To create a new polymophic_allocator, ppl can use some default resources
std::pmr::polymorphic_allocator<char> newdel { std::pmr::new_delete_resource() };
std::pmr::polymorphic_allocator<char> synced { std::pmr::synchronized_pool_resource() };
std::pmr::polymorphic_allocator<char> nonsync { std::pmr::unsynchronized_pool_resource() };
My question is the new_delete_resource thread-safe?