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<div class="titlepage"><div><div><h1 style="clear: both"><code>lazy<T>/atomic_lazy<T></code></h1></div></div></div>
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<p>This is very similar to <a href="../../reference/aliases/eager.html" class="api-reference"><code>eager<T></code></a>
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, except that execution of the
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<code>lazy<T></code> returning function suspends immediately. Functions which return <code>lazy<T></code>
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are therefore suitable for tasks which you need to instantiate right now, but whose
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execution will occur elsewhere e.g. in a separate kernel thread. Because of the very
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common use case of using worker threads to execute the body of lazily executed
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coroutines, most people will want to use <code>atomic_lazy<T></code> instead of <code>lazy<T></code>.</p>
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<p><code>atomic_lazy<T></code> is like <code>lazy<T></code>, except that the setting of the coroutine result
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performs an atomic release, whilst the checking of whether the coroutine has finished
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is an atomic acquire.</p>
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<p><code>lazy<T></code> has similar semantics to <code>std::lazy<T></code>, which is being standardised. See
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<a href="https://wg21.link/P1056">https://wg21.link/P1056</a> <em>Add lazy coroutine (coroutine task) type</em>.</p>
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<p>Example of use (must be called from within a coroutinised function):</p>
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<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>
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<span class="p">{</span>
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<span class="n">co_return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="p">...</span>
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<span class="c1">// Always suspends perhaps causing other coroutines to execute, then resumes.
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</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>
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</code></pre></div>
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<p><code>lazy<T></code> has special semantics if <code>T</code> is a type capable of constructing from
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an <code>exception_ptr</code> or <code>error_code</code> – any exceptions thrown during the function’s body
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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>
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<code>)</code>
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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>
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or <a href="../../reference/types/basic_outcome.html" class="api-reference"><code>basic_outcome<T, EC, EP, NoValuePolicy></code></a>
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where one of its types is
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is compatible will have its <code>.error()</code> or <code>.exception()</code> set.</p>
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<p>Note that <code>lazy<T></code> does not otherwise transport exception throws, and rethrows
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any exceptions thrown within the coroutine body through the coroutine machinery.
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This does not produce reliable consequences in current C++ compilers. You should
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therefore wrap the coroutine body in a <code>try...catch</code> if <code>T</code> is not able to transport
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exceptions on its own.</p>
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<p><em>Requires</em>: C++ coroutines to be available in your compiler.</p>
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<p><em>Namespace</em>: <code>BOOST_OUTCOME_V2_NAMESPACE::awaitables</code></p>
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<p><em>Header</em>: <code><boost/outcome/coroutine_support.hpp></code></p>
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</div><p><small>Last revised: October 04, 2019 at 15:58:37 +0100</small></p>
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