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13 <div class="titlepage"><div><div><h1 style="clear: both"><code>lazy<T>/atomic_lazy<T></code></h1></div></div></div>
14 <p>This is very similar to <a href="../../reference/aliases/eager.html" class="api-reference"><code>eager<T></code></a>
15 , except that execution of the
16 <code>lazy<T></code> returning function suspends immediately. Functions which return <code>lazy<T></code>
17 are therefore suitable for tasks which you need to instantiate right now, but whose
18 execution will occur elsewhere e.g. in a separate kernel thread. Because of the very
19 common use case of using worker threads to execute the body of lazily executed
20 coroutines, most people will want to use <code>atomic_lazy<T></code> instead of <code>lazy<T></code>.</p>
22 <p><code>atomic_lazy<T></code> is like <code>lazy<T></code>, except that the setting of the coroutine result
23 performs an atomic release, whilst the checking of whether the coroutine has finished
24 is an atomic acquire.</p>
26 <p><code>lazy<T></code> has similar semantics to <code>std::lazy<T></code>, which is being standardised. See
27 <a href="https://wg21.link/P1056">https://wg21.link/P1056</a> <em>Add lazy coroutine (coroutine task) type</em>.</p>
29 <p>Example of use (must be called from within a coroutinised function):</p>
30 <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">lazy</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">func</span><span class="p">(</span><span class="kt">int</span> <span class="n">x</span><span class="p">)</span>
31 <span class="p">{</span>
32 <span class="n">co_return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span><span class="p">;</span>
33 <span class="p">}</span>
34 <span class="p">...</span>
35 <span class="c1">// Always suspends perhaps causing other coroutines to execute, then resumes.
36 </span><span class="c1"></span><span class="kt">int</span> <span class="n">r</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">func</span><span class="p">(</span><span class="mi">5</span><span class="p">);</span>
38 <p><code>lazy<T></code> has special semantics if <code>T</code> is a type capable of constructing from
39 an <code>exception_ptr</code> or <code>error_code</code> – any exceptions thrown during the function’s body
40 are sent via <code>T</code>, preferably via the error code route if <a href="../../reference/functions/error_from_exception.html" class="api-reference"><code>error_from_exception(</code></a>
42 successfully matches the exception throw. This means that a <a href="../../reference/types/basic_result.html" class="api-reference"><code>basic_result<T, E, NoValuePolicy></code></a>
44 or <a href="../../reference/types/basic_outcome.html" class="api-reference"><code>basic_outcome<T, EC, EP, NoValuePolicy></code></a>
45 where one of its types is
46 is compatible will have its <code>.error()</code> or <code>.exception()</code> set.</p>
48 <p>Note that <code>lazy<T></code> does not otherwise transport exception throws, and rethrows
49 any exceptions thrown within the coroutine body through the coroutine machinery.
50 This does not produce reliable consequences in current C++ compilers. You should
51 therefore wrap the coroutine body in a <code>try...catch</code> if <code>T</code> is not able to transport
52 exceptions on its own.</p>
54 <p><em>Requires</em>: C++ coroutines to be available in your compiler.</p>
56 <p><em>Namespace</em>: <code>BOOST_OUTCOME_V2_NAMESPACE::awaitables</code></p>
58 <p><em>Header</em>: <code><boost/outcome/coroutine_support.hpp></code></p>
61 </div><p><small>Last revised: October 04, 2019 at 15:58:37 +0100</small></p>
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