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<div class="titlepage"><div><div><h3 class="title">
<a name="beast.more_examples.head_request_client"></a><a class="link" href="head_request_client.html" title="HEAD request (Client)">HEAD request
(Client)</a>
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<p>
The <a href="https://tools.ietf.org/html/rfc7231#section-4.3.2" target="_top">HEAD request</a>
method indicates to the server that the client wishes to receive the entire
header that would be delivered if the method was GET, except that the body
is omitted. When a client wishes to receive the response to a HEAD request,
it is necessary to inform the parser not to expect a body. This is done by
calling <a class="link" href="../ref/boost__beast__http__basic_parser/skip.html" title="http::basic_parser::skip"><code class="computeroutput"><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">skip</span></code></a> with the value <code class="computeroutput"><span class="keyword">true</span></code>, as shown in this example:
</p>
<pre class="programlisting"><span class="comment">/** Send a HEAD request for a resource.
This function submits a HEAD request for the specified resource
and returns the response.
@param res The response. This is an output parameter.
@param stream The synchronous stream to use.
@param buffer The buffer to use.
@param target The request target.
@param ec Set to the error, if any occurred.
@throws std::invalid_argument if target is empty.
*/</span>
<span class="keyword">template</span><span class="special"><</span>
<span class="keyword">class</span> <span class="identifier">SyncStream</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">DynamicBuffer</span>
<span class="special">></span>
<span class="identifier">response</span><span class="special"><</span><span class="identifier">empty_body</span><span class="special">></span>
<span class="identifier">do_head_request</span><span class="special">(</span>
<span class="identifier">SyncStream</span><span class="special">&</span> <span class="identifier">stream</span><span class="special">,</span>
<span class="identifier">DynamicBuffer</span><span class="special">&</span> <span class="identifier">buffer</span><span class="special">,</span>
<span class="identifier">string_view</span> <span class="identifier">target</span><span class="special">,</span>
<span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">ec</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// Do some type checking to be a good citizen</span>
<span class="keyword">static_assert</span><span class="special">(</span><span class="identifier">is_sync_stream</span><span class="special"><</span><span class="identifier">SyncStream</span><span class="special">>::</span><span class="identifier">value</span><span class="special">,</span>
<span class="string">"SyncStream requirements not met"</span><span class="special">);</span>
<span class="keyword">static_assert</span><span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">is_dynamic_buffer</span><span class="special"><</span><span class="identifier">DynamicBuffer</span><span class="special">>::</span><span class="identifier">value</span><span class="special">,</span>
<span class="string">"DynamicBuffer requirments not met"</span><span class="special">);</span>
<span class="comment">// The interfaces we are using are low level and do not</span>
<span class="comment">// perform any checking of arguments; so we do it here.</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">target</span><span class="special">.</span><span class="identifier">empty</span><span class="special">())</span>
<span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">invalid_argument</span><span class="special">(</span><span class="string">"target may not be empty"</span><span class="special">);</span>
<span class="comment">// Build the HEAD request for the target</span>
<span class="identifier">request</span><span class="special"><</span><span class="identifier">empty_body</span><span class="special">></span> <span class="identifier">req</span><span class="special">;</span>
<span class="identifier">req</span><span class="special">.</span><span class="identifier">version</span><span class="special">(</span><span class="number">11</span><span class="special">);</span>
<span class="identifier">req</span><span class="special">.</span><span class="identifier">method</span><span class="special">(</span><span class="identifier">verb</span><span class="special">::</span><span class="identifier">head</span><span class="special">);</span>
<span class="identifier">req</span><span class="special">.</span><span class="identifier">target</span><span class="special">(</span><span class="identifier">target</span><span class="special">);</span>
<span class="identifier">req</span><span class="special">.</span><span class="identifier">set</span><span class="special">(</span><span class="identifier">field</span><span class="special">::</span><span class="identifier">user_agent</span><span class="special">,</span> <span class="string">"test"</span><span class="special">);</span>
<span class="comment">// A client MUST send a Host header field in all HTTP/1.1 request messages.</span>
<span class="comment">// https://tools.ietf.org/html/rfc7230#section-5.4</span>
<span class="identifier">req</span><span class="special">.</span><span class="identifier">set</span><span class="special">(</span><span class="identifier">field</span><span class="special">::</span><span class="identifier">host</span><span class="special">,</span> <span class="string">"localhost"</span><span class="special">);</span>
<span class="comment">// Now send it</span>
<span class="identifier">write</span><span class="special">(</span><span class="identifier">stream</span><span class="special">,</span> <span class="identifier">req</span><span class="special">,</span> <span class="identifier">ec</span><span class="special">);</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">ec</span><span class="special">)</span>
<span class="keyword">return</span> <span class="special">{};</span>
<span class="comment">// Create a parser to read the response.</span>
<span class="comment">// We use the `empty_body` type since</span>
<span class="comment">// a response to a HEAD request MUST NOT</span>
<span class="comment">// include a body.</span>
<span class="identifier">response_parser</span><span class="special"><</span><span class="identifier">empty_body</span><span class="special">></span> <span class="identifier">p</span><span class="special">;</span>
<span class="comment">// Inform the parser that there will be no body.</span>
<span class="identifier">p</span><span class="special">.</span><span class="identifier">skip</span><span class="special">(</span><span class="keyword">true</span><span class="special">);</span>
<span class="comment">// Read the message. Even though fields like</span>
<span class="comment">// Content-Length or Transfer-Encoding may be</span>
<span class="comment">// set, the message will not contain a body.</span>
<span class="identifier">read</span><span class="special">(</span><span class="identifier">stream</span><span class="special">,</span> <span class="identifier">buffer</span><span class="special">,</span> <span class="identifier">p</span><span class="special">,</span> <span class="identifier">ec</span><span class="special">);</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">ec</span><span class="special">)</span>
<span class="keyword">return</span> <span class="special">{};</span>
<span class="comment">// Transfer ownership of the response to the caller.</span>
<span class="keyword">return</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span>
<span class="special">}</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2016, 2017 Vinnie Falco<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>)
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