typically leave that data in the dynamic buffer sequence
for a subsequent async_read_until operation to examine.
</p>
<p>
The default implementation of the <code class="computeroutput">is_match_condition</code> type
trait evaluates to true for function pointers and function objects with
a <code class="computeroutput">result_type</code> typedef. It must be specialised for other user-defined
function objects.
</p>
<h6>
<a name="boost_asio.reference.async_read_until.overload4.h2"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_until.overload4.examples"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_read_until.overload4.examples">Examples</a>
</h6>
<p>
To asynchronously read data into a <code class="computeroutput">std::string</code> until whitespace
is encountered:
</p>
<pre class="programlisting">typedef boost::asio::buffers_iterator<
boost::asio::const_buffers_1> 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);
...
std::string data;
boost::asio::async_read_until(s, data, match_whitespace, handler);
</pre>
<p>
To asynchronously read data into a <code class="computeroutput">std::string</code> until a matching
character is found:
</p>
<pre class="programlisting">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);
...
std::string data;
boost::asio::async_read_until(s, data, match_char('a'), handler);
</pre>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2018 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
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