If I remember right, FixedBufferAllocator requires you to set an upper bound for the amount of memory you allocate -- you don't have to get the number exactly r

If I remember right, FixedBufferAllocator requires you to set an upper bound for the amount of memory you allocate -- you don't have to get the number exactly right or anything. Runtime allocators also have a limit to how much memory you can allocate. It's usually higher, but there's still a limit. Your computer doesn't have an infinite amount of memory.