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<span class="m-breadcrumb"><a href="tutorial.html">Tutorial</a> »</span>
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Awaiting continuables
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</h1>
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<p>Explains how to use the <a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> together with <code>co_await</code>.</p>
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<h3>Contents</h3>
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<ul>
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<li><a href="#tutorial-awaiting-continuables-usage">Using co_await on continuables</a></li>
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<li><a href="#tutorial-awaiting-continuables-await">Using co_await with exceptions</a></li>
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<li><a href="#tutorial-awaiting-continuables-noexcept">Using co_await with disabled exceptions</a></li>
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<li><a href="#tutorial-awaiting-continuables-return">Using continuables as return type from coroutines</a></li>
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<section id="tutorial-awaiting-continuables-usage"><h2><a href="#tutorial-awaiting-continuables-usage">Using co_await on continuables</a></h2><p>Coroutines (<code>co_await</code>) are supported by continuables when the underlying toolchain supports the TS. Currently this works in MSVC 2017 and Clang 5.0.</p><aside class="m-note m-warning"><h4>Attention</h4><p>You have to enable this feature through defining the <code>CONTINUABLE_WITH_EXPERIMENTAL_COROUTINE</code> preprocessor definition.</p></aside><p>It is possible to await for any <a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> as shown below:</p><pre class="m-code"><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">cti</span><span class="o">::</span><span class="n">make_continuable</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">([](</span><span class="k">auto</span><span class="o">&&</span> <span class="n">promise</span><span class="p">)</span> <span class="p">{</span>
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<span class="n">promise</span><span class="p">.</span><span class="n">set_value</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
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<span class="p">});</span></pre><p>As described in <a href="classcti_1_1continuable__base.html#ac79b103d5c000a3a3d1b98d8adcf58e8" class="m-dox">continuable_base::<wbr />operator co_await()</a> a continuable with multiple arguments as result will wrap its result into a <code>std::tuple</code>:</p><pre class="m-code"><span class="n">std</span><span class="o">::</span><span class="n">tuple</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="o">></span> <span class="n">i</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">cti</span><span class="o">::</span><span class="n">make_ready_continuable</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">);</span></pre></section><section id="tutorial-awaiting-continuables-await"><h2><a href="#tutorial-awaiting-continuables-await">Using co_await with exceptions</a></h2><p>When a <a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> was resolved through an exception the exception is rethrown from the <code>co_await</code> expression:</p><pre class="m-code"><span class="k">try</span> <span class="p">{</span>
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<span class="k">auto</span> <span class="n">response</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">http_request</span><span class="p">(</span><span class="s">"github.com"</span><span class="p">);</span>
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<span class="p">}</span> <span class="k">catch</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">exception</span> <span class="k">const</span><span class="o">&</span> <span class="n">e</span><span class="p">)</span> <span class="p">{</span>
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<span class="c1">// Handle the exception</span>
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<span class="p">}</span></pre></section><section id="tutorial-awaiting-continuables-noexcept"><h2><a href="#tutorial-awaiting-continuables-noexcept">Using co_await with disabled exceptions</a></h2><p>In case the library is configured to use error codes or a custom error type the return type of the co_await expression is changed.</p><p>The result is returned through an internal proxy object which may be queried for the error object:</p><table class="m-table"><thead><tr><th>Continuation type</th><th>co_await returns</th></tr></thead><tbody><tr><td><code><a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> with <></code></td><td><code>unspecified<void></code></td></tr><tr><td><code><a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> with <Arg></code></td><td><code>unspecified<Arg></code></td></tr><tr><td><code><a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> with <Args...></code></td><td><code>unspecified<std::tuple<Args...>></code></td></tr></tbody></table><p>The interface of the proxy object is similar to the one proposed in the <code>std::expected</code> proposal:</p><pre class="m-code"><span class="k">if</span> <span class="p">(</span><span class="k">auto</span><span class="o">&&</span> <span class="n">result</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">http_request</span><span class="p">(</span><span class="s">"github.com"</span><span class="p">))</span> <span class="p">{</span>
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<span class="k">auto</span> <span class="n">value</span> <span class="o">=</span> <span class="o">*</span><span class="n">result</span><span class="p">;</span>
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<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
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<span class="n">cti</span><span class="o">::</span><span class="n">error_type</span> <span class="n">error</span> <span class="o">=</span> <span class="n">result</span><span class="p">.</span><span class="n">get_exception</span><span class="p">();</span>
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<span class="p">}</span>
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<span class="k">auto</span> <span class="n">result</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">http_request</span><span class="p">(</span><span class="s">"github.com"</span><span class="p">);</span>
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<span class="kt">bool</span><span class="p">(</span><span class="n">result</span><span class="p">);</span>
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<span class="n">result</span><span class="p">.</span><span class="n">is_value</span><span class="p">();</span>
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<span class="n">result</span><span class="p">.</span><span class="n">is_exception</span><span class="p">();</span>
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<span class="o">*</span><span class="n">result</span><span class="p">;</span> <span class="c1">// Same as result.get_value()</span>
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<span class="n">result</span><span class="p">.</span><span class="n">get_value</span><span class="p">();</span>
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<span class="n">result</span><span class="p">.</span><span class="n">get_exception</span><span class="p">();</span></pre></section><section id="tutorial-awaiting-continuables-return"><h2><a href="#tutorial-awaiting-continuables-return">Using continuables as return type from coroutines</a></h2><p>It is possible to use a <a href="classcti_1_1continuable__base.html" class="m-dox">continuable_<wbr />base</a> as return type from coroutines.</p><pre class="m-code"><span class="n">cti</span><span class="o">::</span><span class="n">continuable</span><span class="o"><></span> <span class="n">resolve_async_void</span><span class="p">()</span> <span class="p">{</span>
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<span class="n">co_await</span> <span class="n">http_request</span><span class="p">(</span><span class="s">"github.com"</span><span class="p">);</span>
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<span class="c1">// ...</span>
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<span class="n">co_return</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="n">cti</span><span class="o">::</span><span class="n">continuable</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">resolve_async</span><span class="p">()</span> <span class="p">{</span>
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<span class="n">co_await</span> <span class="n">http_request</span><span class="p">(</span><span class="s">"github.com"</span><span class="p">);</span>
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<span class="c1">// ...</span>
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<span class="n">co_return</span> <span class="mi">0</span><span class="p">;</span>
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<span class="p">}</span></pre><p>Additionally it's possible to return multiple return values from coroutines by wrapping those in a tuple like type:</p><pre class="m-code"><span class="n">cti</span><span class="o">::</span><span class="n">continuable</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="o">></span> <span class="n">resolve_async_multiple</span><span class="p">()</span> <span class="p">{</span>
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<span class="n">co_await</span> <span class="n">http_request</span><span class="p">(</span><span class="s">"github.com"</span><span class="p">);</span>
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<span class="c1">// ...</span>
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<span class="n">co_return</span> <span class="n">std</span><span class="o">::</span><span class="n">make_tuple</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">);</span>
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<span class="p">}</span></pre></section>
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