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<h1>Basic Server</h1>
<p>In the previous chapter, you sent requests from a client to a test server. Now let's walk through how that server actually works.</p>
<h2>Starting the Server</h2>
<p>Once you've registered your routes, call <code>svr.listen()</code> to start the server.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">listen</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">0.0.0.0</span><span style="color:#c0c5ce;">&quot;, </span><span style="color:#d08770;">8080</span><span style="color:#c0c5ce;">);
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.listen(</span><span style="color:#183691;">&quot;0.0.0.0&quot;</span><span style="color:#323232;">, </span><span style="color:#0086b3;">8080</span><span style="color:#323232;">);
</span></pre>
</div></div>
<p>The first argument is the host, and the second is the port. <code>"0.0.0.0"</code> listens on all network interfaces. Use <code>"127.0.0.1"</code> if you want to accept connections from your own machine only.</p>
<p><code>listen()</code> is a blocking call. It won't return until the server stops. The server keeps running until you press <code>Ctrl+C</code> in your terminal or call <code>svr.stop()</code> from another thread.</p>
<h2>Routing</h2>
<p>Routing is the heart of any server. It's how you tell cpp-httplib: when a request comes in for this URL with this HTTP method, run this code.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">httplib::Server svr;
</span><span style="color:#c0c5ce;">
</span><span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/hi</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const</span><span style="color:#c0c5ce;"> httplib::Request &amp;req, httplib::Response &amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Hello!</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">httplib::Server svr;
</span><span style="color:#323232;">
</span><span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/hi&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const</span><span style="color:#323232;"> httplib::Request </span><span style="font-weight:bold;color:#a71d5d;">&amp;</span><span style="color:#323232;">req, httplib::Response </span><span style="font-weight:bold;color:#a71d5d;">&amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Hello!&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p><code>svr.Get()</code> registers a handler for GET requests. The first argument is the path, the second is the handler function. When a GET request arrives at <code>/hi</code>, your lambda runs.</p>
<p>There's a method for each HTTP verb.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/path</span><span style="color:#c0c5ce;">&quot;, handler); </span><span style="color:#65737e;">// GET
</span><span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/path</span><span style="color:#c0c5ce;">&quot;, handler); </span><span style="color:#65737e;">// POST
</span><span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Put</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/path</span><span style="color:#c0c5ce;">&quot;, handler); </span><span style="color:#65737e;">// PUT
</span><span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Delete</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/path</span><span style="color:#c0c5ce;">&quot;, handler); </span><span style="color:#65737e;">// DELETE
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/path&quot;</span><span style="color:#323232;">, handler); </span><span style="font-style:italic;color:#969896;">// GET
</span><span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/path&quot;</span><span style="color:#323232;">, handler); </span><span style="font-style:italic;color:#969896;">// POST
</span><span style="color:#323232;">svr.Put(</span><span style="color:#183691;">&quot;/path&quot;</span><span style="color:#323232;">, handler); </span><span style="font-style:italic;color:#969896;">// PUT
</span><span style="color:#323232;">svr.Delete(</span><span style="color:#183691;">&quot;/path&quot;</span><span style="color:#323232;">, handler); </span><span style="font-style:italic;color:#969896;">// DELETE
</span></pre>
</div></div>
<p>The handler signature is <code>(const httplib::Request &amp;req, httplib::Response &amp;res)</code>. You can use <code>auto</code> to keep it short.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/hi</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Hello!</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/hi&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Hello!&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>The handler only runs when the path matches. Requests to unregistered paths automatically return 404.</p>
<h2>The Request Object</h2>
<p>The first parameter <code>req</code> gives you everything the client sent.</p>
<h3>Body</h3>
<p><code>req.body</code> holds the request body as a <code>std::string</code>.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/post</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#65737e;">// Echo the body back to the client
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(req.</span><span style="color:#bf616a;">body</span><span style="color:#c0c5ce;">, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/post&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-style:italic;color:#969896;">// Echo the body back to the client
</span><span style="color:#323232;"> res.set_content(req.body, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h3>Headers</h3>
<p>Use <code>req.get_header_value()</code> to read a request header.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/check</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> auth = req.</span><span style="color:#8fa1b3;">get_header_value</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Authorization</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Auth: </span><span style="color:#c0c5ce;">&quot; + auth, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/check&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> auth </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.get_header_value(</span><span style="color:#183691;">&quot;Authorization&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Auth: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> auth, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h3>Query Parameters and Form Data</h3>
<p><code>req.get_param_value()</code> retrieves a parameter by name. It works for both GET query parameters and POST form data.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/search</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> q = req.</span><span style="color:#8fa1b3;">get_param_value</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">q</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Query: </span><span style="color:#c0c5ce;">&quot; + q, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/search&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> q </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.get_param_value(</span><span style="color:#183691;">&quot;q&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Query: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> q, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>A request to <code>/search?q=cpp-httplib</code> gives you <code>"cpp-httplib"</code> for <code>q</code>.</p>
