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I'm reading docs on DirectXMath, and stumbled upon next passage:

As an alternative to enforcing alignment in your C++ class directly by overloading new/delete, you can use the pImpl idiom. If you ensure your Impl class is aligned via __aligned_malloc internally, you can then freely use aligned types within the internal implementation. This is a good option when the 'public' class is a Windows Runtime ref class or intended for use with std::shared_ptr<>, which can otherwise disrupt careful alignment.

I don't understand how shared_ptr can do any change in alignment strategy, it only have a pointer, it doesn't allocate an object.

Yola
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  • I agree. It seems to me that the docs are outdated. in c++11 you can specify the alignment of your data type via attributes. – Richard Hodges Jul 05 '15 at 08:53

2 Answers2

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You're right, std::shared_ptr doesn't affect alignment. It just takes in a pointer to an already allocated object, so if that allocation resulted in a misaligned object, the problem isn't with std::shared_ptr, it's with that allocation.

But std::shared_ptr is often used with std::make_shared. std::make_shared<T> performs a single allocation to reserve memory both for the std::shared_ptr control structure and the T instance. This allocation isn't done using any class-specific operator new (and shouldn't be). If the class-specific operator new sets up stricter alignment than what the default allocator does, then it's easy to see how this can fail when the default allocator gets used.

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    Also even if the ``operator new`` is using 16-byte alignment (such as with x64 native), the standard doesn't require the extra space allocated with ``std::make_shared`` respects the alignment of the class itself. – Chuck Walbourn Jul 05 '15 at 21:41
  • Took me a thousand hours of debugging to finally find the culprit. Thanks! – rwols May 12 '16 at 14:03
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This is why sometimes intrusive ref-count is preferred over std::shared_ptr, you can explicitly declare the alignment requirement for your class.

Also, remember to declare the destructor as private to forbid (de)allocation on stack.

andoryu-
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