doc/html/boost_asio/reference/async_read/overload6.html

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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read.overload6"></a><a class="link" href="overload6.html" title="async_read (6 of 8 overloads)">async_read
  28         (6 of 8 overloads)</a>
  29 </h4></div></div></div>
  30 <p>
  31           Start an asynchronous operation to read a certain amount of data from a
  32           stream.
  33         </p>
  34 <pre class="programlisting">template&lt;
  35     typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
  36     typename Allocator,
  37     typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
  38     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.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>&gt;
  39 <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(
  40     AsyncReadStream &amp; s,
  41     basic_streambuf&lt; Allocator &gt; &amp; b,
  42     CompletionCondition completion_condition,
  43     ReadHandler &amp;&amp; handler = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
  44 </pre>
  45 <p>
  46           This function is used to asynchronously read a certain number of bytes
  47           of data from a stream. The function call always returns immediately. The
  48           asynchronous operation will continue until one of the following conditions
  49           is true:
  50         </p>
  51 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  52 <li class="listitem">
  53               The supplied buffer is full (that is, it has reached maximum size).
  54             </li>
  55 <li class="listitem">
  56               The completion_condition function object returns 0.
  57             </li>
  58 </ul></div>
  59 <p>
  60           This operation is implemented in terms of zero or more calls to the stream's
  61           async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
  62           The program must ensure that the stream performs no other read operations
  63           (such as async_read, the stream's async_read_some function, or any other
  64           composed operations that perform reads) until this operation completes.
  65         </p>
  66 <h6>
  67 <a name="boost_asio.reference.async_read.overload6.h0"></a>
  68           <span class="phrase"><a name="boost_asio.reference.async_read.overload6.parameters"></a></span><a class="link" href="overload6.html#boost_asio.reference.async_read.overload6.parameters">Parameters</a>
  69         </h6>
  70 <div class="variablelist">
  71 <p class="title"><b></b></p>
  72 <dl class="variablelist">
  73 <dt><span class="term">s</span></dt>
  74 <dd><p>
  75                 The stream from which the data is to be read. The type must support
  76                 the AsyncReadStream concept.
  77               </p></dd>
  78 <dt><span class="term">b</span></dt>
  79 <dd><p>
  80                 A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
  81                 object into which the data will be read. Ownership of the streambuf
  82                 is retained by the caller, which must guarantee that it remains valid
  83                 until the handler is called.
  84               </p></dd>
  85 <dt><span class="term">completion_condition</span></dt>
  86 <dd>
  87 <p>
  88                 The function object to be called to determine whether the read operation
  89                 is complete. The signature of the function object must be:
  90 </p>
  91 <pre class="programlisting">std::size_t completion_condition(
  92   // Result of latest async_read_some operation.
  93   const boost::system::error_code&amp; error,
  94
  95   // Number of bytes transferred so far.
  96   std::size_t bytes_transferred
  97 );
  98 </pre>
  99 <p>
 100                 A return value of 0 indicates that the read operation is complete.
 101                 A non-zero return value indicates the maximum number of bytes to
 102                 be read on the next call to the stream's async_read_some function.
 103               </p>
 104 </dd>
 105 <dt><span class="term">handler</span></dt>
 106 <dd>
 107 <p>
 108                 The handler to be called when the read operation completes. Copies
 109                 will be made of the handler as required. The function signature of
 110                 the handler must be:
 111 </p>
 112 <pre class="programlisting">void handler(
 113   const boost::system::error_code&amp; error, // Result of operation.
 114
 115   std::size_t bytes_transferred           // Number of bytes copied into the
 116                                           // buffers. If an error occurred,
 117                                           // this will be the  number of
 118                                           // bytes successfully transferred
 119                                           // prior to the error.
 120 );
 121 </pre>
 122 <p>
 123                 Regardless of whether the asynchronous operation completes immediately
 124                 or not, the handler will not be invoked from within this function.
 125                 On immediate completion, invocation of the handler will be performed
 126                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 127               </p>
 128 </dd>
 129 </dl>
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