<p>To loop over all parameters, use <code>req.params</code>.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/submit</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> std::string result;
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">for </span><span style="color:#c0c5ce;">(</span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;[key, val] : req.</span><span style="color:#bf616a;">params</span><span style="color:#c0c5ce;">) {
</span><span style="color:#c0c5ce;"> result += key + &quot;</span><span style="color:#a3be8c;"> = </span><span style="color:#c0c5ce;">&quot; + val + &quot;</span><span style="color:#96b5b4;">\n</span><span style="color:#c0c5ce;">&quot;;
</span><span style="color:#c0c5ce;"> }
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(result, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/submit&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> std::string result;
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">for </span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">[key, val] </span><span style="font-weight:bold;color:#a71d5d;">:</span><span style="color:#323232;"> req.params) {
</span><span style="color:#323232;"> result </span><span style="font-weight:bold;color:#a71d5d;">+=</span><span style="color:#323232;"> key </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot; = &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> val </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot;</span><span style="color:#0086b3;">\n</span><span style="color:#183691;">&quot;</span><span style="color:#323232;">;
</span><span style="color:#323232;"> }
</span><span style="color:#323232;"> res.set_content(result, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h3>File Uploads</h3>
<p>Files uploaded via multipart form data are available through <code>req.form.get_file()</code>.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/upload</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> f = req.</span><span style="color:#bf616a;">form</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">get_file</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">file</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> content = f.</span><span style="color:#bf616a;">filename </span><span style="color:#c0c5ce;">+ &quot;</span><span style="color:#a3be8c;"> (</span><span style="color:#c0c5ce;">&quot; + </span><span style="color:#8fa1b3;">std::to_string</span><span style="color:#c0c5ce;">(f.</span><span style="color:#bf616a;">content</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">size</span><span style="color:#c0c5ce;">()) + &quot;</span><span style="color:#a3be8c;"> bytes)</span><span style="color:#c0c5ce;">&quot;;
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(content, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/upload&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> f </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.form.get_file(</span><span style="color:#183691;">&quot;file&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> content </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> f.filename </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot; (&quot; </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#323232;">std::to_string(f.content.size()) </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot; bytes)&quot;</span><span style="color:#323232;">;
</span><span style="color:#323232;"> res.set_content(content, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p><code>f.filename</code> gives you the filename, and <code>f.content</code> gives you the file data.</p>
<h2>Path Parameters</h2>
<p>Sometimes you want to capture part of the URL as a variable -- for example, the <code>42</code> in <code>/users/42</code>. Use the <code>:param</code> syntax to do that.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/users/:id</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> id = req.</span><span style="color:#bf616a;">path_params</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">at</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">id</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">User ID: </span><span style="color:#c0c5ce;">&quot; + id, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/users/:id&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> id </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.path_params.at(</span><span style="color:#183691;">&quot;id&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;User ID: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> id, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>A request to <code>/users/42</code> gives you <code>"42"</code> from <code>req.path_params.at("id")</code>. <code>/users/100</code> gives you <code>"100"</code>.</p>
<p>You can capture multiple segments at once.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/users/:user_id/posts/:post_id</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> user_id = req.</span><span style="color:#bf616a;">path_params</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">at</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">user_id</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> post_id = req.</span><span style="color:#bf616a;">path_params</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">at</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">post_id</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">User: </span><span style="color:#c0c5ce;">&quot; + user_id + &quot;</span><span style="color:#a3be8c;">, Post: </span><span style="color:#c0c5ce;">&quot; + post_id, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/users/:user_id/posts/:post_id&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> user_id </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.path_params.at(</span><span style="color:#183691;">&quot;user_id&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> post_id </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.path_params.at(</span><span style="color:#183691;">&quot;post_id&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;User: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> user_id </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot;, Post: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> post_id, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h3>Regex Patterns</h3>
<p>You can also write a regular expression directly in the path instead of <code>:param</code>. Capture group values are available via <code>req.matches</code>, which is a <code>std::smatch</code>.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#65737e;">// Only accept numeric IDs
