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<h1 class="page">Hello Task</h1>
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>The minimal Capy program: a task that prints a message.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_what_you_will_learn"><a class="anchor" href="#_what_you_will_learn"></a>What You Will Learn</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Creating a <code>task&lt;&gt;</code> coroutine</p>
</li>
<li>
<p>Using <code>thread_pool</code> as an execution context</p>
</li>
<li>
<p>Launching tasks with <code>run_async</code></p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_prerequisites"><a class="anchor" href="#_prerequisites"></a>Prerequisites</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>C&#43;&#43;20 compiler</p>
</li>
<li>
<p>Capy library installed</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_source_code"><a class="anchor" href="#_source_code"></a>Source Code</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-cpp hljs" data-lang="cpp">#include &lt;boost/capy.hpp&gt;
#include &lt;iostream&gt;
using namespace boost::capy;
task&lt;&gt; say_hello()
{
std::cout &lt;&lt; "Hello from Capy!\n";
co_return;
}
int main()
{
thread_pool pool;
run_async(pool.get_executor())(say_hello());
return 0;
}</code></pre>
</div>
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<div class="sect1">
<h2 id="_build"><a class="anchor" href="#_build"></a>Build</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-cmake hljs" data-lang="cmake">add_executable(hello_task hello_task.cpp)
target_link_libraries(hello_task PRIVATE capy)</code></pre>
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<h2 id="_walkthrough"><a class="anchor" href="#_walkthrough"></a>Walkthrough</h2>
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<h3 id="_the_task"><a class="anchor" href="#_the_task"></a>The Task</h3>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-cpp hljs" data-lang="cpp">task&lt;&gt; say_hello()
{
std::cout &lt;&lt; "Hello from Capy!\n";
co_return;
}</code></pre>
</div>
</div>
<div class="paragraph">
<p><code>task&lt;&gt;</code> is equivalent to <code>task&lt;void&gt;</code>—a coroutine that completes without returning a value. The <code>co_return</code> keyword marks this as a coroutine.</p>
</div>
<div class="paragraph">
<p>Tasks are lazy: calling <code>say_hello()</code> creates a task object but does not execute the body. The <code>"Hello"</code> message is not printed until the task is launched.</p>
</div>
</div>
<div class="sect2">
<h3 id="_the_thread_pool"><a class="anchor" href="#_the_thread_pool"></a>The Thread Pool</h3>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-cpp hljs" data-lang="cpp">thread_pool pool;</code></pre>
</div>
</div>
<div class="paragraph">
<p><code>thread_pool</code> provides an execution context with worker threads. By default, it creates one thread per CPU core.</p>
</div>
<div class="paragraph">
<p>The pool&#8217;s destructor waits for all work to complete before returning. This ensures the program doesn&#8217;t exit while tasks are running.</p>
</div>
</div>
<div class="sect2">
<h3 id="_launching"><a class="anchor" href="#_launching"></a>Launching</h3>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-cpp hljs" data-lang="cpp">run_async(pool.get_executor())(say_hello());</code></pre>
</div>
</div>
<div class="paragraph">
<p><code>run_async</code> bridges non-coroutine code (like <code>main</code>) to coroutine code. The two-call syntax:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>run_async(pool.get_executor())</code> — Creates a launcher with the executor</p>
</li>
<li>
<p><code>(say_hello())</code> — Accepts the task and starts execution</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>The task runs on one of the pool&#8217;s worker threads.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_output"><a class="anchor" href="#_output"></a>Output</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-asciidoc hljs" data-lang="asciidoc">Hello from Capy!</code></pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_exercises"><a class="anchor" href="#_exercises"></a>Exercises</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Modify <code>say_hello</code> to accept a <code>std::string_view</code> parameter and print it</p>
</li>
<li>
<p>Create multiple tasks and launch them all</p>
</li>
<li>
<p>Add a handler to <code>run_async</code> that prints when the task completes</p>
</li>
</ol>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_next_steps"><a class="anchor" href="#_next_steps"></a>Next Steps</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p><a href="7b.producer-consumer.html" class="xref page">Producer-Consumer</a> — Multiple tasks communicating</p>
</li>
</ul>
</div>
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