If a class T has an alignment requirement, such as one specified by the alignas keyword, are std::optional<T> and boost::optional<T> guaranteed to respect said alignment?
If they were simply wrapper classes for a T object and a bool initialized, then they would automatically align their T member as required, but the standard and boost documentation state that they can hold no object and deal well with expensive to construct objects. From this I understand that they don't simply contain a T. Rather, they seem to allocate a buffer upon which the T is constructed or destroyed manually. Therefore, the C++ language will not automatically align the buffer because it is not of type T.
So, do std::optional<T> and boost::optional<T> properly align their managed T object? Do they also provide optional<T>::operator new and optional<T>::operator new[] that respect the alignment requirement?
A conforming implementation of std::optional must respect the alignment requirements of its value_type according to the C++17 standard draft:
23.6.3 Class template optional [optional.optional]
(...) The contained value shall be allocated in a region of the optional storage suitably aligned for the type T.
An implementor could use a union potentially containing the value as member, ensuring proper alignment.
As Thomas already mentioned while I was looking it up, boost::optional does ensure proper alignment by use of aligned_storage<T>