</span><span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(</span><span style="color:#b48ead;">R</span><span style="color:#c0c5ce;">&quot;(</span><span style="color:#a3be8c;">/files/(\d+)</span><span style="color:#c0c5ce;">)&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> id = req.</span><span style="color:#bf616a;">matches</span><span style="color:#c0c5ce;">[</span><span style="color:#d08770;">1</span><span style="color:#c0c5ce;">]; </span><span style="color:#65737e;">// First capture group
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">File ID: </span><span style="color:#c0c5ce;">&quot; + </span><span style="color:#8fa1b3;">std::string</span><span style="color:#c0c5ce;">(id), &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="font-style:italic;color:#969896;">// Only accept numeric IDs
</span><span style="color:#323232;">svr.Get(</span><span style="font-weight:bold;color:#a71d5d;">R</span><span style="color:#183691;">&quot;(/files/(\d+))&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> id </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.matches[</span><span style="color:#0086b3;">1</span><span style="color:#323232;">]; </span><span style="font-style:italic;color:#969896;">// First capture group
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;File ID: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#323232;">std::string(id), </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p><code>/files/42</code> matches, but <code>/files/abc</code> doesn't. This is handy when you want to constrain what values are accepted.</p>
<h2>Building a Response</h2>
<p>The second parameter <code>res</code> is how you send data back to the client.</p>
<h3>Body and Content-Type</h3>
<p><code>res.set_content()</code> sets the body and Content-Type. That's all you need for a 200 response.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/hi</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Hello!</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/hi&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Hello!&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h3>Status Code</h3>
<p>To return a different status code, assign to <code>res.status</code>.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/not-found</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#bf616a;">status </span><span style="color:#c0c5ce;">= </span><span style="color:#d08770;">404</span><span style="color:#c0c5ce;">;
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Not found</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/not-found&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.status </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#0086b3;">404</span><span style="color:#323232;">;
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Not found&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h3>Response Headers</h3>
<p>Add response headers with <code>res.set_header()</code>.</p>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/with-header</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_header</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">X-Custom</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">my-value</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Hello!</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/with-header&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.set_header(</span><span style="color:#183691;">&quot;X-Custom&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;my-value&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Hello!&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<h2>Walking Through the Test Server</h2>
<p>Now let's use what we've learned to read through the test server from the previous chapter.</p>
<h3>GET /hi</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/hi</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Hello!</span><span style="color:#c0c5ce;">&quot;, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/hi&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Hello!&quot;</span><span style="color:#323232;">, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>The simplest possible handler. We don't need any information from the request, so the <code>req</code> parameter is left unnamed. It just returns <code>"Hello!"</code>.</p>
<h3>GET /search</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/search</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> q = req.</span><span style="color:#8fa1b3;">get_param_value</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">q</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">Query: </span><span style="color:#c0c5ce;">&quot; + q, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/search&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> q </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.get_param_value(</span><span style="color:#183691;">&quot;q&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;Query: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> q, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p><code>req.get_param_value("q")</code> pulls out the query parameter <code>q</code>. A request to <code>/search?q=cpp-httplib</code> returns <code>"Query: cpp-httplib"</code>.</p>
<h3>POST /post</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/post</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(req.</span><span style="color:#bf616a;">body</span><span style="color:#c0c5ce;">, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/post&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> res.set_content(req.body, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>An echo server. Whatever body the client sends, <code>req.body</code> holds it, and we send it straight back.</p>
<h3>POST /submit</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/submit</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> std::string result;
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">for </span><span style="color:#c0c5ce;">(</span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;[key, val] : req.</span><span style="color:#bf616a;">params</span><span style="color:#c0c5ce;">) {
</span><span style="color:#c0c5ce;"> result += key + &quot;</span><span style="color:#a3be8c;"> = </span><span style="color:#c0c5ce;">&quot; + val + &quot;</span><span style="color:#96b5b4;">\n</span><span style="color:#c0c5ce;">&quot;;
</span><span style="color:#c0c5ce;"> }
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(result, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/submit&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> std::string result;
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">for </span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">[key, val] </span><span style="font-weight:bold;color:#a71d5d;">:</span><span style="color:#323232;"> req.params) {
