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PODoes the standard behavior for deleters differ between shared_ptr and unique_ptr in the case of null pointers?
 

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  1. PODoes the standard behavior for deleters differ between shared_ptr and unique_ptr in the case of null pointers?
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    <p>OK, so first some things that might be relevant:</p> <p>I'm using the Clang 3.1 compiler, in C++11 mode, with the standard library set to libc++.</p> <p>I'm trying to familiarize myself with C++11, and in so doing I ran across behavior that seems odd. It may be a quirk of Clang or libc++ but I can't speak C++ standardese and I have no access to other compilers with C++11 support so I can't really check it, and I've searched the internet and Stack Overflow to the best of my ability without finding anything related...so here we go:</p> <p>When using shared_ptr / unique_ptr to implement RAII for a simple resource, it seems that their behavior differs with respect to null pointers upon deletion. I realize that normally it's not necessary to delete a null pointer, but I had expected the behavior to at least match between the two STL smart pointers.</p> <p>For the specific case, consider the following code:</p> <pre><code>{ auto Deleter = [](void *){cout &lt;&lt; "It's later!" &lt;&lt; endl;}; shared_ptr&lt;void&gt; spDoSomethingLater(nullptr, Deleter); unique_ptr&lt;void, void (*)(void *)&gt; upDoSomethingLater(nullptr, Deleter); cout &lt;&lt; "It's now!" &lt;&lt; endl; } </code></pre> <p>I would have expected one of the following outputs from this:</p> <p>a) If both deleters are called even though the pointer is null:</p> <pre><code>"It's now!" "It's later!" "It's later!" </code></pre> <p>b) If neither deleter is called because the pointer is null:</p> <pre><code>"It's now!" </code></pre> <p>But I observe neither of these cases. Instead, I observe:</p> <pre><code>"It's now!" "It's later!" </code></pre> <p>Which means one but not the other of the deleters is being called. Upon further investigation, I found that the deleter for shared_ptr is called regardless of whether it holds a null value, but unique_ptr's deleter is only called if it does not hold a null value.</p> <p>My questions: Is this actually the correct behavior as specified by the standard? If so, why does the specified behavior differ between the two STL types in this manner? If not, is this a bug I should report to libc++?</p>
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