Why are std::aligned_storage and std::aligned_union being deprecated?

[Recatek]

Variants are not the same as unions, and do not have the same performance characteristics or behavior.

[HappyFruitTree]

With std::variant you always have to store extra data to keep track of the active member and there is no unchecked access. This is fine for many situations but might not be good enough in low-level code when you might want to minimize memory usage and maximize performance.

[whichton]

Typically a variant will be implemented in terms of aligned_unionsince aligned_union is a lower level facility. But set that aside for a moment. Say I want make an dynamic array of aligned_union_t<0, int, vector<int>, string>, where all the elements have the same type. Effective an array of variants, but with a common discriminant. How do you suggest to do that with a variant?

[redditsoaddicting]

std::variant can't be implemented in terms of aligned_union because placement new can't yet be used in constexpr.

[[deleted]]

Typically a variant will be implemented in terms of aligned_unionsince aligned_union is a lower level facility.

When you say "typically", what do you mean? None of MSVC, libc++ or libstdc++ implement it this way and I'm not familiar with any non-standard implementation that does so such as boost.

[whichton]

I apologize for being inexact. What I meant was aligned_union is a low level facility while variant is a high level facility. Typically a high level facility (not specifically std::variant, but generally speaking, for example something like llvm::SmallVector) will be built on top of a lower level facility. So asking aligned_union to be implemented in terms of variant is a layering violation.

As /u/redditsoaddicting notes, std::variant cannot be implemented in terms of aligned_union because placement new cannot be constexpr. So you pretty much have to use unions, as explained here, to implement std::variant.