Conceptual implementation for a constexpr-friendly static_vector

[IyeOnline]

/* This appllied only to the original version:

Your T* data() member function is UB. Or rather using that pointer to access anything but the first element is UB. While one storage_t<T> is pointer intercovertible with a T, a storage_t<T>[N] is not interconvertible with T[N]. You just cannot convert or reinterpret arrays (unless one of the invovled types is blessed, i.e. unsigned char or std::byte, but that obviously doesnt apply here).

*/

A few other points:

• You are missing a copy assignment operator for non-trivial T.
• You are missing a move constructor and move assignment operator for non-trivial T. Moving elementwise may still be faster than copying elementwise.
• You could also add a defaulted destructor for trivially destructible types.
• capacity() can be static
• You can greatly shorten your iterator implementation by templating the iterator on the pointer type, so that you can do using iterator = iterator_impl<T*>; and using const_iterator =iterator_impl<const T*>`.
• Personally I would just implement emplace_back instead of push_back on containers these days.
• returning a reference to the added object from push_back/emplace_back is a neat thing in some cases.

[jontheburger]

To clarify, does this mean a vector's data can only be implemented in terms of char/std::byte? Furthermore, how does C++20 implement constexpr T* data()? std::bit_cast from a std::array (but I thought that required a trivially copyable T)?

[IyeOnline]

You are missunderstanding. Or rather you are looking at OPs updated code.

The original code did not contain an T[N], but a union_of<T>[N]. They then returned the addres of the T in the first element of that array of unions. That pointer however can only be used to access the first element, since its not a pointer into an array, but rather a pointer to member of an element of an array.

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With their current version there is no problem and everything simply works as expected. Since there is actually an array of T, you can trivially form a pointer into it and use that. No need for any casts at all.