</span><span style="color:#323232;"> result </span><span style="font-weight:bold;color:#a71d5d;">+=</span><span style="color:#323232;"> key </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot; = &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> val </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot;</span><span style="color:#0086b3;">\n</span><span style="color:#183691;">&quot;</span><span style="color:#323232;">;
</span><span style="color:#323232;"> }
</span><span style="color:#323232;"> res.set_content(result, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>Loops over the form data in <code>req.params</code> using structured bindings (<code>auto &amp;[key, val]</code>) to unpack each key-value pair.</p>
<h3>POST /upload</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Post</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/upload</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> f = req.</span><span style="color:#bf616a;">form</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">get_file</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">file</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> content = f.</span><span style="color:#bf616a;">filename </span><span style="color:#c0c5ce;">+ &quot;</span><span style="color:#a3be8c;"> (</span><span style="color:#c0c5ce;">&quot; + </span><span style="color:#8fa1b3;">std::to_string</span><span style="color:#c0c5ce;">(f.</span><span style="color:#bf616a;">content</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">size</span><span style="color:#c0c5ce;">()) + &quot;</span><span style="color:#a3be8c;"> bytes)</span><span style="color:#c0c5ce;">&quot;;
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(content, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Post(</span><span style="color:#183691;">&quot;/upload&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> f </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.form.get_file(</span><span style="color:#183691;">&quot;file&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> content </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> f.filename </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot; (&quot; </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#323232;">std::to_string(f.content.size()) </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#183691;">&quot; bytes)&quot;</span><span style="color:#323232;">;
</span><span style="color:#323232;"> res.set_content(content, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>Receives a file uploaded via multipart form data. <code>req.form.get_file("file")</code> fetches the field named <code>"file"</code>, and we respond with the filename and size.</p>
<h3>GET /users/:id</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">/users/:id</span><span style="color:#c0c5ce;">&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> id = req.</span><span style="color:#bf616a;">path_params</span><span style="color:#c0c5ce;">.</span><span style="color:#8fa1b3;">at</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">id</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">User ID: </span><span style="color:#c0c5ce;">&quot; + id, &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="color:#183691;">&quot;/users/:id&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> id </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.path_params.at(</span><span style="color:#183691;">&quot;id&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;User ID: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+</span><span style="color:#323232;"> id, </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p><code>:id</code> is the path parameter. <code>req.path_params.at("id")</code> retrieves its value. <code>/users/42</code> gives you <code>"42"</code>, <code>/users/alice</code> gives you <code>"alice"</code>.</p>
<h3>GET /files/(\d+)</h3>
<div class="code-block-wrapper"><div data-code-theme="dark"><pre style="background-color:#2b303b;">
<span style="color:#c0c5ce;">svr.</span><span style="color:#8fa1b3;">Get</span><span style="color:#c0c5ce;">(</span><span style="color:#b48ead;">R</span><span style="color:#c0c5ce;">&quot;(</span><span style="color:#a3be8c;">/files/(\d+)</span><span style="color:#c0c5ce;">)&quot;, [](</span><span style="color:#b48ead;">const auto </span><span style="color:#c0c5ce;">&amp;req, </span><span style="color:#b48ead;">auto </span><span style="color:#c0c5ce;">&amp;res) {
</span><span style="color:#c0c5ce;"> </span><span style="color:#b48ead;">auto</span><span style="color:#c0c5ce;"> id = req.</span><span style="color:#bf616a;">matches</span><span style="color:#c0c5ce;">[</span><span style="color:#d08770;">1</span><span style="color:#c0c5ce;">];
</span><span style="color:#c0c5ce;"> res.</span><span style="color:#8fa1b3;">set_content</span><span style="color:#c0c5ce;">(&quot;</span><span style="color:#a3be8c;">File ID: </span><span style="color:#c0c5ce;">&quot; + </span><span style="color:#8fa1b3;">std::string</span><span style="color:#c0c5ce;">(id), &quot;</span><span style="color:#a3be8c;">text/plain</span><span style="color:#c0c5ce;">&quot;);
</span><span style="color:#c0c5ce;">});
</span></pre>
</div><div data-code-theme="light"><pre style="background-color:#ffffff;">
<span style="color:#323232;">svr.Get(</span><span style="font-weight:bold;color:#a71d5d;">R</span><span style="color:#183691;">&quot;(/files/(\d+))&quot;</span><span style="color:#323232;">, [](</span><span style="font-weight:bold;color:#a71d5d;">const auto &amp;</span><span style="color:#323232;">req, </span><span style="font-weight:bold;color:#a71d5d;">auto &amp;</span><span style="color:#323232;">res) {
</span><span style="color:#323232;"> </span><span style="font-weight:bold;color:#a71d5d;">auto</span><span style="color:#323232;"> id </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> req.matches[</span><span style="color:#0086b3;">1</span><span style="color:#323232;">];
</span><span style="color:#323232;"> res.set_content(</span><span style="color:#183691;">&quot;File ID: &quot; </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#323232;">std::string(id), </span><span style="color:#183691;">&quot;text/plain&quot;</span><span style="color:#323232;">);
</span><span style="color:#323232;">});
</span></pre>
</div></div>
<p>The regex <code>(\d+)</code> matches numeric IDs only. <code>/files/42</code> hits this handler, but <code>/files/abc</code> returns 404. <code>req.matches[1]</code> retrieves the first capture group.</p>
<h2>Next Steps</h2>
<p>You now have the full picture of how a server works. Routing, reading requests, building responses -- that's enough to build a real API server.</p>
<p>Next, let's look at serving static files. We'll build a server that delivers HTML and CSS.</p>
<p><strong>Next:</strong> <a href="../04-static-file-server">Static File Server</a></p>
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