Constant time array indexing depends on a fixed array member size. So, the actual memory layout of storing the nodes with non-fixed size could not use an array

Constant time array indexing depends on a fixed array member size. So, the actual memory layout of storing the nodes with non-fixed size could not use an array of nodes. I suppose you could (should) write your own allocator to ensure nodes are allocated in a contiguous piece of memory, and have a separate array of indexes / pointers that tells you where the n-th node starts. That is another array that'll have to be kept in cache though.