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POAccess to callable object inside an async task conflicts with use of std::cin
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  1. POAccess to callable object inside an async task conflicts with use of std::cin
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    copied!<pre><code>#include &lt;iostream&gt; #include &lt;functional&gt; #include &lt;future&gt; #include &lt;tchar.h&gt; void StartBackground(std::function&lt;void()&gt; notify) { auto background = std::async([&amp;] { notify(); // (A) }); } int _tmain(int argc, _TCHAR* argv[]) { StartBackground([](){}); char c; std::cin &gt;&gt; c; // (B) while (1); return 0; } </code></pre> <p>1) Build and run the code above using Visual Studio 2012.</p> <p>2) Line (A) triggers an Access Violation in <code>_VARIADIC_EXPAND_P1_0(_CLASS_FUNC_CLASS_0, , , , )</code>:</p> <blockquote> <p>First-chance exception at 0x0F96271E (msvcp110d.dll) in ConsoleApplication1.exe: 0xC0000005: Access violation writing location 0x0F9626D8</p> </blockquote> <p>Most confusingly, the exception can be avoided by removing line (B).</p> <h2>Questions</h2> <ul> <li>Why does the callable object <code>notify</code> apparently conflict with the use of <code>std::cin</code>?</li> <li>What's wrong with this code?</li> </ul> <p>The real world scenario for this simplified example is a function that executes some code in parallel and have that code call a user-supplied notify function when done.</p> <h2>Edit</h2> <p>I found at least one problem im my code: The <code>background</code> variable is destroyed as soon as <code>StartBackground()</code> exits. Since <code>std::async</code> may or may not start a separate thread, and <code>std::thread</code> calls <code>terminate()</code> if the thread is still joinable, this might be causing the problem. The following variant works because it gives the task enough time to complete:</p> <pre><code>void StartBackground(std::function&lt;void()&gt; notify) { auto background = std::async([&amp;] { notify(); // (A) }); std::this_thread::sleep_for(std::chrono::seconds(1)); } </code></pre> <p>Keeping the <code>std::future</code> object alive over a longer period instead of sleeping should also work. But the following code also causes the same access violation:</p> <pre><code>std::future&lt;void&gt; background; void StartBackground(std::function&lt;void()&gt; notify) { background = std::async([&amp;] { notify(); // (A) }); } </code></pre> <p>whereas using a <code>std::thread</code> in the same manner works as expected:</p> <pre><code>std::thread background; void StartBackground(std::function&lt;void()&gt; notify) { background = std::thread([&amp;] { notify(); // (A) }); } </code></pre> <p>I'm completely puzzled. I must be missing some very crucial points here regarding <code>std::async</code> and <code>std::thread</code>.</p>
 

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