In my example, the T is one specific type. So you could have std::vector<Cat>. If you also have Dogs, you just make another vector std::vector<Dog>. It works fine with the standard allocator. You don't have to do anything special.

You can do a Vector<TaggedUnion<Union>>. https://pagghiu.github.io/SaneCppLibraries/library_foundatio...

I have not been working (yet) on custom allocators, but that's on the roadmap: https://pagghiu.github.io/SaneCppLibraries/library_container...

If you wanted a factory that allocated vectors of a variety of known types, you'd probably declare template <typename T> before the generator function, so that on compilation a separate version of that function would be emitted for each type you passed to it.

(Not really a c++ expert, but that's my understanding; someone more knowledgeable can correct me).

Why would you need a variant? If you have one of something, put it on the stack. If you have a lot of the a type, put them in a vector.

Isn't that an arena allocator at that point?